Effect of changing dietary sodium on the airway response to histamine

Why Is Your Patient Still Short of Breath? Understanding the Complex Pathophysiology of Dyspnea in Chronic Kidney Disease

Dyspnea is one of the most common symptoms associated with CKD. It has a profound influence on the quality of life of CKD patients, and its underlying causes are often associated with a negative prognosis. However, its pathophysiology is poorly understood. While hemodialysis may address fluid overload, it often does not significantly improve breathlessness, suggesting multiple and co-existing alternative issues exist. The aim of this article is to discuss the main pathophysiologic mechanisms and the most important putative etiologies underlying dyspnea in CKD patients. Congestive heart failure, unrecognized chronic lung disease, pulmonary hypertension, lung fibrosis, air microembolism, dialyzer bio-incompatibility, anemia, sodium, and fluid overload are potential frequent causes of breathing disorders in this population. However, the relative contributions in any one given patient are poorly understood. Systemic inflammation is a common theme and contributes to the development of endothelial dysfunction, lung fibrosis, anemia, malnutrition, and muscle wasting. The introduction of novel multimodal imaging techniques, including pulmonary functional magnetic resonance imaging with inhaled contrast agents, could provide new insights into the pathophysiology of dyspnea in CKD patients and ultimately contribute to improving our clinical management of this symptom.

Dietary sodium manipulation and asthma

BACKGROUND

There is a wide geographical variation in the prevalence of asthma and observational studies have suggested that dietary sodium may play a role.

OBJECTIVES

To assess the effect of dietary sodium manipulation on asthma control.

SEARCH STRATEGY

We carried out a search using the Cochrane Airways Group asthma register. We searched the bibliographies of included randomised controlled trials (RCTs) for additional studies. We carried out the most recent search in November 2010.

SELECTION CRITERIA

We considered only RCTs that involved dietary sodium reduction or increased sodium intake in patients with asthma.

DATA COLLECTION AND ANALYSIS

Both review authors assessed study and extracted data. We conducted data analyses in RevMan 5 using mean differences and random effects.

MAIN RESULTS

We identified a total of nine studies in relation to sodium manipulation and asthma, of which five were in people with asthma (318 participants), and four in people with exercise-induced asthma (63 participants). There were no significant benefits of salt restriction on the control of asthma. There was some evidence from the exercise-induced asthma studies that a low sodium diet may improve lung function after exercise and possibly baseline lung function, but this is based on findings from a very small numbers of participants.

AUTHORS' CONCLUSIONS

This review did not find any evidence that dietary sodium reduction significantly improves asthma control. Although dietary sodium reduction may result in improvements in lung function in exercise-induced asthma, the clinical significance of this effect is unclear.

Salt intake, asthma, and exercise-induced bronchoconstriction: a review

Despite the heterogeneous treatment options for patients with asthma, there remains a substantial burden of unaddressed disease, even with optimal treatment. Epidemiological studies indicate that patients frequently resort to complimentary and alternative therapies when being treated for asthma and other chronic health conditions. Changes in diet associated with the development of a more affluent lifestyle is one of the environmental factors considered to contribute to the increased prevalence of asthma in the past few decades. Dietary sodium in particular has been considered to be a dietary constituent implicated in this phenomenon. This article reviews the studies conducted that have questioned whether reducing dietary salt intake potentially improves pulmonary function and airway hyper-responsiveness in asthmatics, as well as studies evaluating dietary salt intake on the severity of exercise-induced bronchoconstriction (EIB). The data presented supporting dietary salt restriction for reducing airway hyper-responsiveness in asthmatics is encouraging, though not clinically convincing. Studies conducted previously have been limited for a variety of reasons, including limitations related to the experiment and populations studied. However, in studies that evaluated the severity of EIB in asthmatic individuals and involved altered dietary salt intake, data have been more convincing. A low-sodium diet maintained for 1 to 2 weeks decreases bronchoconstriction in response to exercise in individuals with asthma. There are no data regarding the longer-term effects of a low-sodium diet on either the prevalence or severity of asthma or on EIB. As a low-sodium diet has other beneficial health effects, it can be considered a therapeutic option for adults with asthma, although it should be considered as an adjunctive intervention to supplement optimal pharmacotherapy, and not as an alternative.

A multivariate analysis of serum nutrient levels and lung function

Background

There is mounting evidence that estimates of intakes of a range of dietary nutrients are related to both lung function level and rate of decline, but far less evidence on the relation between lung function and objective measures of serum levels of individual nutrients. The aim of this study was to conduct a comprehensive examination of the independent associations of a wide range of serum markers of nutritional status with lung function, measured as the one-second forced expiratory volume (FEV₁).

Methods

Using data from the Third National Health and Nutrition Examination Survey, a US population-based cross-sectional study, we investigated the relation between 21 serum markers of potentially relevant nutrients and FEV₁, with adjustment for potential confounding factors. Systematic approaches were used to guide the analysis.

Results

In a mutually adjusted model, higher serum levels of antioxidant vitamins (vitamin A, beta-cryptoxanthin, vitamin C, vitamin E), selenium, normalized calcium, chloride, and iron were independently associated with higher levels of FEV₁. Higher concentrations of potassium and sodium were associated with lower FEV₁.

Conclusion

Maintaining higher serum concentrations of dietary antioxidant vitamins and selenium is potentially beneficial to lung health. In addition other novel associations found in this study merit further investigation.

Does a low sodium diet improve asthma control? A randomized controlled trial

RATIONALE

Observational studies and initial randomized trials have indicated that a low sodium diet may improve asthma control.

OBJECTIVES

We tested the hypothesis that a low sodium diet would improve asthma control over a 6-week period.

METHODS

Participants with a physician diagnosis of asthma and measurable bronchial reactivity to methacholine entered a randomized double-blind placebo-controlled trial. All adopted a low sodium diet and were randomized to receive either 80 mmol/day of oral sodium supplements (normal sodium intake) or matched placebo (low sodium intake) for 6 weeks. The primary outcome was change in bronchial reactivity to methacholine; secondary outcomes were change in lung function, morning and evening peak expiratory flow, asthma symptoms score, daily bronchodilator use, Juniper Standardized Asthma Quality of Life Questionnaire score, and atopy.

MEASUREMENTS AND MAIN RESULTS

A total of 220 individuals entered the study, of whom 199 completed the protocol. In the low sodium-intake group, mean daily urinary sodium excretion decreased by 20 mmol (SD, 64 mmol) and in the normal-sodium-intake group increased by 28 mmol (SD, 74 mmol). There were no differences between the two groups in the primary or secondary outcome measures; the mean difference in bronchial reactivity between the low- and normal-intake groups was -0.03 doubling doses of methacholine (95% confidence interval, -0.60 to 0.53).

CONCLUSIONS

The use of a low sodium diet as an adjunctive therapy to normal treatment has no additional therapeutic benefit in adults with asthma and bronchial reactivity to methacholine.

Urban/rural differences in diet and atopy in South Africa

BACKGROUND

There are large differences in the prevalence of atopic disease between urban and rural areas of developing countries, without good explanation. Diet has been associated with atopic disease, but studies of specific nutrients are contradictory, cross-sectional studies often being unsupported by trials. We investigated diet as an explanation for the difference in the prevalence of atopy between urban and rural areas in South Africa.

METHODS

A cross-sectional analysis of food frequency questionnaires and allergen skin tests from 698 children aged 8-13 years, recruited from 24 schools in Cape Province, South Africa, who were taking part in a case-control study of exercise-induced bronchospasm. Food frequency data were analysed with a principal components analysis (PCA).

RESULTS

The first two principal components of diet explained 25% of the variance, and discriminated almost perfectly between urban and rural subjects. The 'urban' component of diet was strongly associated with positive skin tests even after adjusting for urban residence. There were no significant associations between individual foods or nutrients and positive skin tests, allowing for multiple testing.

CONCLUSIONS

Diet explains part of the difference in prevalence between urban and rural areas. The ability to demonstrate this using PCA, but not by exhaustive analysis of all foods, reflects the value of reducing the number of dietary dimensions. The number of foods and nutrients which can be assessed, and the possibility of confounding and effect modification, make it difficult to identify the features of diet most directly implicated in disease. This may explain inconsistencies in dietary studies.

Dietary sodium intake and asthma: an epidemiological and clinical review

The changes in diet associated with the development of a more affluent lifestyle have been considered one of the environmental factors that may have contributed to the rise in the prevalence of asthma over the past few decades, and dietary sodium has been considered to be a dietary constituent which may be implicated in this phenomenon. The data presented in this review demonstrate that adoption of a low sodium diet for a period of 2-5 weeks may improve lung function and decrease bronchial reactivity in adults with asthma, while sodium loading appears to have a detrimental effect. Similarly, a low sodium diet maintained for 1-2 weeks decreases bronchoconstriction in response to exercise in individuals with asthma. There is no data as to the longer-term effect of a low sodium diet on either the prevalence or severity of asthma or on exercise-induced bronchoconstriction. As a low sodium diet has other beneficial health effects, it can be considered as a therapeutic option for adults with asthma, although it should be considered as an adjunctive intervention to supplement optimal pharmacological management of asthma and not as an alternative. If the relationship between higher sodium intake and increased prevalence and severity of asthma is causal, then there are potential population benefits for asthma as well as cardiovascular disease to be derived from public health measures to reduce sodium consumption.

Sodium intake and bronchial hyperresponsiveness in adults

BACKGROUND

Several investigations suggested a relationship between sodium intake and asthma and bronchial hyperresponsiveness (BHR), respectively. However, clinical and epidemiological studies did not show consistent finding.

OBJECTIVE

We analysed the association between dietary sodium intake and BHR to methacholine among 613 adults aged 20-65 years as one of the two German centres of the European Community Respiratory Health Survey (ECRHS).

METHODS

Dietary sodium intake was estimated from a 3-day weighed record of food intake. We applied multiple logistic regression models contrasting the three higher quartiles of sodium intake versus the lowest to assess the risk of BHR and mild BHR estimated by PD20 and PD10, respectively, controlling for potential confounders and stratified for sex. In addition, we analysed PD20 (dose of methacholine causing a fall of 20% in forced expiratory volume in 1s) as continuous variable expressed as transformed dose-response slope (tDRS) in the linear model.

RESULTS

Women were as expected more likely to be bronchial hyperresponsive (PD20: 26.1%; PD10: 52.2%) than men (PD20:15.8%; PD10: 34.8%) and had a lower mean daily sodium intake (2.36 g) compared with men (3.15 g). Logistic regression did not show any significant relationship between sodium intake and BHR in terms of PD20 after adjustment for age group, education, smoking status, body mass index and height in men or women. However, mild BHR assessed as PD10 was statistically significant positively related to the third (OR: 2.35; CI: 1.11-5.00) and highest quartile of sodium intake (OR: 2.28; CI: 1.06-4.88) in women, but not in men for third quartile (OR: 1.29; CI: 0.68-2.44) and for fourth quartile (OR: 1.07; CI: 0.56-2.07), respectively.

CONCLUSION

Sodium intake by several food items does not alter BHR assessed as PD20 to methacholine but may increase mild BHR assessed as PD10. We conclude that, in addition, PD10 has to be considered when the effect of sodium intake on BHR is studied.

Should we restrict chloride rather than sodium?

Low-salt diets have potential for prevention and treatment of hypertension, and may also reduce risk for stroke, left ventricular hypertrophy, osteoporosis, renal stones, asthma, cataract, gastric pathology, and possibly even senile dementia. Nonetheless, the fact that salt restriction evokes certain counter-regulatory metabolic responses-- increased production of renin and angiotensin II, as well as increased sympathetic activity--that are potentially inimical to vascular health, has suggested to some observers that salt restriction might not be of unalloyed benefit, and might in fact be contraindicated in some "salt-resistant" subjects. Current epidemiology indicates that lower-salt diets tend to reduce coronary risk quite markedly in obese subjects, whereas the impact of such diets on leaner subjects (who are less likely to be salt sensitive) is equivocal--seemingly consistent with the possibility that salt restriction can exert countervailing effects on vascular health. There is considerable evidence that sodium chloride, rather than sodium per se, is responsible for the known adverse effects of dietary salt. Other non-halide sodium salts, such as sodium citrate or bicarbonate, do not raise plasma volume, increase blood pressure, boost urinary calcium loss, or promote stroke in stroke-prone rats. Nonetheless, these compounds have been shown to blunt the impact of salt restriction on renin, angiotensin II, and sympathetic activity in humans. This may rationalize limited clinical evidence that organic sodium salts can decrease blood pressure in salt-restricted hypertensives. Furthermore, organic sodium salts have an alkalinizing metabolic impact favorable to bone health. These considerations suggest that restricting dietary salt to the extent feasible, while encouraging consumption of organic sodium salts in mineral waters, soft drinks, or other nutraceuticals--preferably in conjunction with organic potassium salts and taurine--may represent a superior strategy for controlling blood pressure, promoting vascular health, and preserving bone density. Further clinical studies should determine whether a moderately salt-restricted diet supplemented with organic sodium salts has a better and more uniform impact on hypertension than salt restriction alone, while rodent studies should examine the comparative impact of these regimens on rodents prone to vascular disease.

Dietary salt reduction or exclusion for allergic asthma

BACKGROUND

There is a wide geographical variation in asthma prevalence and one explanation may be in dietary salt consumption.

OBJECTIVES

To assess the effect of dietary sodium reduction in patients with asthma.

SEARCH STRATEGY

A search was conducted using the Cochrane Airways Group asthma register. Bibliographies of included randomised controlled trials (RCTs) were searched for additional studies. Authors of identified RCTs were contacted for other studies. The most recent search was carried out in February 2004.

SELECTION CRITERIA

All studies were to be RCTs that involved dietary salt reduction or increased salt intake in patients with asthma. Studies of other allergic conditions such as hay fever, allergic rhinitis and eczema were considered patients with asthma were separately identified.

DATA COLLECTION AND ANALYSIS

Study quality was assessed and data extracted by two reviewers. All data analysis was conducted using the Cochrane Collaboration software (RevMan).

MAIN RESULTS

Six RCTs were included in this review. All studies were small size and of short duration. Data from only four studies could be pooled. Low sodium diet was associated with a significantly lower urine sodium excretion than normal or high salt diets. There were no significant differences in any asthma outcome between low salt and normal or high salt diets, however the confidence intervals were wide. FEV(1) with low salt compared to normal diet showed a WMD 0.09 L with a 95% confidence interval (95%CI) -0.19 L to 0.38 L, and compared to a high salt diet WMD 0.18 L; 95%CI -0.11 L to 0.48 L. Daily PEFR was also non-significantly higher with low salt diet compared to normal (WMD 19.52 L/min; 95% CI -21.22 to 60.25) and high salt diet (WMD 7.57 L/min; 95% CI -37.52 to 52.67). Reliever bronchodilator with the low salt diet when compared to both the normal and high salt diets showed WMD -0.07 puffs/day; 95%CI -0.94 to 0.81 & WMD -0.65 puffs/day; 95%CI -1.75 to 0.45, respectively.

REVIEWERS' CONCLUSIONS

Based on currently available evidence it is not possible to conclude whether dietary salt reduction has any place in the treatment or management of asthma. The results of this review do indicate an improvement in pulmonary function with low salt diet, however further large scales trials are required before any firm conclusions can be reach.

Dietary components with demonstrated effectiveness in decreasing the severity of exercise-induced asthma

Exercise-induced asthma (EIA) occurs in up to 90% of individuals with asthma and approximately 10% of the general population without asthma. EIA describes a condition in which vigorous physical activity triggers acute airway narrowing with heightened airway reactivity resulting in reductions in forced expiratory volume in 1 second of greater than 10% compared with pre-exercise values. Treatment of EIA almost exclusively involves the use of pharmacological medications. However, there is accumulating evidence that a dietary excess of salt and omega-6 fatty acids, and a dietary deficiency of antioxidant vitamins and omega-3 fatty acids, can modify the severity of EIA. The modification of these dietary factors has the potential to reduce the incidence and prevalence of this disease. The dietary component most studied to date is dietary salt. Recent studies have supported a role for dietary salt as a modifier of the severity of EIA, suggesting that salt-restrictive diets can reduce the severity of EIA. Since EIA is part of the asthmatic diathesis, it is possible that EIA may serve as a useful model for investigation of potential dietary interventions for reducing airway hyperresponsiveness.

Food and nutrient intakes and asthma risk in young adults

BACKGROUND

Some aspects of diet are relatively newly recognized potential risk factors for asthma, but the evidence to date is conflicting.

OBJECTIVE

The goal was to determine whether the food and nutrient intakes of adults with asthma differ from those of adults without asthma.

DESIGN

This was a community-based, cross-sectional study of 1601 young adults ( +/- SD age: 34.6 +/- 7.1 y) who were initially recruited by random selection from the federal electoral rolls in Melbourne in 1999. Subjects completed a detailed respiratory questionnaire, a validated semiquantitative food-frequency questionnaire, skin-prick testing, and lung function tests, including a methacholine challenge test for bronchial hyperreactivity (BHR). A total of 25 nutrients and 47 food groups were analyzed by using multiple logistic regression with alternate definitions of asthma and atopy as the outcomes.

RESULTS

Whole milk appeared to protect against current asthma (odds ratio: 0.66; 95% CI: 0.46, 0.97), doctor-diagnosed asthma (0.73; 0.54, 0.99), BHR (0.68; 0.48, 0.92), and atopy (0.71; 0.54, 0.94). Conversely, soy beverage was associated with an increased risk of current asthma (2.05; 1.19, 3.53), doctor-diagnosed asthma (1.69; 1.04, 2.77), and BHR (1.65; 1.00, 2.71). Apples and pears appeared to protect against current asthma (0.83; 0.71, 0.98), asthma (0.88; 0.78, 1.00), and BHR (0.88; 0.77, 1.00).

CONCLUSIONS

The consumption of dairy products, soy beverages, and apples and pears, but not of nutrients per se, was associated with a range of asthma definitions. Dietary modification after diagnosis is one possible explanation for this finding. Intervention studies using whole foods are required to ascertain whether such modifications of food intake could be beneficial in the prevention or amelioration of asthma.

Harmful effects of dietary salt in addition to hypertension

In addition to raising the blood pressure dietary salt is responsible for several other harmful effects. The most important are a number which, though independent of the arterial pressure, also harm the cardiovascular system. A high salt intake increases the mass of the left ventricle, thickens and stiffens conduit arteries and thickens and narrows resistance arteries, including the coronary and renal arteries. It also increases the number of strokes, the severity of cardiac failure and the tendency for platelets to aggregate. In renal disease, a high salt intake accelerates the rate of renal functional deterioration. Apart from its effect on the cardiovascular system dietary salt has an effect on calcium and bone metabolism, which underlies the finding that in post-menopausal women salt intake controls bone density of the upper femur and pelvis. Dietary salt controls the incidence of carcinoma of the stomach and there is some evidence which suggests that salt is associated with the severity of asthma in male asthmatic subjects.

Dietary salt reduction or exclusion for allergic asthma

BACKGROUND

There is a wide geographical variation in asthma prevalence and one explanation may be in dietary salt consumption.

OBJECTIVES

To assess the effect of dietary sodium reduction in patients with asthma.

SEARCH STRATEGY

A search was conducted using the Cochrane Airways Group asthma register. Bibliographies of included randomised controlled trials (RCTs) were searched for additional studies. Authors of identified RCTs were contacted for other studies.

SELECTION CRITERIA

All studies were to be RCTs that involved dietary salt reduction or increased salt intake in patients with asthma. Studies of other allergic conditions such as hay fever, allergic rhinitis and eczema were considered patients with asthma were separately identified.

DATA COLLECTION AND ANALYSIS

Study quality was assessed and data extracted by two reviewers. All data analysis was conducted using the Cochrane Collaboration software (RevMan 4.1.1).

MAIN RESULTS

Fifty-six abstracts were identified and 15 studies were reviewed in full text. Five fulfilled the inclusion criteria. Nine were excluded. One was published in duplicate. Complete agreement was reached between the reviewers on inclusion or exclusion of all studies. All studies were small and of short duration. Data from only three could be pooled. Low sodium diet was associated with a significantly lower urine sodium excretion than normal or high salt diets. There were no significant differences in any asthma outcome between low salt and normal or high salt diets, but FEV1 was slightly higher with low salt compared to normal, WMD 0.09 L (95% confidence interval (CI) -0.26, 0.44 L, n=88), as was daily PEFR, WMD 11 l/min (95% CI -81, 103 l/min, n=78). With low compared to high salt, FEV1 was slightly higher WMD 0.22 L (95% CI -0.14, 0.59 L, n=88), as was daily PEFR, WMD 14 l/min (95% CI -41, 68 l/min, n=78). Bronchodilator use was slightly lower, WMD 0.7 puffs/day (95% CI -1.8, 0.5 puffs/day, n=62).

REVIEWER'S CONCLUSIONS

Based on currently available evidence it is not possible to conclude whether dietary salt reduction has any place in the treatment or management of asthma.

Dietary salt restriction improves pulmonary function in exercise-induced asthma

PURPOSE

Exercise-induced asthma (EIA) occurs in approximately 90% of persons with asthma. The mechanism has not been delineated. Epidemiological studies have suggested that dietary salt may play a role in airway responsiveness. Therefore, the purpose of this study was to determine the influence of both elevated and restricted salt diets on pulmonary function in subjects with EIA.

METHODS

Eight subjects with EIA and eight subjects without EIA (control) participated in a double-blind crossover study. Pulmonary function was determined pre- and post-exercise challenge before and after 2 wk on a normal salt, sodium chloride, diet (NSD), a low salt diet (LSD), and a high salt diet (HSD). A 1-wk washout occurred between diets.

RESULTS

Diet had no effect on preexercise pulmonary function values in either group and had no effect on postexercise pulmonary function values in control subjects. However, LSD improved and HSD worsened postexercise pulmonary function values in EIA subjects. Forced expiratory volume in 1 s (FEV1) decreased by at least 10% in EIA subjects with exercise. In EIA subjects, FEV1 decreased by 14+/-6% on LSD, 20+/-7% on NSD, and 24+/-6% on HSD at 15 min postexercise. Similar patterns were observed for forced vital capacity and peak expiratory flow rates. Although LSD did not normalize pulmonary function in EIA, it did improve it.

CONCLUSIONS

These data suggest that individuals with EIA might benefit from lower salt diets.

Diet and asthma

The role of food intolerance in asthma is well recognized, and where food avoidance measures are instituted considerable improvement in asthma symptoms and in reduction in drug therapy and hospital admissions can result. These benefits may have a greater impact in those patients with greater symptoms. However, the promise of such benefits should not result in an approach which ignores inhaled drug therapy, or in a dietary regime which is inappropriate in the face of mild symptoms. Whilst sub-optimal intake of dietary nutrients is also a recently recognized potential risk factor for asthma, available data are insufficient to implicate any as casual. A number of studies have sought to establish the role of the antioxidant vitamins, A, C and E and selenium, yet others of the elements sodium and magnesium. Sub-optimal nutrient intake may enhance asthmatic inflammation, consequently contributing to bronchial hyperreactivity. Prospective studies of supplementation therapy are needed to confirm this.

Asthma, oxidant stress, and diet

It has been suggested that the increased prevalence of atopy and asthma observed in many developed countries over the past 30 y is in part the result of a decrease in the incidence and severity of early childhood infections. The immunologic consequence of this phenomenon has been the expansion of T-lymphocyte populations away from the T-helper 1 (Th1) subset and in the direction of the Th2 subset. This leads to the creation of a cytokine-mediated propensity for the development of an intense inflammatory response in the airways, resulting in oxidative stress, airway tissue injury, and the development of atopy and asthmatic symptomatology. Over this same period, there has been a decreased intake of dietary substances that contribute to antioxidant defense, and this appears to have contributed to the rise of atopy and asthma. Studies evaluating the efficacy of these antioxidant substances in the prevention of asthma and as adjuvants in the treatment of asthma are reviewed, and suggestions are made for the direction of future studies.

Prevalence of atopy, asthma symptoms and diagnosis, and the management of asthma: comparison of an affluent and a non-affluent country

BACKGROUND

The prevalence of childhood asthma and of atopy varies widely between countries. However, few studies have compared the pattern of diagnosis and management of asthma, or the role of atopy in predisposing to asthma between a less affluent country and a more affluent country. The aim of this study was to compare the prevalence of symptoms, diagnosis, and management of asthma, and the prevalence of atopy as measured by skin prick tests in Nigeria and Australia using a standardised methodology.

METHODS

Respiratory history was collected using a validated questionnaire administered to parents, and atopy was measured with skin prick tests in 654 Australian and 566 Nigerian children aged 8-11 years (70% consent rate in Australia, 60% in Nigeria).

RESULTS

Wheeze and persistent cough were less prevalent in Nigeria (10.2% and 5.1%, respectively) than in Australia (21.9% and 9.6%, respectively), caused less morbidity, and were less likely to be labelled or treated as asthma than in Australia. There was no significant difference in the overall prevalence of atopy between the two countries (Australia 32. 5%, Nigeria 28.2%). Atopy was a strong risk for wheeze in both countries (odds ratio (OR) 3.4 (95% CI 2.3 to 5.1) in Australia, 1.8 (95% CI 1.0 to 3.3) in Nigeria), especially atopy to house dust mites (OR 3.1 (95% CI 2.1 to 4.7) in Australia, 2.4 (95% CI 1.3 to 4. 3) in Nigeria).

CONCLUSION

Although there was a similar prevalence of atopy in both countries, Australian children had a higher prevalence of asthma symptoms. Further studies are needed to determine why atopic children in Australia are more at risk of developing asthma. Such studies will have important implications for the prevention of asthma.

Dietary influences on chronic obstructive lung disease and asthma: a review of the epidemiological evidence

The epidemiological evidence for a relationship between diet and indicators of asthma and chronic obstructive pulmonary disease (COPD) is evaluated. The review focuses on the intake of Na, n-3 fatty acids, and antioxidant vitamins as well as fruit and vegetables. Experimental studies suggest that a high-Na diet has a small adverse effect on airway reactivity in asthma patients. However, observational studies provide no clear evidence that high Na intake has adverse effects on airway reactivity or asthma symptoms in open populations. n-3 Polyunsaturated fatty acids, which are present in fish oils, are metabolized into less broncho-constricting and inflammatory mediators than n-6 polyunsaturated fatty acids. Studies in the general adult population suggest that a high fish intake has a beneficial effect on lung function, but the relationship with respiratory symptoms and clinically-manifest asthma or COPD is less evident. Also, experimental studies in asthma patients have not demonstrated an improvement in asthma severity after supplementations with fish oil. Several studies showed a beneficial association between fruit and vegetable intake and lung function, but the relationship with respiratory symptoms and the clinically-manifest disease was less convincing. A similar pattern was found for vitamin C in relation to indicators of asthma and COPD, but there are still conflicting results with respect to vitamin E and beta-carotene. In conclusion, the epidemiological evidence for a beneficial effect on indicators of asthma and COPD of eating fish, fruit and vegetables is increasing. However, the effectiveness of dietary supplementation in open-population samples is often not demonstrated. Several unresolved questions are raised, which should be addressed in future studies on the relationship between diet and respiratory disease.

New insights into risk factors of asthma

The prevalences of bronchial asthma in the Asia-Pacific region range from 0 to 24% and appear to be increasing in some countries. The increased prevalence of asthma may be related to the urbanization of these countries or areas. Risk factors relating to the development of asthma are multiple and complex. These include: Predisposing or genetic factors: (atopy and bronchial hyperresponsiveness; BHR) that increase an individuals susceptibility. Longitudinal studies in children have shown that BHR precedes asthma in some individuals. A gene governing BHR is located near a major locus that regulates serum IgE levels on chromosome 5 q. An additional gene that determines the specificity of the immune response located in the human leukocyte antigen complex (HLA) may govern the specificity of the immune response to common aeroallengens in some individuals. Causal Factors: inhaled allergens are the most important causal factors of asthma, which include indoor allergens (domestic mites, animal, cockroach and fungus allergens), domestic mites being the most common potential allergen, and outdoor allergens (pollens from trees, grasses and weeds). Owing to the geographic location and different sensitivity to allergen between races, allergens vary from area to area. Certain drugs, food and food additives are also the cause of asthma attack. Contributing factors: smoking is an important trigger and a serious problem in most Asian-Pacific countries. Air pollution in particular sulfur dioxide (SO2), nitrogen dioxide (NO2) and respirating particles are common contributing factors. Very low concentration of SO2 (0.5 ppm) can cause bronhospasm in asthmatics. A combination of low concentration of SO2 and NO2 often encountered in heavy traffic further enhances the airway responsiveness to inhaled allergen. In addition, respiratory virus infection is closely associated to the development of asthma in childhood.

Consumption of oily fish and childhood asthma risk

OBJECTIVE

To investigate the association between diet and airway disease in children in the light of epidemiological studies suggesting that consumption of fish more than once a week reduces the risk of developing airway hyperresponsiveness (AHR).

DESIGN

Diet was assessed by a detailed food frequency questionnaire and airway disease by respiratory symptoms or airway responsiveness to exercise.

METHODS

A questionnaire, containing questions about the frequency of eating more than 200 foods, was sent to the parents of 574 children in whom we had measured recent wheeze (by questionnaire), AHR (by exercise) and atopy (by skin prick tests) six months before this study. We defined current asthma as the presence of both recent wheeze and AHR.

RESULTS

Response rate to the questionnaire was 81.5% (n=468.) After adjusting for confounders such as sex, ethnicity, country of birth, atopy, respiratory infection in the first two years of life and a parental history of asthma or smoking, children who ate fresh, oily fish (>2% fat) had a significantly reduced risk of current asthma (odds ratio, 0.26; 95% confidence interval, 0.09-0.72; P<0.01). No other food groups or nutrients were significantly associated with either an increased or reduced risk of current asthma.

CONCLUSION

These data suggest that consumption of oily fish may protect against asthma in childhood.

Salt and public health--policies for dietary salt in the Nordic countries

OBJECTIVE

To review current knowledge about excess dietary salt as a risk factor for diseases and to compare and discuss the national policies for dietary salt in the Nordic countries.

DESIGN

Literature review, questionnaire and interviews. Nordic comparative study.

SETTING

The Medline bibliographic system and authorities responsible for the national nutritional policies in the Nordic countries.

SUBJECTS

Scientific articles published since 1988 concerning the impact of excess dietary salt on health and key persons with responsibilities for the formulation of national recommendations about dietary salt in each of the five Nordic countries.

MAIN OUTCOME MEASURES

Articles dealing with (i) epidemiological observations, (ii) sensitive groups, (iii) underlying biological mechanisms concerning the contents of the latest national recommendations and the wording of legislation and decrees about salt in food-stuffs in the Nordic countries.

RESULTS

The impact of excess dietary salt on health mainly concerns hypertension, gastric cancer, osteoporosis and bronchial hyperreactivity. The national policy for dietary salt in Finland differs from the other Nordic countries. In Finland salt is a food additive and a variety of special regulations exist. Finland has chosen a more active way to achieve the existing Nordic recommendation of 5 g salt per day. The daily intake is about 10 g per day in all the Nordic countries.

CONCLUSIONS

There is need for further epidemiological studies of the relationship between excess dietary salt and gastric cancer, osteoporosis, and bronchial hyperreactivity. The knowledge of the relationship between dietary salt and raised blood pressure is sufficient to put force behind the implementation of existing Nordic recommendations.

Effect of dietary sodium on airways responsiveness and its importance in the epidemiology of asthma: an evaluation in three areas of northern England

BACKGROUND

Although several investigations have shown a relationship between asthma (or its surrogate, airways responsiveness) and dietary or urinary sodium, others have not, and the matter remains controversial. This "salt effect" has been investigated during two recent epidemiological surveys of men in northern England. The first assessed the possible effect on airways responsiveness of occupational exposure to welding fumes, and the second characterised airways responsiveness in two geographically distinct residential areas. Thus, three separate study areas/populations were involved.

METHODS

Investigation 1 involved 1059 shipyard workers aged 16-27 years who were exposed variously to welding fumes, and Investigation 2 involved 587 men aged 20-44 years who lived in rural West Cumbria or in urban Newcastle upon Tyne. In Investigation 1, a 24 hour urine specimen was requested from each subject with quantifiable airways responsiveness (PD20 < or = 6400 micrograms methacholine) and from an equal number of subjects without measurable airways responsiveness from the same occupational subgroup. In Investigation 2, every subject was asked to provide a 24 hour urine specimen.

RESULTS

Of the men undergoing methacholine tests, satisfactory 24 hour urine specimens were obtained from 234 (22.1%) in Investigation 1 and 232 (39.5%) in Investigation 2. Analysis using multiple linear regression, multiple linear logistic regression, and multiple regression for censored data produced consistent results within each study population but conflicting results between them, such that there was no hint of a relationship between airways responsiveness and 24 hour urinary sodium excretion in the shipyard workers of Investigation 1 nor in the rural West Cumbrian population of Investigation 2, but an association was found in the urban Newcastle population of Investigation 2. All study populations were sufficiently large to demonstrate anticipated relationships between airways responsiveness and atopy, baseline FEV1, and (Newcastle only) age.

CONCLUSIONS

If airways responsiveness is related to dietary sodium the relationship is not likely to be strong.

Deleterious effects of salt intake other than effects on blood pressure

1. Salt intake is not only known to play an important role in determining blood pressure (BP) but has been shown to have other deleterious effects independent of BP. 2. Epidemiological and animal studies have provided evidence that salt intake may have an adverse effect on stroke mortality independent of BP. 3. Significant correlation between sodium excretion (as a measure of salt intake) and left ventricular (LV) hypertrophy has been shown in many clinical studies. Salt restriction has also been found to produce a significant reduction in LV mass. 4. In animal studies, salt restriction in uninephrectomized spontaneously hypertensive rats retarded renal glomerular injury and suppressed compensatory growth independent of hypertension. Moreover, a high sodium diet accelerated cerebral arterial disease even when no increases in BP could be detected. 5. Epidemiological data have shown an association between asthma mortality and regional purchases of table salt. Furthermore, dietary salt restriction in asthmatic patients results in improvement of symptomatology with lower consumption of bronchodilators. 6. Patients with essential hypertension are known to have increased urinary calcium excretion, and hypertension may be one factor that may increase the likelihood of osteoporosis. High salt intake is also associated with increased hydroxyproline excretion indicating increased resorption of bone. Sodium restriction reduces calcium excretion and may reduce bone demineralization and hip fractures in a similar manner to that seen with diuretics.

Dietary sodium intake and the risk of airway hyperreactivity in a random adult population

BACKGROUND

High dietary sodium intake has been identified as a potential cause of asthma and airway hyperreactivity. This study was designed to test the hypothesis that dietary sodium intake is an independent determinant of the risk of hyperreactivity in the general population, and to assess the role of atopy in the association between these factors.

METHODS

Airway reactivity to methacholine, atopy, 24 hour urinary sodium excretion, and self-reported smoking and symptom history were measured in a random sample of 1702 adults aged 18-70 from an administrative district of Nottingham. Hyperreactivity was defined as a PD20FEV1 of 12.25 mumol or less, and atopy was defined quantitatively as the mean allergen skin weal response to Dermatophagoides pteronyssinus, cat fur, and grass pollen, and categorically as the occurrence of any allergen response 1 mm or greater than the saline control. Multiple logistic regression analysis was used to estimate the independent relative odds of hyperreactivity, atopy, or symptoms in relation to sodium excretion in all 1702 subjects, and multiple linear regression to assess the independent relation between sodium excretion and mean allergen skin weal diameter, and the PD20 value amongst hyperreactive subjects.

RESULTS

There was no relation between the relative odds of hyperreactivity to methacholine and 24 hour urinary sodium excretion, either before or after adjustment for age, smoking, allergen skin weal diameter, and sex, and similarly no relation if the analysis was restricted to men or women only. The relative odds of having at least one allergen skin test response 1 mm greater than the saline control were increased in relation to sodium excretion after adjustment for age, sex, and smoking by a ratio of 2.08 (95% CI 1.04 to 4.15) per log10 unit increase in sodium excretion, but there was no evidence of an association between sodium excretion and the occurrence of self-reported wheeze, hay fever, eczema, or asthma. There was no relation between 24 hour sodium excretion and the magnitude of the mean allergen skin weal response or the PD20 value.

CONCLUSIONS

These findings do not support the hypothesis that a high dietary sodium intake is a risk factor for airway hyperreactivity or atopic disease in the general adult population.

Are asthmatics salt-sensitive? A preliminary controlled study

OBJECTIVE

Epidemiologic evidence suggests that high levels of salt consumption are associated with "spastic" disorders of smooth muscles, ie, essential hypertension and bronchial asthma. Experimentally, it has been shown that high intake of salt leads to increased bronchial hyperreactivity in asthmatics, ie, enhanced contractility of bronchial muscle to spasmogenic stimuli. On the basis of these observations, the following questions were asked: (1) Does salt loading worsen the clinical and functional findings in asthmatics? (2) Is it the sodium or the chloride in salt that is important?

METHODS

To answer these questions, the effect of salt restriction (= 5 to 6 g NaCl/d = 86 to 103 mmol Na), salt loading (+ 6.1 +/- 2.8 g NaCl/d = + 105 +/- 48 mmol Na), and loading with sodium citrate in nearly equimolar concentrations (+ 140 +/- 40 ml Shohl's solution, = + 120 +/- 30 mmol Na) was investigated in 14 asthmatics in a controlled crossover study. The total sodium load during the high salt diet was 191 to 209 mmol of sodium per day and during the sodium-citrate phase, 206 to 223 mmol of sodium per day.

RESULTS

Statistical analysis showed that salt loading worsened symptoms (p = 0.06) and increased the use of inhaled steroids (p < 0.05). The effect on lung function was less equivocal: salt loading worsened the forced expiratory volume in 1 s (p < 0.01) and the peak expiratory flow rate (p < 0.05). This effect was presumably mediated by sodium, not chloride, as is demonstrated by loading with sodium citrate.

CONCLUSION

Patients with bronchial asthma seems to be salt-sensitive, the responsible ion being presumably sodium. A low-salt diet appears to have a favorable effect in patients with asthma and to reduce the need for anti-asthma drugs.

Effect of alterations of dietary sodium on the severity of asthma in men

BACKGROUND

There is some evidence of a positive association between increased dietary salt consumption and both increased bronchial reactivity and mortality from asthma in men. This study assesses the effects of alterations in dietary salt consumption on the clinical severity of asthma in adult male asthmatic patients.

METHODS

A randomised, double blind, placebo controlled, crossover design was employed. Twenty seven mild to moderate asthmatic patients were established on a low sodium diet (80 mmol/day) at the end of a 4-5 day run in period and then randomised to receive 200 mmol/day slow sodium or matching placebo for five weeks, crossing over to the alternative regime for a further five weeks. Patients used diary cards to record twice daily peak expiratory flow rates, daily symptom scores, and bronchodilator consumption. Spirometry and degree of bronchial responsiveness (methacholine challenge test) were measured at screening and at the end of each treatment period. Twenty four hour urinary sodium excretion was measured at screening and in duplicate for each treatment period.

RESULTS

Twenty two patients completed the study. For these patients the mean (95% confidence interval (CI)) difference in 24 hour sodium excretion between treatments was 204 (175 to 235) mmol. Compared with placebo, sodium supplementation resulted in deleterious alterations of all measured parameters. Bronchial reactivity rose on slow sodium with a 0.73 (0.2 to 1.3) doubling dose methacholine difference compared with placebo. Estimated median (95% CI) difference in bronchodilator consumption was 1.3 (0.4 to 2.1) puffs per day, the estimated median difference in symptom score was 0.6 (0.2 to 0.9), and mean forced expiratory volume in one second fell by 0.21 (0.05 to 0.37) 1. The peak expiratory flow rate rose on placebo and fell on slow sodium. Median differences between treatments were 5.6% (2.2% to 9.8%) for morning and 7.8% (3.9% to 12.9%) for evening peak expiratory flow rate.

CONCLUSIONS

Our results suggest that large increases in dietary sodium result in physiological deterioration and increased morbidity in male asthmatic patients.

Asthma mortality: another opinion--is it a matter of life and ... bread?

There is a general consensus that the mortality rates for asthma in much of the developed world have been increasing for the past 10-15 years. This has occurred despite an improved information base regarding diagnosis and management as well as the development of novel and more effective therapeutic modalities. Several explanations have been proposed for this increase, including a statistical artifact based on a change in the coding criteria for asthma from the International Classification of Diseases Version 8 (ICD-8) to ICD-9, worsened pollution, delays in seeking medical help, behavioral changes, deficits in asthma education of both patients and primary care providers, toxicity of beta agonists, and noncompliance with medications. We suggest that although all of these are potential etiological factors, there may exist still another major etiological risk. There have been dramatic changes in our eating habits and food preparation. In general, we eat less total calories, more meals outside the home, and more refined or prepared foods. With the emphasis on reducing the intake of saturated fats and cholesterol, we eat more polyunsaturated fats both by choice and secondary to manufacturers' attempts to improve the appeal of foods to health-conscious government and the public. The drive to remove animal fats and cholesterol from our diet has resulted in the replacement of animal fats with vegetable oil in food manufacture, fast-food frying, and even home preparation. One result of these dietary habits has been a doubling from 8 to 15% of the percentage of the polyunsaturated linoleic acid in body fat. We postulate that this, and other nutritional changes, will render asthma a more difficult syndrome to manage and will contribute adversely to the inflammatory abnormalities in airways.

Effects of Na+ transport inhibitors on guinea-pig tracheal responses to spasmogens

The effects of ouabain, amiloride, K(+)-free solution and low Na+ (25 mM) solution on the responses to CaCl2 (in Ca(2+)-free, K(+)-depolarizing solution), KCl, acetylcholine, histamine and 5-hydroxytryptamine were studied in guinea-pig isolated trachea. Ouabain (10 microM) did not alter the contractile responses to CaCl2, KCl and acetylcholine but depressed those to histamine and 5-hydroxytryptamine produced in normal Ca2+ (2.5 mM) and Ca(2+)-free (EGTA 0.1 mM) media. Amiloride (0.1 mM), K(+)-free solution, and low Na+ solution depressed responses to acetylcholine, histamine and 5-hydroxytryptamine produced in normal Ca2+ and Ca(2+)-free media. Ouabain and amiloride had no effect on responses of skinned strips to Ca2+. The mechanism of the inhibitory effects of these interventions is uncertain but the findings suggest that the availability of Na+ influences the airway smooth muscle responses to spasmogens.

Effect of dietary sodium on the severity of bronchial asthma

BACKGROUND

A high sodium intake has been found to increase bronchial reactivity in men with asthma. The effects of change in sodium intake on peak flow rate have not been determined.

METHODS

The effect of changing dietary salt intake for two weeks on the severity of asthma as measured by peak expiratory flow (PEF) was studied in 17 patients with mild asthma in an open randomised crossover trial. PEF measurements were made by the patients in their own homes. Patients were placed on three levels of dietary sodium intake: normal, low, and high. Sodium intake was assessed by 24 hour urine collection.

RESULTS

The mean (SD) urine sodium was 147 (45), 84 (32), and 201 (73) mmol/24 hours in the normal, low, and high sodium intake periods respectively. There were no significant differences in PEF or PEF amplitude (highest--lowest PEF), an index of asthma lability, between the three dietary salt periods. CONCLUSION A low and high dietary salt intake for two weeks has no effect on peak expiratory flow in patients with mild asthma.

Airway obstruction and bronchial hyperresponsiveness in left ventricular failure and mitral stenosis

Small and large airways narrow in LVF and the term cardiac asthma is often used. However, current usage of this term is inconsistent and its meaning is therefore ambiguous. The term is better avoided despite several emerging similarities with bronchial asthma. Airway narrowing may be precipitated by acute elevation of pulmonary or bronchial vascular pressures. This appears to be mainly due to reflex bronchoconstriction. The afferents of this reflex are C-fibers with their endings in the lung parenchyma, bronchi, and pulmonary blood vessels and RAR in the larger airways, and they run in the vagus nerves, as do the efferent bronchoconstrictor fibers. Chronic elevation of pulmonary vascular pressures, as in mitral stenosis, are also associated with airway narrowing. Pulmonary edema (in the absence of vascular hypertension) also causes reflex bronchoconstriction. Bronchial responsiveness to bronchoconstrictor drugs is increased in LVF, partly, at least, due to reflex mechanisms. Bronchial mucosal swelling may also contribute. Narrowing by nonreflex mechanisms definitely occurs and there is direct evidence that decreased lung volume caused by pulmonary edema may cause this. There is little evidence for bronchial narrowing due to the mechanical effect of peribronchial edema, or by swelling of the bronchial mucosa. However, edema foam may terminally cause grave obstruction. Patients with LVF are commonly treated with bronchodilator drugs, but the basis for this approach needs further clarification.

Atrial natriuretic peptide in bronchial asthma

Regulation of ion transport across the airway mucosa may be involved in the mechanisms producing hyperreactivity and asthma. Atrial Natriuretic Peptide (ANP) has been proposed to participate in the pathogenesis of asthma, and it has been found to have a bronchodilatory effect on asthmatic patients. Experimental evidences suggest that ANP also has some effect on fluid accumulation in the lungs. We hypothesise that ANP may also play a role in the pathogenesis of asthma through changes in the transport of water and ions across the airway epithelium.

Methacholine airway responsiveness and 24-hour urine excretion of sodium and potassium. The Normative Aging Study

Prior studies have suggested a direct relationship between dietary sodium intake and nonspecific airway responsiveness. The relationship of dietary sodium and potassium intake to methacholine airway responsiveness was examined among 273 male participants of the Normative Aging Study (age range 44 to 82 yr) using 24-h urinary excretion of these cations as a surrogate for intake. Methacholine airway responsiveness was analyzed as dose-response slope, a continuous measure of responsiveness that represents the slope of a line connecting the origin to the last point of the dose-response plot. Greater airway responsiveness to methacholine was associated with greater potassium excretion. A significant relationship between methacholine dose-response slope and potassium excretion (p = 0.014) was observed in multivariate analysis that took into account other covariates, including age, percentage of predicted FEV1, cigarette smoking, and skin test reactivity. In contrast, methacholine airway responsiveness did not appear related to urinary sodium excretion. These data suggest that dietary potassium may have an influence on airway responsiveness of middle-aged and older men.