Dietary risk factors of stroke and hypertension in Japan -- Part 2: Validity of urinalysis for dietary salt and protein intakes under a field condition

High sodium intake: review of recent issues on its association with cardiovascular events and measurement methods

There has been a long-known association between high dietary sodium intake and hypertension, as well as the increased risk of cardiovascular disease. Reduction of sodium intake is a major challenge for public health. Recently, there have been several controversial large population-based studies regarding the current recommendation for dietary sodium intake. Although these studies were performed in a large population, they aroused controversies because they had a flaw in the study design and methods. In addition, knowledge of the advantages and disadvantages of the methods is essential in order to obtain an accurate estimation of sodium intake. I have reviewed the current literatures on the association between sodium intake and cardiovascular events, as well as the methods for the estimation of sodium intake.

Estimation of 24-hour urinary sodium excretion using spot urine samples

The present study evaluated the reliability of equations using spot urine (SU) samples in the estimation of 24-hour urine sodium excretion (24-HUNa). Equations estimating 24-HUNa from SU samples were derived from first-morning SU of 101 participants (52.4 ± 11.1 years, range 24–70 years). Equations developed by us and other investigators were validated with SU samples from a separate group of participants (n = 224, 51.0 ± 10.9 years, range 24–70 years). Linear, quadratic, and cubic equations were derived from first-morning SU samples because these samples had a sodium/creatinine ratio having the highest correlation coefficient for 24-HUNa/creatinine ratio (r = 0.728, p < 0.001). In the validation group, the estimated 24-HUNa showed significant correlations with measured 24-HUNa values. The estimated 24-HUNa by the linear, quadratic, and cubic equations developed from our study were not significantly different from measured 24-HUNa, while estimated 24-HUNa by previously developed equations were significantly different from measured 24-HUNa values. The limits of agreement between measured and estimated 24-HUNa by six equations exceeded 100 mmol/24-hour in the Bland-Altman analysis. All equations showed a tendency of under- or over-estimation of 24-HUNa, depending on the level of measured 24-HUNa. Estimation of 24-HUNa from single SU by equations as tested in the present study was found to be inadequate for the estimation of an individual’s 24-HUNa.

Estimation of 24-h urinary sodium excretion using lean body mass and overnight urine collected by a pipe-sampling method

BACKGROUND Although salt intake can be estimated from 24-h urinary sodium excretion (Na24 ), for a long time there has been no precise and easy method for its measurement.

OBJECTIVE

To investigate the possibility of estimating Na24 using pipe-sampling of overnight urine and lean body mass.

PARTICIPANTS AND METHODS

Body height, body weight and body fat determined by bioelectrical impedance (lean body mass = body weight body fat) was measured in 351 healthy individuals (126 men, 225 women). Twenty-four-hour urine was collected and creatinine and sodium were measured. To predict 24-h urinary creatinine excretion (Cr24 ), the relationship between Cr24 and lean body mass was investigated. Both 24-h urine and overnight urine specimens were collected in 149 individuals (71 men, 78 women) using a sampling pipe (semi-automatic proportional urine sampling device; height 16 cm, width 1.5 cm). Multiple regression analysis was used to estimate Na24.

RESULTS

The prediction of Cr24 (Pr.UCr24 ) was derived from lean body mass. Using Pr.UCr24 and the overnight urinary Na/Cr excretion ratio (Na n /Cr n ), Na24 was estimated as 0.634 (Na n /Cr n ) Pr.UCr 24 + 104.7 mmol/day for men and 0.682 (Na n /Cr n ) Pr.UCr 24 + 62.6 mmol/day for women. The correlation coefficient (r) between true Na24 and Na24 estimated by these formulae was r = 0.78 ( P<0.001; mean difference SD-0.03 39.0 mmol/day).

CONCLUSIONS

A new pipe-sampling method using overnight urine and lean body mass was easy and reliable for the estimation of Na 24. Furthermore, this method is convenient and may enable counselling on salt intake.

Relation of urinary urea to blood pressure: interaction with urinary sodium

A previous study reported that urinary markers of protein intake are inversely related to blood pressure via unknown mechanisms. In man and rats, protein intake affects renal function and increases renal sodium excretion. The present study investigates the relation between markers of protein intake and blood pressure and the possible role of sodium in this relation. Blood pressure status, overnight urinary urea as index of protein intake, urinary and plasma sodium, and other variables were measured in a population sample of 3705 men and women, aged 25-74 years, without high plasma creatinine. Urinary urea was inversely related to blood pressure and hypertension: in multivariate analyses, 6.5 mmol/h higher urinary urea (about one s.d. in men and women) was related to 4.25 mm Hg lower systolic blood pressure (95% confidence interval = 1.34-8.49), and to 0.65 lower risk of hypertension (95% CI 0.34-0.87). An interaction was found between overnight urinary sodium and the relation of urinary urea to blood pressure: the relation was significant only in persons with overnight urinary sodium above the median. Urinary urea was significantly and inversely also related to plasma sodium. Data confirm an inverse relation to blood pressure of protein intake as measured by urinary urea. The possibility of sodium-related mechanisms is supported by the interaction of urinary sodium with the relation and by the inverse association of urinary urea with plasma sodium. The hypothesis is made that high protein intake could counteract sodium-dependent blood pressure rise via stimulation of renal sodium excretion.

Validity of a self-administered diet history questionnaire for assessment of sodium and potassium: comparison with single 24-hour urinary excretion

We developed a self-administered diet history questionnaire (DHQ) for use in prevention and control of cardiovascular diseases and cancer, and validated it by comparison with single 24-h urinary excretion of sodium (Na) and potassium (K). The subjects were 154 male and 69 female freshmen university students. Mean intakes (mmol/day) assessed by DHQ and the urinary excretion of Na were 196 and 165 respectively for men and 179 and 136 respectively for women. Those of K were 61.5 and 43.9 respectively for men and 56.8 and 41.6 respectively for women. The ratios of urinary excretion to dietary intake of Na were 0.97 in men and 0.84 in women. Those of K were 0.78 in men and 0.80 in women. The results for both Na and K were reasonable, except for Na in men. When Pearson correlation was examined between dietary and urinary Na and K, no significant correlations for Na in men (r=0.14) or women (r=0.23, p=0.06), or significant correlations for K in men (r=0.34, p<0.001) or women (r=0.40, p<0.001) were observed. The results suggest a reasonable ability to estimate a subject mean for Na in women, K in both sexes, and individual level for K for both sexes. The validity for individual level for Na intake is not conclusive because the duration of urine collection was too short.

Interrelationships between blood pressure, sodium, potassium, serum cholesterol, and protein intake in Japanese

Interrelationships among blood pressure (BP), sodium (Na), potassium (K), dietary protein, and serum cholesterol level (Chol) were examined in 62% (1120) of 1818 Japanese inhabitants of both sexes aged over 30 years who lived in a rural village in Japan. Fasting single-spot urine specimens were collected in the morning to measure Na, K, urea nitrogen (UN), inorganic sulfate (SO4), and creatinine (Cr). The Cr ratios of Na, K, UN, SO4, Na/K, and SO4/UN were analyzed by multiple regression analysis to determine independent associations with BP together with age, obesity index, hematocrit (Hct), Chol, triglyceride (TG), and fasting serum glucose level (Glu). Except for Na/Cr in men, Na/Cr and Na/K were found to be independently and positively related to BP, particularly to systolic BP (SBP). In contrast, K/Cr and SO4/UN (an index related to the dietary score of sulphur-containing amino acids derived mainly from animal protein) were both negatively associated with SBP, and UN/Cr (an index of total protein intake) was positively associated with SBP in men. Chol was linked to BP negatively in men but positively in women. Age, obesity index, TG, and Hct were generally positively and significantly related to BP in both sexes. The results confirmed on epidemiological grounds the positive link of Na and the negative link of K to BP within a single population in Japan. They further suggest, although only in men, that there is a negative relationship of Chol and dietary animal protein with BP.