Insulin sensitivity and intake of vitamins E and C in African American, Hispanic, and non-Hispanic white men and women: the Insulin Resistance and Atherosclerosis Study (IRAS)

An Open Pilot Study of the Effect and Tolerability of Add-On Multivitamin Therapy in Patients with Intractable Focal Epilepsy

Observational studies have investigated the potential modulatory effect of neuronal excitability by vitamins in epilepsy. We aimed to investigate whether the addition of multivitamin therapy (B6/B9, D, E and Q) to regular antiepileptic drug therapy could ameliorate seizures in patients with refractory focal epilepsy. We conducted a prospective cohort open study to investigate the effect and tolerability of add-on multivitamin therapy (daily dose: B6 100 mg, B9 5 mg, D 1000 IU, E 400 IU and coenzyme Q10 100 mg) in patients with intractable focal epilepsy. All patients had effect and safety assessments at baseline and after one, three and six months of the supplementation. Thirty patients (11 men and 19 women) with a mean age of 42.37 ± 9.40 years were recruited and four patients discontinued. The seizure frequency significantly decreased after the six-month supplementation (9.04 ± 18.16/month and 2.06 ± 3.89/month, p = 0.045). At the final visit, 62.5% of the patients showed a ≥50% reduction in seizure frequency, and 12.5% were seizure-free. As to safety and tolerability, most patients did not experience significant adverse events, although three patients reported seizure worsening. In conclusion, this pilot study demonstrated the therapeutic potential and essentially good tolerability of add-on multivitamin therapy in patients with refractory focal epilepsy.

Development of type 2 diabetes mellitus in people with intermediate hyperglycaemia

BACKGROUND

Intermediate hyperglycaemia (IH) is characterised by one or more measurements of elevated blood glucose concentrations, such as impaired fasting glucose (IFG), impaired glucose tolerance (IGT) and elevated glycosylated haemoglobin A1c (HbA1c). These levels are higher than normal but below the diagnostic threshold for type 2 diabetes mellitus (T2DM). The reduced threshold of 5.6 mmol/L (100 mg/dL) fasting plasma glucose (FPG) for defining IFG, introduced by the American Diabetes Association (ADA) in 2003, substantially increased the prevalence of IFG. Likewise, the lowering of the HbA1c threshold from 6.0% to 5.7% by the ADA in 2010 could potentially have significant medical, public health and socioeconomic impacts.

OBJECTIVES

To assess the overall prognosis of people with IH for developing T2DM, regression from IH to normoglycaemia and the difference in T2DM incidence in people with IH versus people with normoglycaemia.

SEARCH METHODS

We searched MEDLINE, Embase, ClincialTrials.gov and the International Clinical Trials Registry Platform (ICTRP) Search Portal up to December 2016 and updated the MEDLINE search in February 2018. We used several complementary search methods in addition to a Boolean search based on analytical text mining.

SELECTION CRITERIA

We included prospective cohort studies investigating the development of T2DM in people with IH. We used standard definitions of IH as described by the ADA or World Health Organization (WHO). We excluded intervention trials and studies on cohorts with additional comorbidities at baseline, studies with missing data on the transition from IH to T2DM, and studies where T2DM incidence was evaluated by documents or self-report only.

DATA COLLECTION AND ANALYSIS

One review author extracted study characteristics, and a second author checked the extracted data. We used a tailored version of the Quality In Prognosis Studies (QUIPS) tool for assessing risk of bias. We pooled incidence and incidence rate ratios (IRR) using a random-effects model to account for between-study heterogeneity. To meta-analyse incidence data, we used a method for pooling proportions. For hazard ratios (HR) and odds ratios (OR) of IH versus normoglycaemia, reported with 95% confidence intervals (CI), we obtained standard errors from these CIs and performed random-effects meta-analyses using the generic inverse-variance method. We used multivariable HRs and the model with the greatest number of covariates. We evaluated the certainty of the evidence with an adapted version of the GRADE framework.

MAIN RESULTS

We included 103 prospective cohort studies. The studies mainly defined IH by IFG5.6 (FPG mmol/L 5.6 to 6.9 mmol/L or 100 mg/dL to 125 mg/dL), IFG6.1 (FPG 6.1 mmol/L to 6.9 mmol/L or 110 mg/dL to 125 mg/dL), IGT (plasma glucose 7.8 mmol/L to 11.1 mmol/L or 140 mg/dL to 199 mg/dL two hours after a 75 g glucose load on the oral glucose tolerance test, combined IFG and IGT (IFG/IGT), and elevated HbA1c (HbA1c5.7: HbA1c 5.7% to 6.4% or 39 mmol/mol to 46 mmol/mol; HbA1c6.0: HbA1c 6.0% to 6.4% or 42 mmol/mol to 46 mmol/mol). The follow-up period ranged from 1 to 24 years. Ninety-three studies evaluated the overall prognosis of people with IH measured by cumulative T2DM incidence, and 52 studies evaluated glycaemic status as a prognostic factor for T2DM by comparing a cohort with IH to a cohort with normoglycaemia. Participants were of Australian, European or North American origin in 41 studies; Latin American in 7; Asian or Middle Eastern in 50; and Islanders or American Indians in 5. Six studies included children and/or adolescents.Cumulative incidence of T2DM associated with IFG5.6, IFG6.1, IGT and the combination of IFG/IGT increased with length of follow-up. Cumulative incidence was highest with IFG/IGT, followed by IGT, IFG6.1 and IFG5.6. Limited data showed a higher T2DM incidence associated with HbA1c6.0 compared to HbA1c5.7. We rated the evidence for overall prognosis as of moderate certainty because of imprecision (wide CIs in most studies). In the 47 studies reporting restitution of normoglycaemia, regression ranged from 33% to 59% within one to five years follow-up, and from 17% to 42% for 6 to 11 years of follow-up (moderate-certainty evidence).Studies evaluating the prognostic effect of IH versus normoglycaemia reported different effect measures (HRs, IRRs and ORs). Overall, the effect measures all indicated an elevated risk of T2DM at 1 to 24 years of follow-up. Taking into account the long-term follow-up of cohort studies, estimation of HRs for time-dependent events like T2DM incidence appeared most reliable. The pooled HR and the number of studies and participants for different IH definitions as compared to normoglycaemia were: IFG5.6: HR 4.32 (95% CI 2.61 to 7.12), 8 studies, 9017 participants; IFG6.1: HR 5.47 (95% CI 3.50 to 8.54), 9 studies, 2818 participants; IGT: HR 3.61 (95% CI 2.31 to 5.64), 5 studies, 4010 participants; IFG and IGT: HR 6.90 (95% CI 4.15 to 11.45), 5 studies, 1038 participants; HbA1c5.7: HR 5.55 (95% CI 2.77 to 11.12), 4 studies, 5223 participants; HbA1c6.0: HR 10.10 (95% CI 3.59 to 28.43), 6 studies, 4532 participants. In subgroup analyses, there was no clear pattern of differences between geographic regions. We downgraded the evidence for the prognostic effect of IH versus normoglycaemia to low-certainty evidence due to study limitations because many studies did not adequately adjust for confounders. Imprecision and inconsistency required further downgrading due to wide 95% CIs and wide 95% prediction intervals (sometimes ranging from negative to positive prognostic factor to outcome associations), respectively.This evidence is up to date as of 26 February 2018.

AUTHORS' CONCLUSIONS

Overall prognosis of people with IH worsened over time. T2DM cumulative incidence generally increased over the course of follow-up but varied with IH definition. Regression from IH to normoglycaemia decreased over time but was observed even after 11 years of follow-up. The risk of developing T2DM when comparing IH with normoglycaemia at baseline varied by IH definition. Taking into consideration the uncertainty of the available evidence, as well as the fluctuating stages of normoglycaemia, IH and T2DM, which may transition from one stage to another in both directions even after years of follow-up, practitioners should be careful about the potential implications of any active intervention for people 'diagnosed' with IH.

Micronutrient status in type 2 diabetes: a review

Type 2 diabetes is characterized by significant losses of important micronutrients due to metabolic basis of the disease and its complications. Evidence of changes in trace mineral and vitamin metabolism as a consequence of type 2 diabetes is reviewed in this chapter. This review is not a meta-analysis but an overview of the micronutrient status, metabolic needs, and potential micronutrient requirements in type 2 diabetics. This chapter will not concentrate on vitamin D and type 2 diabetes as this is a topic that has been extensively reviewed before. The less well-known micronutrients notably zinc, magnesium, chromium, copper, manganese, iron, selenium, vanadium, B-group vitamins, and certain antioxidants are assessed. While some evidence is available to demonstrate the positive influence of micronutrient supplementation on glycemic control, much remains to be investigated. Additional research is necessary to characterize better biomarkers of micronutrient status and requirements in type 2 diabetics. The optimal level of micronutrient supplementation to achieve glucose homeostasis in type 2 diabetics remains a challenge.

Does vitamin C deficiency increase lifestyle-associated vascular disease progression? Evidence based on experimental and clinical studies

SIGNIFICANCE

Despite continuous advances in the prevention of cardiovascular disease (CVD), critical issues associated with an unhealthy lifestyle remain an increasing cause of morbidity and mortality in industrialized countries.

RECENT ADVANCES

A growing body of literature supports a specific role for vitamin C in a number of reactions that are associated with vascular function and control including, for example, nitric oxide bioavailability, lipid metabolism, and vascular integrity.

CRITICAL ISSUES

A large body of epidemiological evidence supports a relationship between poor vitamin C status and increased risk of developing CVD, and the prevalence of deficiency continues to be around 10%-20% of the general Western population although this problem could easily and cheaply be solved by supplementation. However, large intervention studies using vitamin C have not found a beneficial effect of supplementation. This review outlines the proposed mechanism by which vitamin C deficiency worsens CVD progression. In addition, it discusses problems with the currently available literature, including the discrepancies between the large intervention studies and the experimental and epidemiological literature.

FUTURE DIRECTIONS

Increased insights into vitamin C deficiency-mediated CVD progression will enable the design of future randomized controlled trials that are better suited to test the efficacy of vitamin C in disease prevention as well as the identification of high-risk individuals which could possibly benefit from supplementation.

Effects of total and green vegetable intakes on glycated hemoglobin A1c and triglycerides in elderly patients with type 2 diabetes mellitus: the Japanese Elderly Intervention Trial

AIM

Many reports have shown that vegetable intake is effective in inhibiting the onset and progression of diabetes mellitus, although the amount of vegetable intake required to be effective remains as unclear. The present study therefore aimed to clarify the relationship between the amount of vegetable intake and glycated hemoglobin A1c (HbA1c) and other metabolic parameters using male Japanese type 2 diabetic patients aged 65 years or older as subjects.

METHOD

Participants were 417 male type 2 diabetic patients aged 65 years or older enrolled in the Japanese Elderly Diabetes Intervention Trial. Dietary intakes were measured by using the Food Frequency Questionnaires method. The patients were divided into five groups by their daily total vegetable intake (A1: ~100 g, A2: 100~150 g, A3: 150~200 g, A4: 200~300 g, A5: 300 g~), and compared HbA1c and other metabolic parameters. Furthermore, the relationship between daily green vegetable intake and HbA1c and other metabolic parameters were examined among five groups divided by quintile methods.

RESULTS

There were significant decreases in HbA1c, triglycerides and waist circumference with an increase of total vegetable intake. A significant decrease of HbA1c levels was observed in patients with a daily total vegetable intake of 150 g or more. Furthermore, there was a significant decrease of serum triglyceride levels in patients with a total vegetable intake of 200 g or more. HbA1c levels showed a decreasing tendency with the increase of green vegetable intake, and HbA1c levels in the Q1 group (green vegetable intake: less than 40 g) was significantly higher than those in the other four groups (anovaP = 0.025). In addition, there were significant decreases of body mass index, triglyceride levels and waist circumference with the increase of green vegetable intake. Triglyceride levels decreased significantly from the Q3 group (green vegetable intake: 70 g or more) to the Q5 group (green vegetable intake: 130 g or more; anovaP = 0.016). In the group with a lower intake of total vegetables and green vegetables, the protein energy ratio decreased significantly. As a result, the fat energy ratio and energy intake tended to increase with the decrease of total and green vegetable intakes. Furthermore, intake of grains, sweets and alcoholic beverages increased with the decrease of total vegetable intake. In contrast, intake of nuts, potatoes, sugar, legumes, fruit, seaweed and fish increased with the increase of total vegetable intake

CONCLUSIONS

Daily total vegetable intake of 200 g or more, and green vegetable intake of 70 g or more correlated with improved control of HbA1c and triglyceride levels in elderly type 2 diabetes patients through achieving a well-balanced diet.

Vitamin E and adiponectin: proposed mechanism for vitamin E-induced improvement in insulin sensitivity

Insulin resistance and type 2 diabetes have been treated with the PPARγ agonists thiazolidinediones, or TZDs, since the 1990s. One mechanism by which these drugs may work is through PPARγ-mediated upregulation of adiponectin, an endogenous adipokine that has been shown to increase insulin sensitivity. Interestingly, α- and γ-tocopherol, two vitamin E vitamers, have structural similarities to the TZDs and have also been linked to enhanced insulin sensitivity. A recent study identified a novel function of α- and γ-tocopherol in 3T3-L1 preadipocytes: upregulation of an endogenous ligand involved in activating PPARγ. This study also found that tocopherols dramatically enhanced adiponectin expression and that this effect was mediated through a PPARγ-dependent process. These findings illustrate a possible mechanistic link between vitamin E and insulin sensitivity.

Evaluation of two dietary treatments in obese hyperinsulinemic adolescents

Hyperinsulinemia increases the risk of cardiovascular disease in obese children. Only a few treatments are available to decrease insulin resistance. The reduction of hyperinsulinemia by dietary means would be a simple, physiologic and economic way to reduce the risk of metabolic disease.

OBJECTIVE

To compare the effects of two low-energy diets on serum insulin concentrations and weight loss in obese hyperinsulinemic adolescents.

MATERIALS AND METHODS

Eighty-six randomly assigned insulin-resistant obese adolescents completed a 16 week calorie-restricted diet. The experimental diet had a reduced glycemic index designed to evoke a low insulin response (LIR), with carbohydrates and proteins ingested in separate meals. The control diet was a conventional (CD) with similar proportions (60%, 20% and 20%). Variables studied were blood glucose and insulin concentrations after an oral glucose load, body mass index, waist circumference, and insulin resistance (homeostasis model assessment, HOMA).

RESULTS

Mean weight [+/- Standard Deviation (SD)] was significantly reduced after the LIR (-0.53 +/- 0.5) and the CD (-0.54 +/- 0.4), but a greater decrease of waist circumference (cm) was observed after the LIR (-9.1 +/- 4.8 vs. -6.6 +/- 4.6, p = 0.02). Fasting insulin concentrations (-17.9 +/- 27.9 vs. -9.4 +/- 14.8, p = 0.01) and HOMA dropped significantly more after the LIR than after the CD (-3.5 +/- 4.9SD vs. -2.4 +/- 1SD, p < 0.0001).

CONCLUSIONS

The LIR diet reduces serum insulin concentrations and waist circumference more than conventional treatment and appears to be a promising alternative to a conventional diet in insulin-resistant obese adolescents. Long-term follow-up is needed to evaluate the maintenance of weight loss and metabolic parameters.

Dietary antioxidants and glucose metabolism

PURPOSE OF REVIEW

Oxidative stress seems to play a pathogenic role in the vicious circle linking obesity, insulin resistance and type 2 diabetes. Hypothetically, dietary antioxidants should decrease oxidative stress and therefore improve glucose metabolism. However, many interventional trials evaluating the effect of antioxidant supplementation on insulin resistance, plasma glucose levels and risk of type 2 diabetes gave inconsistent results.

RECENT FINDINGS

Many studies have recently demonstrated a positive effect of vitamin supplementation and of food enriched in antioxidant (seafood, whole nut, etc.) on markers of oxidative stress, insulin resistance, fasting plasma glucose and incidence of diabetes. The present paper critically reviews the consolidated notions on dietary antioxidant in view of the recent evidences.

SUMMARY

Although a definitive estimation of the impact of dietary antioxidants on glucose metabolism is still lacking, food with high antioxidant concentrations seems to have a protective effect, improving oxidative stress-mediated detrimental effects on the vicious circle among obesity, insulin resistance and redox imbalance.

Plasma and dietary vitamin E in relation to insulin secretion and sensitivity

AIM

In the Insulin Resistance and Atherosclerosis Study (IRAS), we have previously shown a protective effect of plasma alpha-tocopherol concentration against diabetes incidence among persons not taking vitamin E supplements. The biologic mechanism for such a protective effect could involve improvement in either insulin sensitivity (S(I)), insulin secretion or both. Thus, we examined vitamin E in relation to insulin secretion and S(I) among persons not taking vitamin E supplements.

METHODS

This analysis included 457 adults aged 40-69 years without a previous diabetes diagnosis or vitamin E supplement use at baseline and seen at the 5-year follow-up examination. Baseline nutrient intake was estimated from a validated 1-year food frequency questionnaire; plasma levels of alpha-tocopherol were also assessed. At follow up, a frequently sampled intravenous glucose tolerance test determined S(I), acute insulin response to glucose (AIR), and the disposition index (DI) was calculated as the sum of the log-transformed AIR and S(I) to reflect pancreatic compensation for insulin resistance.

RESULTS

In multivariable regression analyses, no relationship was observed for vitamin E intake and either S(I), AIR or DI. However, plasma alpha-tocopherol concentration was positively associated with log-transformed S(I) (beta= 0.27 +/- 0.09, p < 0.01) and DI (beta= 0.41 +/- 0.14, p < 0.01), but not with log-transformed AIR.

CONCLUSIONS

Plasma concentration of alpha-tocopherol may improve S(I) and pancreatic compensation for insulin resistance, although it does not seem to be related to acute insulin response.

Intake of fruit, vegetables, and antioxidants and risk of type 2 diabetes: systematic review and meta-analysis

BACKGROUND

The public health recommendation is to consume five or more servings of fruit and vegetables daily, but the association between the intake of fruit and vegetables and the risk of type 2 diabetes is unclear.

METHODS

To examine the association between the intake of fruit, vegetables, and antioxidants and the risk of type 2 diabetes, prospective cohort studies were sought, with measures of intakes at baseline and incident diabetes at follow-up.

RESULTS

Five cohort studies of fruit and vegetables intake and the risk of diabetes were included, incorporating 167,128 participants and 4858 incident cases of type 2 diabetes, with a mean follow-up of 13 years. The relative risk of type 2 diabetes for consuming five or more servings of fruit and vegetables daily was 0.96 (95% CI, 0.79-1.17, P = 0.96), 1.01 (0.88-1.15, P = 0.88) for three or more servings of fruit, and 0.97 (0.86-1.10, P = 0.59) for three or more servings of vegetables. Nine cohort studies of antioxidant intake and the risk of diabetes were also identified, incorporating 139,793 participants and 8813 incident cases of type 2 diabetes, with a mean follow-up of 13 years. The pooled relative risk was 0.87 (0.79-0.98, P = 0.02) for the highest compared with the lowest antioxidant intake.

CONCLUSIONS

The consumption of three or more daily servings of fruit or vegetables was not associated with a substantial reduction in the risk of type 2 diabetes. The intake of antioxidants was associated with a 13% reduction in risk, mainly attributed to vitamin E. These results are not supported by randomized, controlled trials of vitamin supplements, therefore the findings might not be causal, and the discrepancy remains unclear.

Associations between concentrations of α- and γ-tocopherol and concentrations of glucose, glycosylated haemoglobin, insulin and C-peptide among US adults

Our objective was to study the cross-sectional associations between concentrations of α- and γ-tocopherol and concentrations of glucose, glycosylated haemoglobin, insulin and C-peptide among US adults. We used data for 1289 participants without self-reported diabetes who were aged ≥20 years in the National Health and Nutrition Examination Survey 1999–2000. α-Tocopherol concentration was inversely associated with glucose concentration (β per mmol/l=−0·01064, se 0·00356, P=0·004) after adjusting for age, sex, race or ethnicity, education, smoking status, concentrations of total cholesterol and triacylglycerols, systolic blood pressure, waist circumference, alcohol use, physical activity, time watching television or videos or using a computer, and use of vitamin/mineral/dietary supplements. Among 659 participants who did not report using supplements, this association was no longer significant whereas the concentration of α-tocopherol was inversely associated with concentration of C-peptide (β per mmol/l=−0·01121, se 0·00497, P=0·024). γ-Tocopherol concentration was positively associated with concentration of glucose (β per mmol/l=0·09169, se 0·02711, P=0·001) and glycosylated haemoglobin (β per mmol/l=0·04954, se 0·01284, P<0·001), but not insulin or C-peptide. The relationships between physiologic concentrations of the various forms of vitamin E and measures of glucose intolerance deserve additional investigation.

Nutritional correlates of dietary glycaemic index: new aspects from a population perspective

The role of dietary glycaemic index (GI) and glycaemic load (GL) in disease aetiology is of increasing interest. However, nutritional factors related to dietary GI and GL are not well understood from a population perspective. We aimed to investigate the relation ship between GI and GL and dietary intake at the food and nutrient level. Study subjects were 1071 non-diabetic adults from the Insulin Resistance Atherosclerosis Study, Exam I, 1992–4. Usual dietary intake was assessed with a 114-item modified Block food frequency questionnaire. Published GI values were assigned to food line items. Correlation and regression analyses were conducted. Intake of white bread, beer, meats and fries/fried potatoes was positively associated with average GI, as was fat, starch and alcohol intake (before and after energy adjustment). Intake of fruits and low-fat milk was inversely associated with GI, as were intakes of mono- and disaccharides, and fibre. GL was positively correlated with carbohydrate foods and inversely with non-carbohydrate foods. Gender-specific regression models identified eight food groups explaining 63 % (men) and 55 % (women) total GI variation after adjusting for demographics; 70 % of variation in GL was explained by eleven (men) and nine (women) food groups, respectively. Although the GI of a food is an indicator of the ability of carbohydrates to raise blood glucose, dietary GI, unlike GL, appears to reflect more dimensions of diet than just carbohydrates, such as the combination of foods consumed. This may have implications for the interpretation of dietary GI in epidemiologic studies.

Food consumption and the incidence of type II diabetes mellitus

OBJECTIVE

The consumption of different foods was studied for their ability to predict type II diabetes mellitus.

DESIGN

The study design was a cohort study, based on the Finnish Mobile Clinic Health Examination Survey.

SETTING

A total of 30 communities from different parts of Finland.

SUBJECTS

A total of 4304 men and women, 40-69 y of age and free of diabetes at baseline in 1967-1972 and followed up for incidence of diabetes medication during 23 y (383 incident cases).

RESULTS

Higher intakes of green vegetables, fruit and berries, oil and margarine, and poultry were found to predict a reduced risk of type II diabetes. The relative risks of developing type II diabetes between the extreme quartiles of the intakes were 0.69 (95% confidence interval (CI) = 0.50-0.93; P for trend (P) = 0.02) for green vegetables, 0.69 (CI = 0.51-0.92; P = 0.03) for fruit and berries, 0.71 (CI = 0.52-0.98; P = 0.01) for margarine and oil, and 0.71 (CI = 0.54-0.94; P = 0.01) for poultry.

CONCLUSION

The results suggest that prevention of type II diabetes might be aided by consumption of certain foods that are rich in nutrients with hypothesized health benefits.

Dietary antioxidant intake and risk of type 2 diabetes

OBJECTIVE

The intake of antioxidants was studied for its ability to predict type 2 diabetes.

RESEARCH DESIGN AND METHODS

A cohort of 2,285 men and 2,019 women 40-69 years of age and free of diabetes at baseline (1967-1972) was studied. Food consumption during the previous year was estimated using a dietary history interview. The intake of vitamin C, four tocopherols, four tocotrienols, and six carotenoids was calculated. During a 23-year follow-up, a total of 164 male and 219 female incident cases occurred.

RESULTS

Vitamin E intake was significantly associated with a reduced risk of type 2 diabetes. The relative risk (RR) of type 2 diabetes between the extreme quartiles of the intake was 0.69 (95% CI 0.51-0.94, P for trend = 0.003). Intakes of alpha-tocopherol, gamma-tocopherol, delta-tocopherol, and beta-tocotrienol were inversely related to a risk of type 2 diabetes. Among single carotenoids, beta-cryptoxanthin intake was significantly associated with a reduced risk of type 2 diabetes (RR 0.58, 95% CI 0.44-0.78, P < 0.001). No association was evident between intake of vitamin C and type 2 diabetes risk.

CONCLUSIONS

This study supports the hypothesis that development of type 2 diabetes may be reduced by the intake of antioxidants in the diet.

Nutrition and prevention of type 2 diabetes

In recent years, the prevalence of type 2 diabetes has increased alarmingly worldwide, giving diabetes the dimension of an epidemic. Striking parallel increases in the prevalence of obesity reflect the importance of body fatness as a contributing factor to diabetes incidence. Moreover, it has been estimated that up to 75% of the risk of type 2 diabetes is attributable to obesity. Recent clinical trials and observational epidemiologic studies demonstrate the efficacy of lifestyle changes, including decreased energy intake, decreased fat intake, and weight loss, as well as regular participation in physical activity, in improving insulin sensitivity (SI) and reducing the risk of diabetes. This review evaluates evidence of the effect of diet on insulin resistance, insulin secretion, and glucose tolerance, and reflects on directions for future work toward primary prevention of type 2 diabetes.

The metabolic syndrome and antioxidant concentrations: findings from the Third National Health and Nutrition Examination Survey

Oxidative stress may play a role in the pathophysiology of diabetes and cardiovascular disease, but little is known about antioxidant status among individuals with the metabolic syndrome who are at high risk for developing these conditions. Using data from the Third National Health and Nutrition Examination Survey (1988-1994), we compared circulating concentrations of vitamins A, C, and E; retinyl esters; five carotenoids; and selenium in 8,808 U.S. adults aged > or = 20 years with and without the metabolic syndrome. After adjusting for age, sex, race or ethnicity, education, smoking status, cotinine concentration, physical activity, fruit and vegetable intake, and vitamin or mineral use, participants with the metabolic syndrome had significantly lower concentrations of retinyl esters, vitamin C, and carotenoids, except lycopene. With additional adjustment for serum lipid concentrations, vitamin E concentrations were significantly lower in participants with the metabolic syndrome than those without the syndrome. Retinol concentrations were similar between the two groups. After excluding participants with diabetes, the results were very similar. Consumption of fruits and vegetables was also lower among people with the metabolic syndrome. Adults with the metabolic syndrome have suboptimal concentrations of several antioxidants, which may partially explain their increased risk for diabetes and cardiovascular disease.

Dietary intakes and plasma concentrations of carotenoids and tocopherols in relation to glucose metabolism in subjects at high risk of type 2 diabetes: the Botnia Dietary Study

BACKGROUND

The role of antioxidants in the pathogenesis of type 2 diabetes is uncertain.

OBJECTIVE

We evaluated cross-sectional relations of dietary intakes and plasma concentrations of antioxidants with glucose metabolism in a high-risk population.

DESIGN

The subjects were 81 male and 101 female first- and second-degree, nondiabetic relatives of patients with type 2 diabetes. Antioxidant intake data were based on 3-d food records. Subjects taking supplements containing beta-carotene or alpha-tocopherol were excluded. Plasma antioxidant concentrations were measured by HPLC. By using multiple linear regression analysis and adjusting for demographic, anthropometric, and lifestyle covariates, we studied whether dietary and plasma alpha- and beta-carotene, lycopene, and alpha- and gamma-tocopherol were related to fasting and 2-h concentrations of glucose and nonesterified fatty acids during an oral-glucose-tolerance test, to the homeostasis model assessment index of insulin resistance, and to measures of beta cell function (incremental 30-min serum insulin concentration during an oral-glucose-tolerance test and first-phase insulin secretion during an intravenous-glucose-tolerance test).

RESULTS

In men, dietary carotenoids were inversely associated with fasting plasma glucose concentrations (P < 0.05), plasma beta-carotene concentrations were inversely associated with insulin resistance (P = 0.003), and dietary lycopene was directly related to baseline serum concentrations of nonesterified fatty acids (P = 0.034). In women, dietary alpha-tocopherol and plasma beta-carotene concentrations were inversely and directly associated, respectively, with fasting plasma glucose concentrations (P < 0.05). In both sexes, cholesterol-adjusted alpha-tocopherol concentrations were directly associated with 2-h plasma glucose concentrations (P < 0.05).

CONCLUSION

The data suggest an advantageous association of carotenoids, which are markers of fruit and vegetable intake, with glucose metabolism in men at high risk of type 2 diabetes.

Plasma and dietary vitamin E in relation to incidence of type 2 diabetes: The Insulin Resistance and Atherosclerosis Study (IRAS)

OBJECTIVE

To evaluate the association of vitamin E with incidence of type 2 diabetes and to do so separately among individuals who did and those who did not report regular use of vitamin supplementation.

RESEARCH DESIGN AND METHODS

The Insulin Resistance Atherosclerosis Study (IRAS) included 895 nondiabetic adults at baseline (including 303 with impaired glucose tolerance [IGT]), 148 of whom developed type 2 diabetes according to World Health Organization (WHO) criteria during the 5-year follow-up. At baseline, dietary vitamin E was estimated by a validated food frequency interview, usual supplement use was confirmed by supplement label, and plasma alpha-tocopherol was measured. Analyses were conducted separately for individuals who did (n = 318) and did not (n = 577) use vitamin E supplements.

RESULTS

Among supplement nonusers, reported mean intake of vitamin E (mg alpha-tocopherol equivalents [alpha-TE]) did not differ between those who remained nondiabetic (n = 490) and those who developed diabetes (n = 87) (10.5 +/- 5.5 vs. 9.5 +/- 4.8 [means +/- SD], respectively, NS). After adjustment for demographic variables, obesity, physical activity, and other nutrients, the association remained nonsignificant (odds ratio [OR] 0.80, 95% CI 0.13-5.06) for the highest level of intake (> or =20 mg alpha-TE) compared with the lowest level (1-4 alpha-TE). However, results for plasma concentration of alpha-tocopherol showed a significant protective effect both before and after adjustment for potential confounders (adjusted OR 0.12, 95% CI 0.02-0.68, for the highest quintile vs. the lowest quintile; overall test for trend, P < 0.01). Among individuals who reported habitual use of vitamin E supplements (at least once per month in the year before baseline; 259 remained nondiabetic and 59 developed diabetes), no protective effect was observed for either reported intake of vitamin E or plasma concentration of alpha-tocopherol

CONCLUSIONS

A protective effect of vitamin E may exist within the range of intake available from food. This effect may go undetected within studies of high-dose supplement use, which appears to hold no additional protective benefit.

Antioxidant vitamins and their influence in diabetes mellitus

Diabetes mellitus is a chronic disease associated with serious complications. A number of studies have suggested that enhanced oxidation is the underlying abnormality responsible for some of the complications of diabetes. It is not known whether the ingestion of antioxidant vitamins could retard or perhaps reverse the oxidative damage. The information regarding the benefit of antioxidant vitamin supplementation is conflicting some trials have demonstrated adverse effects of excessive consumption of vitamin supplements. In this article, we review the available literature on the association of cardiovascular events and ingestion of vitamins with antioxidant properties. Given the lack of data to substantiate the benefit and safety of ingestion of antioxidant vitamins in excess of the recommended dietary allowance, physicians should avoid the recommendation of vitamin supplementation to their patients.

Effect of whole grains on insulin sensitivity in overweight hyperinsulinemic adults

BACKGROUND

Epidemiologic studies have found whole-grain intake to be inversely associated with the risk of type 2 diabetes and heart disease.

OBJECTIVE

We tested the hypothesis that whole-grain consumption improves insulin sensitivity in overweight and obese adults.

DESIGN

This controlled experiment compared insulin sensitivity between diets (55% carbohydrate, 30% fat) including 6-10 servings/d of breakfast cereal, bread, rice, pasta, muffins, cookies, and snacks of either whole or refined grains. Total energy needs were estimated to maintain body weight. Eleven overweight or obese [body mass index (in kg/m(2)): 27-36] hyperinsulinemic adults aged 25-56 y participated in a randomized crossover design. At the end of each 6-wk diet period, the subjects consumed 355 mL (12 oz) of a liquid mixed meal, and blood samples were taken over 2 h. The next day a euglycemic hyperinsulinemic clamp test was administered.

RESULTS

Fasting insulin was 10% lower during consumption of the whole-grain than during consumption of the refined-grain diet (mean difference: -15 +/- 5.5 pmol/L; P = 0.03). After the whole-grain diet, the area under the 2-h insulin curve tended to be lower (-8832 pmol.min/L; 95% CI: -18720, 1062) than after the refined-grain diet. The rate of glucose infusion during the final 30 min of the clamp test was higher after the whole-grain diet (0.07 x 10(-4) mmol.kg(-1).min(-1) per pmol/L; 95% CI: 0.003 x 10(-4), 0.144 x 10(-4)).

CONCLUSION

Insulin sensitivity may be an important mechanism whereby whole-grain foods reduce the risk of type 2 diabetes and heart disease.

Dietary treatment of the metabolic syndrome--the optimal diet

The treatment of the metabolic syndrome aims to improve insulin sensitivity and correct/prevent the associated metabolic and cardiovascular abnormalities. Since many individuals with the metabolic syndrome are overweight, dietary treatment should be primarily focused on weight reduction. This approach can improve insulin sensitivity and exert beneficial effects on all the other abnormalities clustering in the syndrome. Insulin sensitivity can also be influenced by diet composition. In this respect, the specific effects of the quality of dietary fat are of great interest, given the considerable evidence in experimental animals that saturated fat in the diet may lead to insulin resistance. In man, there is indirect evidence that a higher saturated fat intake is associated with impaired insulin action. Human studies have also attempted to evaluate the relationship between total fat intake and insulin sensitivity. They are consistent in showing that fat intake is correlated with both plasma insulin values (positively) and insulin sensitivity (negatively). However, these correlations are largely mediated by body weight. Conversely, intervention studies are consistent in showing that when total fat intake is moderately increased (from 20 to 40%), no major effect is observed on insulin sensitivity. We have recently undertaken a large, multicentre intervention study in 162 healthy individuals given either a high-saturated-fat or a high-monounsaturated-fat diet for 3 months. It shows that a high-monounsaturated-fat diet significantly improves insulin sensitivity compared to a high-saturated-fat diet. However, this beneficial effect of monounsaturated fat disappears when total fat intake exceeds 38% of total energy. Independently of its effects on insulin sensitivity, diet composition can influence the factors clustering in the metabolic syndrome. Dietary carbohydrate increases blood glucose levels, particularly in the postprandial period, and consequently also insulin levels and plasma triglycerides. The detrimental effects of a high-carbohydrate diet on plasma glucose/insulin, triglyceride/HDL or fibrinolysis occur only when carbohydrate foods with a high glycaemic index are consumed, while they are abolished if the diet is based largely on fibre-rich, low-glycaemic-index foods. In conclusion, weight reduction is a powerful measure for the treatment of metabolic syndrome. Moreover, the diet for the treatment of the metabolic syndrome should be limited in the intake of saturated fat, while high fibre/low-glycaemic-index foods should be used without specific limitations. Moderate amounts of monounsaturated fat could be permitted as they do not induce detrimental metabolic effects.