The Role of Vitamin D and Its Molecular Bases in Insulin Resistance, Diabetes, Metabolic Syndrome, and Cardiovascular Disease: State of the Art

In the last decade, an increasing awareness was directed to the role of Vitamin D in non-skeletal and preventive roles for chronic diseases. Vitamin D is an essential hormone in regulating calcium/phosphorous balance and in the pathogenesis of inflammation, insulin resistance, and obesity. The main forms of vitamin D, Cholecalciferol (Vitamin D3) and Ergocalciferol (Vitamin D2) are converted into the active form (1,25-dihydroxyvitamin D) thanks to two hydroxylations in the liver, kidney, pancreas, and immune cells. Some anti-inflammatory cytokines are produced at higher levels by vitamin D, while some pro-inflammatory cytokines are released at lower levels. Toll-Like Receptor (TLR) expression is increased, and a pro-inflammatory state is also linked to low levels of vitamin D. Regardless of how it affects inflammation, various pathways suggest that vitamin D directly improves insulin sensitivity and secretion. The level of vitamin D in the body may change the ratio of pro- to anti-inflammatory cytokines, which would impact insulin action, lipid metabolism, and the development and function of adipose tissue. Many studies have demonstrated an inverse relationship between vitamin D concentrations and pro-inflammatory markers, insulin resistance, glucose intolerance, metabolic syndrome, obesity, and cardiovascular disease. It is interesting to note that several long-term studies also revealed an inverse correlation between vitamin D levels and the occurrence of diabetes mellitus. Vitamin D supplementation in people has controversial effects. While some studies demonstrated improvements in insulin sensitivity, glucose, and lipid metabolism, others revealed no significant effect on glycemic homeostasis and inflammation. This review aims to provide insight into the molecular basis of the relationship between vitamin D, insulin resistance, metabolic syndrome, type 1 and 2 diabetes, gestational diabetes, and cardiovascular diseases.

Egg and Dietary Cholesterol Consumption and the Prevalence of Metabolic Syndrome: Findings from a Population-Based Nationwide Cohort

BACKGROUND

Although the Chinese Dietary Guidelines (2016) removed restrictions on dietary cholesterol intake, evidence of egg and dietary cholesterol intake and cardiometabolic diseases is inconsistent. Associations between egg and cholesterol consumption and metabolic syndrome (MetS) in non-Western populations are still poorly documented.

OBJECTIVE

Our aim was to assess egg and dietary cholesterol intake in relation to the prevalence of MetS among participants in a Chinese nationwide study.

DESIGN

This cross-sectional study used data from the China Health and Nutrition Survey (1991-2009).

PARTICIPANTS/SETTING

The sample consisted of 8,241 healthy Chinese adults (20 years and older).

MAIN OUTCOME MEASURES

MetS cases were defined according to the National Cholesterol Education Program Adult Treatment Panel III criteria.

STATISTICAL ANALYSIS

Cumulative means of egg and cholesterol consumption were calculated in accordance with 3 consecutive 24-hour dietary recalls in each examination cycle. Logistic regression models were conducted to assess the associations with prevalent MetS.

RESULTS

Overall, 2,580 (31.3%) participants were identified as MetS cases in 2009. After multivariate adjustment, total egg consumption (>1 egg/d) was associated with 20% higher odds of MetS (odds ratio [OR] 1.20, 95% CI 1.06 to 1.37; P trend = .001) compared with consumption of ≤1/2 egg/d. Examining cooking methods, a positive association was observed between fried egg consumption and MetS odds (OR comparing the highest category [>1/2 egg/d] with the lowest category [≤1/7 egg/d] 1.22, 95% CI 1.08 to 1.39; P trend = .001), and nonfried egg intake was not associated with MetS odds (P trend = .08). Total dietary intake and egg-sourced cholesterol intake were both positively correlated with MetS odds (OR 1.31, 95% CI 1.12 to 1.53; P trend = .005) comparing the highest consumption (>371 mg · 2,000 kcal^-1 · d^-1) with the lowest consumption (≤132 mg · 2,000 kcal^-1 · d^-1) for total dietary cholesterol (OR 1.36; 95% CI 1.17 to 1.58; P trend < .001) and comparing the highest consumption (>232 mg · 2,000 kcal^-1 · d^-1) with the lowest consumption (≤46 mg · 2,000 kcal^-1 · d^-1) for egg-sourced cholesterol; similar associations were not observed for non-egg-sourced cholesterol consumption (P trend = .83). Substituting eggs and fried eggs for other protein sources, including low-fat and whole-fat dairy products; nuts and legumes; total red meat; processed meat; poultry meat; or seafood, was still associated with higher odds of MetS.

CONCLUSIONS

Consumption of >1 egg/d and >1/2 fried egg/d was associated with a higher prevalence of MetS than consumption of ≤1/2 egg/d and ≤1/7 fried egg/d. Future longitudinal cohort studies and randomized controlled trials are needed to further investigate the relationship between egg consumption and MetS and explore possible mechanisms of action.

Dietary cholesterol and egg intake are associated with the risk of gestational diabetes: a prospective study from Southwest China

Background

An increasing body of evidence suggests that cholesterol intake increases during pregnancy and may influence the risk of gestational diabetes mellitus (GDM). However, existing evidence remains controversial and limited. The present study aimed to determine the relation among dietary cholesterol, specifically egg consumption, in pregnant Chinese women and their risk of GDM.

Methods

A population-based study that included 1617 pregnant women was conducted in 2017. At baseline, dietary information was collected by 24-hour dietary recalls over three days. GDM was diagnosed by a 75 g 2-hr oral glucose tolerance test (OGTT) at 24-28 weeks of gestation. Logistic regression models were used to examine the associations of dietary cholesterol and egg intake with GDM. In addition, path analysis including cholesterol intake, plasma lipid profiles and GDM risk was conducted.

Results

The average total cholesterol intake was 340.8 mg/d, and cholesterol from eggs accounted for 59.2%. The odds ratio (OR) of GDM risk was 1.48 for the highest quartile of total cholesterol intake compared to the lowest quartile (95% CI 1.10-2.00; P_trend = 0.015) after adjustment for potential risk factors for GDM. Moreover, cholesterol from eggs rather than from other foods was positively associated with incident GDM (OR=1.09, 95% CI 1.03-1.17). Each additional egg consumed per day was positively correlated with a higher risk of GDM (OR=1.32, 95% CI 1.11-1.58). Path analysis indicated that cholesterol intake not only increased the risk of GDM by elevating plasma total cholesterol (TC), but also increased the risk of GDM through other non hyperlipidemia pathways.

Conclusions

Maternal dietary cholesterol intake was significantly associated with incident GDM, and egg consumption was a major driver of the association in this population. More studies are needed to substantiate these findings and to explore the underlying mechanisms.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12884-022-04382-y.

The role of nutrition in the development and management of gestational diabetes among Iranian women: a systematic review and meta-analysis

Purpose

We aimed to review existing evidence on nutrition associations with gestational diabetes mellitus (GDM) development and management among Iranian women.

Methods

Web of Science, PubMed, Scopus, SID, and Magiran were searched up to August 2020. Observational studies on associations between circulatory markers of vitamins and minerals, dietary intakes, and GDM and clinical trials of the effects of nutritional supplementations or dietary modifications on management or prevention of GDM among Iranian women were selected.

Results

We reviewed 49 publications. Pooled analyses revealed that GDM women had lower serum vitamin D (-8.31 nmol/l (95% CIs= -14.4, -2.19), higher serum iron (26.2 μg/dl (95% CIs= 2.52, 49.8), ferritin (24.1 ng/ml (95% CIs= 15.0, 33.4), and haemoglobin (1.14 g/dl (95% CIs: 0.32, 1.96) levels than non-GDM women. Single studies found the inverse associations of the Mediterranean diet, dietary approaches to stop hypertension diet, plant-based dietary index, and the direct associations of dietary acid load and dietary inflammatory index with the odds of GDM. Vitamin D supplementation early in pregnancy prevented GDM in two clinical trials. In two studies, iron supplementation's effect on GDM in non-anemic women was inconsistent. Pooled analyses of probiotic supplementation in women with GDM showed no significant impact on maternal glycemia.

Conclusions

Vitamin D supplementation early in pregnancy may reduce the incidence of GDM. There is no compelling evidence that vitamin D or probiotics can help with GDM management. There is currently inadequate data to recommend a specific dietary pattern to prevent GDM in Iranian women.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40200-021-00944-7.

Association between egg intake and blood pressure in the USA: the INTERnational study on MAcro/micronutrients and blood Pressure (INTERMAP)

OBJECTIVES

To investigate associations of egg intake with blood pressure (BP) and the role of dietary variables and other macro- and micro-nutrients in the association.

DESIGN

We used cross-sectional data for the USA as part of the INTERnational study on MAcro/micronutrients and blood Pressure (INTERMAP). INTERMAP was surveyed between 1996 and 1999, including four 24-h dietary recalls, two 24-h urine collections and eight measurements of systolic BP and diastolic BP (SBP, DBP). Average egg intake (g/d) was calculated. Multivariable linear regression models were used to estimate the association between egg intake (per each 50 g/d or per quintile) and BP. The roles of dietary variables and other macro- and micro-nutrients in this association were also investigated.

SETTING

In the USA.

PARTICIPANTS

In total, 2195 US INTERMAP men and women aged 40-59 years.

RESULTS

Participants were 50 % female, 54 % non-Hispanic White and 16 % non-Hispanic Black. Mean egg intake (sd) in men and women was 30·4(29·8) and 21·6(20·5) g/d, respectively. Adjusting for demographics, socio-economics, lifestyle and urinary Na:K excretion ratios, we found non-linear associations with BP in non-obese women (P-quadratic terms: 0·004 for SBP and 0·035 for DBP).The associations remained after adjusting for dietary variables, macro/micro nutrients or minerals. Dietary cholesterol was highly correlated with egg intake and may factor in the association. No association was found in obese women and in obese or non-obese men.

CONCLUSION

Egg intake was non-linearly associated with SBP and DBP in non-obese women, but not in obese women or men. Underlying mechanisms require additional study regarding the role of obesity and sex.

Beyond protein intake: does dietary fat intake in the year preceding pregnancy and during pregnancy have an impact on gestational diabetes mellitus?

Purpose

Studies regarding the association between dietary fat intake and gestational diabetes mellitus (GDM) are limited and provide conflicting findings. Thus, the study aims to examine the association of dietary fat intake in the year preceding pregnancy and during pregnancy with the risk of GDM, taking the relevance of dietary protein intake on GDM into consideration.

Methods

A prospective study was conducted in 6299 singleton pregnancies, using the data from the Nutrition in Pregnancy and Growth in Southwest China (NPGSC). A validated food frequency questionnaire was used to assess dietary fat intake in the year preceding pregnancy and during the first and second trimesters of pregnancy. Logistic regression analysis was used to assess the prospective associations of dietary fat intake and the type and source of dietary fats in different time windows with GDM risk.

Results

Higher intake of total fat [OR (95% CI): 2.21 (1.19–4.20), P = 0.02] during 12–22 weeks of gestation was associated with higher GDM risk. However, adjustment for animal protein intake greatly attenuated this association [OR (95% CI): 1.81 (0.93, 3.64), P = 0.11]. Total fat intake neither in the year preceding pregnancy nor during the early pregnancy was associated with GDM risk. Moreover, insignificant associations were observed between intakes of vegetable fat, animal fat, cholesterol, saturated fatty acid, monounsaturated fatty acid and polyunsaturated fatty acid one year before pregnancy and during the first and second trimesters and GDM risk.

Conclusion

Our study indicated that dietary fat intake one year before pregnancy and across the two pregnancy trimesters preceding the diagnosis of GDM has no relevance on GDM risk among Chinese women, particularly those with normal BMI, low, or normal calorie intake.

Dietary Cholesterol Intake and Risk of Gestational Diabetes Mellitus: A Meta-Analysis of Observational Studies

This meta-analysis aimed to understand the association between dietary cholesterol intake and the risk of gestational diabetes mellitus (GDM). We systematically searched the PubMed, Web of Science, and EMBASE databases to identify observational studies, published before September 2020, investigating the relationship between dietary cholesterol intake and risk of GDM. After evaluating the heterogeneity among studies, we used fixed- or random-effects models to calculate pooled values of relative risk (RR) and 95% confidence intervals (CIs) in order to estimate the effect of dietary cholesterol on the risk of GDM. We included a total of 9 studies (3 cohort, 4 cross-sectional, and 2 case-control) involving 30123 participants and 3237 cases. The pooled RR of GDM comparing highest versus lowest category of dietary cholesterol intake was 1.49 (95% CI, 1.18-1.88). The dose-response relationship showed that the risk of GDM increased by 32% (RR = 1.32; 95% CI, 1.20-1.45) for every increase of 100 mg per day in dietary cholesterol intake. This meta-analysis indicates that an increase in dietary cholesterol intake is associated with increased risk of GDM. Detailed cohort studies are essential to gain a better understanding of this association. Key teaching pointsStudies on animal models have suggested that a cholesterol-rich diet can promote systemic inflammation and insulin resistance, both of which can contribute to T2DM and GDM.Several epidemiological studies have investigated a potential relationship between dietary cholesterol intake and GDM risk, and the results are controversial.Our meta-analysis indicates that increased dietary cholesterol intake is associated with increased GDM risk.Dose-response analysis showed a linear relationship between dietary cholesterol intake and risk of GDM.

Temporal Trends in Maternal Food Intake Frequencies and Associations with Gestational Diabetes: The Cambridge Baby Growth Study

Previous studies have suggested that in the first decade of this century the incidence of gestational diabetes (GDM) in pregnancy rose worldwide. In the Cambridge Baby Growth Study cohort we observed that this temporal trend was associated with an index of multiple deprivation and reductions in indices of insulin secretion. Deprivation level was not directly associated with GDM, suggesting that the temporal trend may relate more to other factors linked to it, such as dietary composition. In this study we investigated temporal trends in perceived food intake frequencies, derived from a qualitative, short questionnaire, in 865 pregnant Cambridge Baby Growth Study (CBGS) recruits. A number of food frequency ranks showed both temporal trends and associations with GDM, but of note is the frequency of egg consumption (negative temporal trend p = 0.03, slope = -6.2 ranks/year; negative association with GDM p = 3.0 × 10-8, slope = -0.002 increased risk/rank) as it was also positively associated with the insulin disposition index (p = 1.17 × 10-3, slope = 0.42 ranks. L/mmoL). These results are consistent with a potential protective effect of factors related to the frequency of egg consumption in pregnancy. Such factors may have contributed to the observed temporal trend in GDM risk but the overall detectable effect appears to have been small.