Insulin resistance, low-fat diets, and low-carbohydrate diets: time to test new menus

Improving the scientific rigour of nutritional recommendations for adults with type 2 diabetes: A comprehensive review of the American Diabetes Association guideline-recommended eating patterns

AIMS

The global rate of type 2 diabetes (T2D) continues to rise. Guidelines that influence the worldwide treatment of this disease are central to changing this trajectory. We sought in this review to evaluate the appropriateness of sources cited in the American Diabetes Association's (ADA) guidelines on eating patterns for T2D management, identify additional relevant sources, and evaluate the evidence.

MATERIALS AND METHODS

We reviewed the evidence behind the ADA's recommendations on eating patterns in the 2018 and 2019 ADA Standards of Care and the 2014 ADA Nutrition Therapy Recommendations for Adults with Diabetes. Additionally, we conducted a comprehensive search to identify any additional studies not included in the cited evidence. To determine appropriateness of inclusion in the guidelines, the following criteria were applied: 1) it was a clinical trial or systematic review/meta-analysis of clinical trials; 2) it involved persons with T2D; 3) one of the study arms followed one of the eating patterns currently recommended; 4) its reported outcomes included glycaemic control; 5) outcomes were reported separately for persons with T2D.

RESULTS

We found a wide variation in the evidence for each eating pattern. Issues that have hampered the guideline process include: lack of a rigorous literature review, resulting in the omission of pertinent studies; an overreliance on prospective cohort studies; inconsistent standards for evidence; inclusion of studies not on persons with T2D; and bias.

CONCLUSIONS

The ADA Guidelines recommended eating patterns fall short of rigorous standards of scientific review according to state-of-the-art systematic review and guideline creation practices.

Systematic review and meta-analysis of different dietary approaches to the management of type 2 diabetes

BACKGROUND

There is evidence that reducing blood glucose concentrations, inducing weight loss, and improving the lipid profile reduces cardiovascular risk in people with type 2 diabetes.

OBJECTIVE

We assessed the effect of various diets on glycemic control, lipids, and weight loss.

DESIGN

We conducted searches of PubMed, Embase, and Google Scholar to August 2011. We included randomized controlled trials (RCTs) with interventions that lasted ≥6 mo that compared low-carbohydrate, vegetarian, vegan, low-glycemic index (GI), high-fiber, Mediterranean, and high-protein diets with control diets including low-fat, high-GI, American Diabetes Association, European Association for the Study of Diabetes, and low-protein diets.

RESULTS

A total of 20 RCTs were included (n = 3073 included in final analyses across 3460 randomly assigned individuals). The low-carbohydrate, low-GI, Mediterranean, and high-protein diets all led to a greater improvement in glycemic control [glycated hemoglobin reductions of -0.12% (P = 0.04), -0.14% (P = 0.008), -0.47% (P < 0.00001), and -0.28% (P < 0.00001), respectively] compared with their respective control diets, with the largest effect size seen in the Mediterranean diet. Low-carbohydrate and Mediterranean diets led to greater weight loss [-0.69 kg (P = 0.21) and -1.84 kg (P < 0.00001), respectively], with an increase in HDL seen in all diets except the high-protein diet.

CONCLUSION

Low-carbohydrate, low-GI, Mediterranean, and high-protein diets are effective in improving various markers of cardiovascular risk in people with diabetes and should be considered in the overall strategy of diabetes management.

The role of insulin resistance in the pathogenesis of atherosclerotic cardiovascular disease: an updated review

Insulin resistance is the main pathologic mechanism that links the constellation of clinical, metabolic and anthropometric traits with increased risk for cardiovascular disease and type II diabetes mellitus. These traits include hyperinsulinemia, impaired glucose intolerance, endothelial dysfunction, dyslipidemia, hypertension, and generalized and upper body fat redistribution. This cluster is often referred to as insulin resistance syndrome. The progression of insulin resistance to diabetes mellitus parallels the progression of endothelial dysfunction to atherosclerosis leading to cardiovascular disease and its complications. In fact, insulin resistance assessed by homeostasis model assessment (HOMA) has shown to be independently predictive of cardiovascular disease in several studies and one unit increase in insulin resistance is associated with a 5.4% increase in cardiovascular disease risk. This review article addresses the role of insulin resistance as a main causal factor in the development of metabolic syndrome and endothelial dysfunction, and its relationship with cardiovascular disease. In addition to this, we review the type of lifestyle modification and pharmacotherapy that could possibly ameliorate the effect of insulin resistance and reverse the disturbances in insulin, glucose and lipid metabolism.

A low carbohydrate Mediterranean diet improves cardiovascular risk factors and diabetes control among overweight patients with type 2 diabetes mellitus: a 1-year prospective randomized intervention study

BACKGROUND

The appropriate dietary intervention for overweight persons with type 2 diabetes mellitus (DM2) is unclear. Trials comparing the effectiveness of diets are frequently limited by short follow-up times and high dropout rates.

AIM

The effects of a low carbohydrate Mediterranean (LCM), a traditional Mediterranean (TM), and the 2003 American Diabetic Association (ADA) diet were compared, on health parameters during a 12-month period.

METHODS

In this 12-month trial, 259 overweight diabetic patients (mean age 55 years, mean body mass index 31.4 kg/m(2)) were randomly assigned to one of the three diets. The primary end-points were reduction of fasting plasma glucose, HbA1c and triglyceride (TG) levels.

RESULTS

194 patients out of 259 (74.9%) completed follow-up. After 12 months, the mean weight loss for all patients was 8.3 kg: 7.7 kg for ADA, 7.4 kg for TM and 10.1 kg for LCM diets. The reduction in HbA1c was significantly greater in the LCM diet than in the ADA diet (-2.0 and -1.6%, respectively, p < 0.022). HDL cholesterol increased (0.1 mmol/l +/- 0.02) only on the LCM (p < 0.002). The reduction in serum TG was greater in the LCM (-1.3 mmol/l) and TM (-1.5 mmol/l) than in the ADA (-0.7 mmol/l), p = 0.001.

CONCLUSIONS

An intensive 12-month dietary intervention in a community-based setting was effective in improving most modifiable cardiovascular risk factors in all the dietary groups. Only the LCM improved HDL levels and was superior to both the ADA and TM in improving glycaemic control.

Macronutrients and insulin resistance in cholesterol gallstone disease

Cholelithiasis is a major source of digestive morbidity worldwide. Cholesterol stones account for the majority of gallstones in the United States and other Western countries. The pathogenesis of cholesterol gallstone disease is multifactorial with key factors including cholesterol supersaturation of bile, altered biliary motility, and nucleation and growth of cholesterol crystals. Increasing evidence suggests that many, but not all, causative factors of cholesterol gallstones are related to insulin resistance which, in association with obesity, has reached an epidemic level worldwide. Experimental studies show that hyperinsulinemia, a key feature of insulin resistance, may cause increased hepatic cholesterol secretion and cholesterol supersaturation of bile and gallbladder dysmotility, and thereby may enhance gallstone formation. Insulin resistance syndrome can be modified by environmental factors, including dietary factors. The impact of diet on insulin sensitivity is mediated by both dietary composition and its energy content. The contribution of specific dietary elements to the prevalence and incidence of cholesterol gallstone disease has been explored in animal and human studies. There is considerable evidence to suggest that different types of fatty acids, independent of the total amount of fat consumption, affect insulin sensitivity and cholesterol gallstone disease differently. The effects of salt intake, consumption of protein and carbohydrates, and alcohol drinking on insulin resistance are controversial. Additional intervention trials and controlled experimental feeding studies are needed to further clarify these relationships and to provide useful prophylactic and therapeutic strategies.

Weight loss with a low-carbohydrate, Mediterranean, or low-fat diet

BACKGROUND

Trials comparing the effectiveness and safety of weight-loss diets are frequently limited by short follow-up times and high dropout rates.

METHODS

In this 2-year trial, we randomly assigned 322 moderately obese subjects (mean age, 52 years; mean body-mass index [the weight in kilograms divided by the square of the height in meters], 31; male sex, 86%) to one of three diets: low-fat, restricted-calorie; Mediterranean, restricted-calorie; or low-carbohydrate, non-restricted-calorie.

RESULTS

The rate of adherence to a study diet was 95.4% at 1 year and 84.6% at 2 years. The Mediterranean-diet group consumed the largest amounts of dietary fiber and had the highest ratio of monounsaturated to saturated fat (P<0.05 for all comparisons among treatment groups). The low-carbohydrate group consumed the smallest amount of carbohydrates and the largest amounts of fat, protein, and cholesterol and had the highest percentage of participants with detectable urinary ketones (P<0.05 for all comparisons among treatment groups). The mean weight loss was 2.9 kg for the low-fat group, 4.4 kg for the Mediterranean-diet group, and 4.7 kg for the low-carbohydrate group (P<0.001 for the interaction between diet group and time); among the 272 participants who completed the intervention, the mean weight losses were 3.3 kg, 4.6 kg, and 5.5 kg, respectively. The relative reduction in the ratio of total cholesterol to high-density lipoprotein cholesterol was 20% in the low-carbohydrate group and 12% in the low-fat group (P=0.01). Among the 36 subjects with diabetes, changes in fasting plasma glucose and insulin levels were more favorable among those assigned to the Mediterranean diet than among those assigned to the low-fat diet (P<0.001 for the interaction among diabetes and Mediterranean diet and time with respect to fasting glucose levels).

CONCLUSIONS

Mediterranean and low-carbohydrate diets may be effective alternatives to low-fat diets. The more favorable effects on lipids (with the low-carbohydrate diet) and on glycemic control (with the Mediterranean diet) suggest that personal preferences and metabolic considerations might inform individualized tailoring of dietary interventions. (ClinicalTrials.gov number, NCT00160108.)

Comparison of monounsaturated fat with carbohydrates as a replacement for saturated fat in subjects with a high metabolic risk profile: studies in the fasting and postprandial states

BACKGROUND

In subjects with a high prevalence of metabolic risk abnormalities, the preferred replacement for saturated fat is unresolved.

OBJECTIVE

The objective was to study whether carbohydrate or monounsaturated fat is a preferred replacement for saturated fat.

DESIGN

Fifty-two men and 33 women, selected to have any combination of HDL cholesterol < or = 30th percentile, triacylglycerol > or = 70th percentile, or insulin > or = 70th percentile, were enrolled in a 3-period, 7-wk randomized crossover study. The subjects consumed an average American diet (AAD; 36% of energy from fat) and 2 additional diets in which 7% of energy from saturated fat was replaced with either carbohydrate (CHO diet) or monounsaturated fatty acids (MUFA diet).

RESULTS

Relative to the AAD, LDL cholesterol was lower with both the CHO (-7.0%) and MUFA (-6.3%) diets, whereas the difference in HDL cholesterol was smaller during the MUFA diet (-4.3%) than during the CHO diet (-7.2%). Plasma triacylglycerols tended to be lower with the MUFA diet, but were significantly higher with the CHO diet. Although dietary lipid responses varied on the basis of baseline lipid profiles, the response to diet did not differ between subjects with or without the metabolic syndrome or with or without insulin resistance. Postprandial triacylglycerol concentrations did not differ significantly between the diets. Lipoprotein(a) concentrations increased with both the CHO (20%) and MUFA (11%) diets relative to the AAD.

CONCLUSIONS

In the study population, who were at increased risk of coronary artery disease, MUFA provided a greater reduction in risk as a replacement for saturated fat than did carbohydrate.

A novel system for collecting longitudinal self-reported dietary consumption information: the internet data logger (iDL)

As diet has been considered as the major exposure pathway for toxic chemicals, such as heavy metals and pesticides, we have developed a web-based questionnaire system, the internet data logger (iDL), in order to facilitate the collection of detailed self-recording of standardized daily dietary consumption. An iDL prototype optimized for hand-held devices was tested in the on-going Children's Pesticide Exposure Study (CPES) in Atlanta, GA in which 15 consecutive days of dietary consumption information were recorded by 11 children or their parents and transmitted to a server via broadband wireless internet connection. Participants spent an average of 6.2 min in day 1 to enter a meal into iDL and approximately 4 min per meal for the remaining 14 study days. Approximately 75% of 446 total meals were entered into iDL on the date of consumption. A within-person comparison revealed that many food items recorded on paper diary were later classified as "unknown" according to iDL standards due to the lack of detailed information provided by the participants. The development of iDL represents a novel and pioneering tool, which integrates dietary data collection, transmission, and management in a real-time and automated manner. iDL provides the needed flexibility and mobility for research focused on collecting not only dietary consumption data but also other time-sensitive behavior data. This study has demonstrated that iDL can be deployed to collect long-term dietary consumption information in a population-based study, though future modifications/enhancements will undoubtedly improve its usability and application.

Perilipin gene variation determines higher susceptibility to insulin resistance in Asian women when consuming a high-saturated fat, low-carbohydrate diet

OBJECTIVE

To investigate the association between genetic variation in the adipocyte protein perilipin (PLIN) and insulin resistance in an Asian population as well as to examine their modulation by macronutrient intake.

RESEARCH DESIGN AND METHODS

A nationally representative sample (Chinese, Malays, and Indians) was selected in the Singapore National Health Survey following the World Health Organization-recommended model for field surveys of diabetes. A total of 1,909 men and 2,198 women (aged 18-69 years) were studied. Genetic (PLIN 11482G-->A and 14995A-->T), lifestyle, clinical, and biochemical data were obtained. Homeostasis model assessment of insulin resistance (HOMA-IR) was used to evaluate insulin resistance. Diet was measured by a validated food frequency questionnaire in one of every two subjects.

RESULTS

We did not find a significant between-genotype difference in insulin resistance measures. However, in women we found statistically significant gene-diet interactions (recessive model) between PLIN 11482G-->A/14995A-->T polymorphisms (in high linkage disequilibrium) and saturated fatty acids (SFAs; P = 0.003/0.005) and carbohydrate (P = 0.004/0.012) in determining HOMA-IR. These interactions were in opposite directions and were more significant for 11482G-->A, considered the tag polymorphism. Thus, women in the highest SFA tertile (11.8-19%) had higher HOMA-IR (48% increase; P trend = 0.006) than women in the lowest (3.1-9.4%) only if they were homozygotes for the PLIN minor allele. Conversely, HOMA-IR decreased (-24%; P trend = 0.046) as carbohydrate intake increased. These effects were stronger when SFAs and carbohydrate were combined as an SFA-to-carbohydrate ratio. Moreover, this gene-diet interaction was homogeneously found across the three ethnic groups.

CONCLUSIONS

PLIN 11482G-->A/14995A-->T polymorphisms modulate the association between SFAs/carbohydrate in diet and insulin resistance in Asian women.

An update on low-carbohydrate, high-protein diets

PURPOSE OF REVIEW

The use of low-carbohydrate diets in weight loss is an area of great controversy. In recent years, a significant amount of research has been conducted to evaluate the efficacy of these diets. This review aims to discuss mechanisms of action for weight loss; to assess impact on metabolic parameters including insulin sensitivity and cardiovascular risk parameters and to evaluate the data regarding safety and effectiveness for weight loss.

RECENT FINDINGS

Most studies demonstrate that subjects following low carbohydrate diets lose more weight over the first 3-6 months than subjects consuming control diets. This weight loss is not sustained, however, at 1 year. Carbohydrate controlled diets may be associated with increased insulin sensitivity and improved glycemic control. High carbohydrate, low fat diets appear to have a more favorable impact on total and LDL cholesterol, whereas low carbohydrate diets have been shown to significantly decrease triglyceride and increase HDL cholesterol levels in short-term studies.

SUMMARY

Low carbohydrate diets may be effective in helping people to lose weight. More long-term studies need to be performed, however, to better evaluate efficacy, safety, and impact on metabolic profile.

Low-carbohydrate or low-fat diets for the metabolic syndrome?

This report discusses a variety of considerations and reviews recent clinical trial data relevant to the question of which dietary approach is optimal for treatment and prevention of the metabolic syndrome. Special emphasis is given to the question of whether the degree to which metabolic syndrome features are observed in an individual patient should correspond to the degree to which carbohydrate restriction should be recommended. Short-term metabolic studies lend support to this concept, but longer-term effectiveness trials suggest the degree of dietary adherence and associated weight losses, rather than diet type, are the key predictors of metabolic cardiac risk factor reduction.