Dietary strategies for patients with type 2 diabetes in the era of multi-approaches; review and results from the Dietary Intervention Randomized Controlled Trial (DIRECT)

Dietary intervention is recognized as a key component in prevention and management of type 2 diabetes (T2DM) and the debate persists: which dietary strategy is most effective. In the Dietary Intervention Randomized Controlled Trial (DIRECT) 322 moderately obese participants were randomized for 2 years to one of three diet groups: low-fat, Mediterranean and low-carbohydrate. Differential effects were observed in the sub-group of patients with T2DM at 24 months: participants randomized to the Mediterranean diet, which had the highest intake of dietary fibers and unsaturated to saturated fat ratio, achieved greater significant improvements in fasting plasma glucose and insulin levels. Patients who were randomized to the low-carbohydrate diet, which had the minimal intake of carbohydrates, achieved a significant reduction of hemoglobin A1C. Although improvements were observed in all groups, the low-fat diet was likely to be less beneficial in terms of glycemic control and lipid metabolism. Interpretation of results from different studies on dietary strategies may be complex since there is often no consistency in diet compositions, calorie restriction, intensity of intervention, dietary assessment or extent of adherence in the trial. Nevertheless, it seems that low fatrestricted calorie diets are effective for weight loss and are associated with some metabolic benefits; however, some recent trials have shown that low carbohydrate diets are as efficient in inducing weight loss and in some metabolic measures such as serum triglycerides and HDL-cholesterol may be even superior to low fat diets. When addressing the issue of diet quality rather than quantity applying the glycemic index may have some added benefits. Furthermore special features of the Mediterranean diet have apparent additional favorable effects for patients with T2DM.     

Dietary fiber prevents carbohydrate-induced hypertriglyceridemia

Plant foods rich in carbohydrate and dietary fiber have many health benefits. One concern often expressed about higher carbohydrate, lower fat diets is that they may increase fasting serum triglycerides. Recently the importance of hypertriglyceridemia as an independent risk factor for coronary heart disease has been reaffirmed. For 40 years, clinicians have noted “carbohydrate-induced hypertriglyceridemia” when persons were fed high-carbohydrate, low-fiber diets. The role of fiber in protecting from carbohydrate-induced hypertriglyceridemia has not been discussed by many reviewers. Systematic review of the literature documents that high fiber intakes clearly protect from carbohydrate-induced hypertriglyceridemia. These are reviewed. Thus, recent and earlier research indicates that use of a higher carbohydrate, higher fiber diet compared with a lower carbohydrate, higher fat diet is associated with a small reduction in fasting serum triglyceride values.     

Cardiovascular disease: Optimal approaches to risk factor modification of diet and lifestyle

Optional statement Cardiovascular disease (CVD) accounts for almost 50% of all deaths in industrialized nations. As much as 70% of CVD can be prevented or delayed with dietary choices and lifestyle modifications. Western-style diets, sedentary lifestyles, and cigarette smoking are key modifiable CVD risk factors. Although CVD mortality was trending downward for almost 50 years, a resurgence, both nationally and globally, has occurred. A growing epidemic of obesity (“globesity”), decreasing physical activity, and persistent cigarette smoking are major behavioral factors underlying this change. Diet and lifestyle increase CVD risk both directly and indirectly. Direct effects include biological, molecular, and physiologic alterations, including inflammatory stimuli and oxidative stresses. Indirect effects include diabetes, dyslipidemias, and hypertension. However, trials studying links between diet and CVD remain notoriously difficult to execute and interpret. Diet interventions are typically confounded by other aspects of an overall diet as well as by lifestyle. Furthermore, benefits derived from a specific dietary or lifestyle intervention may not be proportional to the degree of risk posed by the unhealthy diet or lifestyle. Nonetheless, therapeutic rationale for diet and lifestyle are supported by basic and clinical research. Key components of a healthy aggregate diet include 1) reduced caloric intake; 2) reduced total fat, saturated fat, trans fat, and cholesterol with proportional increases in monosaturated, n-3 (omega-3), and n-6 fatty acids; 3) increased dietary fiber, fruit, and vegetables; 4) increased micronutrients (eg, folate, B₆, B₁₂); 5) increased plant protein in lieu of animal protein; 6) reduced portions of highly processed foods; and 7) adopting a more Mediterranean or “prudent” dietary pattern over the prevailing “western” dietary pattern. Key lifestyle interventions include increased physical activity and smoking cessation. Translation of the benefits of healthy diet and lifestyle to the wider population requires both individual and public health strategies targeting at-risk groups.     

Ernährungstherapie des Typ-2-Diabetes

During the last decade dietary treatment of type 2 diabetes has become more important than ever before because there has been a further increase in the mean body mass index of patients suffering from this disease. In addition to the classical low fat diet, novel diets have been established for dietary treatment of type 2 diabetes, such as carbohydrate-reduced diets or the traditional Mediterranean diet. However, the scientific evidence supporting very low carbohydrate diets is still limited with respect to long-term effects and is not sufficient to replace the current recommendation for a low fat diet. At present, patients with type 2 diabetes have various options for dietary treatment which are equally improving metabolic control. Common components of these diets include moderate energy restriction, high fibre intake, low glycemic index (GI) and modification of fat intake including a reduction of saturated fats and trans-fatty acids. Patients with nephropathy should restrict and modify protein intake.     

How enthusiastically should the use of monounsaturated fats be encouraged?

The Garg study diet supplied 50% of the calories from fat (33% from monounsaturated fat), which eliminates much of the bulk from the diet. Substituting monounsaturates for carbohydrates in the diet translates into giving up 120 calories from a carbohydrate source (very large potato, 1 1/2 slices of bread, 1 cup of cereal, etc) for a tablespoon of olive oil. Somehow that doesn't seem like a good exchange. Abbott et al substituted complex carbohydrates for saturated fat in the diet and accomplished the same result as Coulston et al with a low-carbohydrate (40%), moderate-fat (40%) diet. The American Diabetes Association suggested that if diabetes was poorly controlled and hypertriglyceridemia a problem, monounsaturates might be useful to maintain caloric balance. However, in light of the high incidence of obesity in persons with NIDDM and recent studies on the ease of dietary fat storage in adipose tissue, should fat of any kind, including monounsaturated fats, be substituted for carbohydrates in the diet of persons with diabetes?     

The low-carbohydrate diet and cardiovascular risk factors: Evidence from epidemiologic studies

AimsObesity is an important public health issue because of its high prevalence and concomitant increase in risk of cardiovascular diseases. Low carbohydrate diets are popular for weight loss and weight management but are not recommended in leading guidelines due to the perception that increases in dietary fat intake may lead to an adverse cardiovascular risk profile. To clarify the effects of a low-carbohydrate diet for weight loss on cardiovascular disease risk factors as compared to a low fat diet for weight loss, we systematically reviewed data from randomized controlled clinical trials and large observational studies.Data synthesisWe searched the MEDLINE database (Jan 1966–Nov 2013) to identify studies that examined a low-carbohydrate diet as compared to a low-fat diet for weight loss or the improvement of cardiovascular disease risk factors.ConclusionsRecent randomized controlled trials document that low-carbohydrate diets not only decrease body weight but also improve cardiovascular risk factors. In light of this evidence from randomized controlled trials, dietary guidelines should be re-visited advocating a healthy low carbohydrate dietary pattern as an alternative dietary strategy for the prevention of obesity and cardiovascular disease risk factors.     

The potential for dietary restriction to increase longevity in humans: extrapolation from monkey studies

Based on results emerging from long-term studies of dietary restriction in rhesus monkeys, we offer our views regarding whether dietary restriction can increase longevity in humans. Because lifespan data in monkeys remain inconclusive currently, we respond that “we do not for sure.” Based on the vast literature regarding the effects of healthy, low calorie diets on health and longevity in a wide range of species, including humans, and based on data emerging from monkey studies suggesting that dietary restriction improves markers of disease risk and health, we respond that “we think so.” Because it is unlikely that an experimental study will ever be designed to address this question in humans, we respond that “we think we will never know for sure. ” We suggest that debate of this question is clearly an academic exercise; thus, we would suggest that the more compelling discussion should focus on whether basic mechanisms of DR can be discovered and if such discoveries can lead to the development of effective DR mimetics. Even if proof that DR or DR mimetics can increase longevity in humans will likely never emerge, we would suggest that endpoints regarding disease risk and disease incidence as well as maintenance of function can be examined in human clinical trials, and that these will be highly relevant for evaluating the effectiveness of such treatments.     

Short-term effects of severe dietary carbohydrate-restriction advice in Type 2 diabetes--a randomized controlled trial.

OBJECTIVE: This study sought to examine the effects of a 3-month programme of dietary advice to restrict carbohydrate intake compared with reduced-portion, low-fat advice in obese subjects with poorly controlled Type 2 diabetes. RESEARCH DESIGN AND METHODS: One hundred and two patients with Type 2 diabetes were recruited across three centres and randomly allocated to receive group education and individual dietary advice. Weight, glycaemic control, lipids and blood pressure were assessed at baseline and 3 months. Dietary quality was assessed at the end of study. RESULTS: Weight loss was greater in the low-carbohydrate (LC) group (-3.55 +/- 0.63, mean +/- sem) vs. -0.92 +/- 0.40 kg, P = 0.001) and cholesterol : high-density lipoprotein (HDL) ratio improved (-0.48 +/- 0.11 vs. -0.10 +/- 0.10, P = 0.01). However, relative saturated fat intake was greater (13.9 +/- 0.71 vs. 11.0 +/- 0.47% of dietary intake, P < 0.001), although absolute intakes were moderate. CONCLUSIONS: Carbohydrate restriction was an effective method of achieving short-term weight loss compared with standard advice, but this was at the expense of an increase in relative saturated fat intake.     

The effects of low-fat,high-carbohydrate diets on plasma lipoproteins,weight loss,and heart disease risk reduction

Although there is consensus about restriction of dietary saturated and trans fatty acids, cholesterol, and sugars, there is debate about what the optimal total fat and carbohydrate content of the diet should be for weight loss and coronary heart disease (CHD) risk reduction. The overall evidence that dietary composition plays an important role in determining caloric intake is limited. Three recent randomized trials have indicated that lowcarbohydrate diets are more effective in promoting weight loss in overweight and obese subjects over 4 to 6 months, but not over 1 year. In our own randomized trial no such differences were noted, and compliance with extreme diets was limited. Moreover little attempt has been made to control for the type of carbohydrate used in the low-fat, high-carbohydrate arms of these trials. Available evidence suggests that restriction of sugars and carbohydrates having a high glycemic index would be preferable to total carbohydrate restriction, and that an increased intake of fiber and essential fats (especially omega-3 fatty acids) is also important for overall heart disease risk reduction.     

Reducing atherothrombotic events in high-risk patients: Recent data on therapy with statins and fatty acids

Traditional treatment of atherosclerotic coronary heart disease by cardiovascular specialists, which has focused on “critical” stenoses, may be less effective in reducing morbidity and mortality than therapies that stabilize plaques and reduce thrombosis and sudden death. Recent data from clinical trials of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor (statin) therapy and modification of dietary fat composition demonstrate that both these approaches can reduce clinical events. Although revascularization therapy is effective in reducing angina caused by high-grade stenotic lesions, this therapy is incomplete because the more-numerous “smaller” plaques that typically cause clinical events remain untreated. Two recent trials suggest that statin therapy may have benefits on stabilizing plaques in high-risk patients within a year. Additional benefit may also be provided by increasing dietary consumption of monounsaturated or omega-3 polyunsaturated fatty acids. Both statin therapy and diets high in monounsaturated or omega-3 fatty acids appear to improve morbidity and mortality by modifying the underlying atherothrombotic disease process.     

Low-carbohydrate diets: An update on current research

The diabetes and obesity epidemics have stimulated research to assess the benefits and potential risks of low-carbohydrate diets. Carbohydrate comprises less than 45% of calories in carbohydrate-restricted diets, but very low carbohydrate ketogenic diets may restrict carbohydrate to 20 g initially with variability in the carbohydrate level subsequently. Some research suggests that low-carbohydrate diets may achieve better early weight loss than comparison diets higher in carbohydrate. Studies of up to 1 year suggest that weight loss on low-carbohydrate diet is comparable with fat-restricted diets with higher carbohydrate content. Limited research has been conducted to evaluate low-carbohydrate diets in managing type 2 diabetes. Although science continues to advance in this field, current research suggests that low-carbohydrate diets can be a viable option for achieving weight loss and may have beneficial effects on glycemic control, triglyceride levels, and high-density lipoprotein cholesterol levels in some patients.     

Is Atkins dead (again)?

Despite consistent epidemiological evidence that weight gain is linked to higher fat and lower carbohydrate consumption, supported by animal evidence and the inescapable truth that fat supplies 9 kcal/g compared to 3.75 kcal/g from carbohydrates, low-carbohydrate "Atkins" style diets are heavily promoted for obesity control. The randomised controlled trial evidence is very small. The totality of the evidence continues to show that low-carbohydrate diets are marginally disadvantageous for long-term health and for weight maintenance. People can lose weight equally well on low-carbohydrate ("Atkins-style") diets, and some groups of obese patients tend to lose a little more than on high-carbohydrate groups. This small difference (1-2 kg) may be explained by rapid loss of (glycogen-associated) body water, or by the influence of extraordinary media coverage leading to elevation of expectation and compliance with low-carbohydrate diets in the short term.     

Carbohydrate restriction and cardiovascular risk

Originally developed as a strategy for weight loss, diets based on restriction of carbohydrates were traditionally of concern because of the assumed increased cardiovascular risk if the carbohydrates were replaced with fat. It now appears that such diets are associated with an improvement in markers of cardiovascular risk, even with higher saturated fat intake and even in the absence of weight loss. Various evidence supports this paradigm shift: 1) carbohydrate restriction improves markers of atherogenic dyslipidemia (triglycerides, high-density lipoprotein cholesterol, apolipoprotein B-apolipoprotein A-1 ratio) and reduces the more atherogenic small, dense low-density lipoprotein cholesterol; 2) high amounts of dietary carbohydrates increase de novo fatty acid synthesis and plasma triglycerides; and 3) large, long-term studies of traditional dietary fat reduction continue to fail to demonstrate the predicted improvement in cardiovascular disease risk. Cardiovascular disease is the leading cause of morbidity and mortality in the Western world. It seems appropriate to consider carbohydrate reduction as a useful, if not the preferred, alternative to low-fat diets, which have met with limited success.     

Effects of diet composition on glucose tolerance and plasma lipids of non insulin-dependent diabetics

Summary The possible influence of qualitative dietary changes on carbohydrate tolerance and plasma lipid levels of non insulin-dependent diabetics was studied in 5 maturity-onset diabetics of normal weight who remained stabilized with 5 to 10 mg glibenclamide. Each patient was submitted to 4 isocaloric qualitatively different diets: standard, high carbohydrate, high fat, and high protein. Each diet was administered for 30 consecutive days in random order. Apart from assessing metabolic control, an oral glucose tolerance test (40 g/m² body surface area) was performed in the morning, at the end of each period. Blood was collected at 0, 60, 120, and 180 min after starting glucose ingestion, for blood glucose and plasma FFA determinations. Fasting total lipids, triglycerides, lipoalbumins, and α- and β-lipoproteins, and 24-h urinary urea were also investigated. The findings did not disclose any differences in carbohydrate tolerance after the various diets. The drop in FFA levels following an oral glucose load was greatest after the high protein diet. Fasting total lipids were lowest after the high fat and high protein diets. Triglycerides fell after the high fat diet. The remaining parameters showed less significant variations. Although no differences were observed in carbohydrate tolerance, the conclusion is that relative carbohydrate restriction together with sufficient protein supply might be most beneficial for non insulin-dependent diabetics.     

Ern?hrungsempfehlungen bei Diabetes und deren Implementierung

Current evidence-based nutrition recommendations for patients with type 1 and type 2 diabetes emphasize the restriction of saturated fatty acids to <7% of total energy intake, a minimization of trans-fats and a fibre consumption of at least 14 g/1000 Kcal, whereby half the daily cereal product intake should comprise whole grains. The use of the glycaemic index may provide modest additional benefit for glycaemic control over carbohydrate monitoring alone. All patients with overweight or obesity should receive advice on reducing energy and fat intake. A low-carbohydrate energy restricted diet may also be effective in the short-term (up to 1 year). Routine supplementation of vitamins, minerals and trace elements is not recommended.     

Adults with type 1 diabetes eat a high-fat atherogenic diet that is associated with coronary artery calcium

Aims/hypothesis  Coronary heart disease is the leading cause of mortality among people with type 1 diabetes. Diet is an important lifestyle factor that relates to risk of CHD. The aim of this study was to examine how diet and adherence to dietary guidelines differ between adults with and without type 1 diabetes, and their correlation with CHD risk factors and coronary artery calcium (CAC). Methods  The study involved 571 people with type 1 diabetes and 696 controls, aged 19 to 56 years, who were asymptomatic for CHD. CAC was measured by electron-beam computed tomography. Results  Compared with the controls, adults with type 1 diabetes reported a diet higher in fat, saturated fat and protein but lower in carbohydrates. Fewer than half of those with type 1 diabetes met dietary guidelines for fat and carbohydrate intake, and only 16% restricted saturated fat to less than 10% of daily energy intake. Adults with type 1 diabetes were significantly less likely to meet dietary guidelines than controls. Fat and saturated fat intakes were positively correlated, but carbohydrate intake was negatively correlated with CHD risk factors and HbA_1c. A high-fat diet and higher intake of protein were associated with greater odds of CAC, while higher carbohydrate intake was associated with reduced odds of CAC. Conclusions/interpretation  Adults with type 1 diabetes reported consuming higher than recommended levels of fat and saturated fat. High fat intake was associated with increased CHD risk factors, worse glycaemic control and CAC. An atherogenic diet may contribute to the risk of CHD in adults with type 1 diabetes.     

The changing roles of dietary carbohydrates: from simple to complex

The dietary recommendations made for carbohydrate intake by many organizations/agencies have changed over time. Early recommendations were based on the need to ensure dietary sufficiency and focused on meeting micronutrient intake requirements. Because carbohydrate-containing foods are a rich source of micronutrients, starches, grains, fruits, and vegetables became the foundation of dietary guidance, including the base of the US Department of Agriculture's Food Guide Pyramid. Dietary sufficiency recommendations were followed by recommendations to reduce cholesterol levels and the risk for cardiovascular disease; reduction in total fat (and hence saturated fat) predominated. Beginning in the 1970s, carbohydrates were recommended as the preferred substitute for fat by the American Heart Association and others to achieve the recommended successive reductions in total fat and low-density lipoprotein cholesterol (LDL-C). Additional research on fats and fatty acids found that monounsaturated fatty acids could serve as an alternative substitution for saturated fats, providing equivalent lowering of LDL-C without concomitant reductions in high-density lipoprotein cholesterol and increases in triglycerides witnessed when carbohydrates replace saturated fat. This research led to a sharper focus in the guidelines in the 1990s toward restricting saturated fat and liberalizing a range of intake of total fat. Higher-fat diets, still low in saturated fatty acids, became alternative strategies to lower-fat diets. As the population has become increasingly overweight and obese, the emergence of the metabolic syndrome and its associated disruptions in glucose and lipid metabolism has led to reconsiderations of the role of carbohydrate-containing foods in the American diet. Consequently, a review of the evidence for and against high-carbohydrate diets is important to put this controversy into perspective. The current dietary recommendations for carbohydrate intake are supported by the evidence.     

Atkins and other low-carbohydrate diets: hoax or an effective tool for weight loss?

CONTEXT: The Atkins diet books have sold more than 45 million copies over 40 years, and in the obesity epidemic this diet and accompanying Atkins food products are popular. The diet claims to be effective at producing weight loss despite ad-libitum consumption of fatty meat, butter, and other high-fat dairy products, restricting only the intake of carbohydrates to under 30 g a day. Low-carbohydrate diets have been regarded as fad diets, but recent research questions this view. STARTING POINT: A systematic review of low-carbohydrate diets found that the weight loss achieved is associated with the duration of the diet and restriction of energy intake, but not with restriction of carbohydrates. Two groups have reported longer-term randomised studies that compared instruction in the low-carbohydrate diet with a low-fat calorie-reduced diet in obese patients (N Engl J Med 2003; 348: 2082-90; Ann Intern Med 2004; 140: 778-85). Both trials showed better weight loss on the low-carbohydrate diet after 6 months, but no difference after 12 months. WHERE NEXT?: The apparent paradox that ad-libitum intake of high-fat foods produces weight loss might be due to severe restriction of carbohydrate depleting glycogen stores, leading to excretion of bound water, the ketogenic nature of the diet being appetite suppressing, the high protein-content being highly satiating and reducing spontaneous food intake, or limited food choices leading to decreased energy intake. Long-term studies are needed to measure changes in nutritional status and body composition during the low-carbohydrate diet, and to assess fasting and postprandial cardiovascular risk factors and adverse effects. Without that information, low-carbohydrate diets cannot be recommended.     

Metabolic effects of high-protein diets

Protein is more satiating than carbohydrate or fat, and high-protein diets (25%–35% of energy) are commonly used for weight loss. High-protein diets usually replace carbohydrate with protein and may be low or high in saturated fat. Invariably, serum triglyceride is lower with the lower intake of carbohydrate, but the effects on high-density lipoprotein cholesterol and low-density lipoprotein cholesterol are strongly dependent on the amount of carbohydrate restriction and the intake of saturated fat, and in some cases low-density lipoprotein cholesterol may rise despite weight loss. In situations of weight stability, higher intakes of protein are associated with lower blood pressures, and in diabetic patients higher intakes of protein are associated with lower glycosylated hemoglobin. The overall effect on long-term atherosclerosis risk is not clear, as the current limited epidemiology provides conflicting data.     

Dietary fatty acids and insulin resistance

High-fat diets have been associated with insulin resistance, a risk factor for both Type II diabetes and heart disease. The effect of dietary fat on insulin varies depending on the type of fatty acid consumed. Saturated fatty acids have been consistently associated with insulin resistance. On the other hand, medium and long-chain fatty acid intakes are associated with insulin sensitivity, as are high intakes of ϕ3 fatty acids. Trans fatty acids appear to potentiate insulin secretion, at least in the short-term, to a greater degree than cis fatty acids. This may reflect chronic alterations in insulin sensitivity, although this remains to be tested. In summary, although it must be emphasized that all diets high in fat cause insulin resistance relative to high-carbohydrate diets, it appears that dietary saturated, short-chain and ϕ6 fatty acids have the most deleterious effects on insulin action.