Should obese patients be counselled to follow a low‐glycaemic index diet? Yes

A reduction in dietary fat has been widely advocated for the prevention and treatment of obesity and related complications. However, the efficacy of low‐fat diets has been questioned in recent years. One potential adverse effect of reduced dietary fat is a compensatory increase in the consumption of high glycaemic index (GI) carbohydrate, principally refined starchy foods and concentrated sugar. Such foods can be rapidly digested or transformed into glucose, causing a large increase in post‐prandial blood glucose and insulin. Short‐term feeding studies have generally found an inverse association between GI and satiety. Medium‐term clinical trials have found less weight loss on high GI or high glycaemic load diets compared to low GI or low glycaemic load diets. Epidemiological analyses link GI to multiple cardiovascular disease risk factors and to the development of cardiovascular disease and type 2 diabetes. Physiologically orientated studies in humans and animal models provide support for a role of GI in disease prevention and treatment. This review examines the mechanisms underlying the potential benefits of a low GI diet, and whether such diets should be recommended in the clinical setting.     

The role of the Mediterranean diet on weight loss and obesity-related diseases

Pandemic obesity is a major public health problem because of its association with non-communicable diseases and all-cause mortality, which can be improved/delayed with weight loss. Thus, several scientific societies and governments have launched guidelines to reduce body weight and adiposity or, at least, to avoid weight gain. In spite of the abundant literature on the topic, there is still controversy on the relative roles of fat and carbohydrate in the diet on weight gain. Present recommendations to avoid weight gain and obesity are directed to reduce intake of total energy variably and of total fat to <30% of energy, in spite on the lack of evidence of protection against cardiovascular disease (CVD) and mortality. By contrast, both high and low carbohydrate diets are associated with CVD and all-cause mortality in prospective studies, with a safe intake level at ≈50% of energy. Many popular diets with widely different macronutrient composition, including the Mediterranean diet, have been used in obesity; when energy-restricted, all result in similar modest weight loss at 6 months, but the effects are largely lost at 12 months. The Mediterranean diet is a plant-based, high-fat, high-unsaturated fat dietary pattern that has been consistently associated with lower rates on non-communicable diseases and total mortality in prospective studies and with reduced CVD in the PREDIMED trial. For this merits above other diets, this dietary pattern might also be used advantageously for weight loss. The results of the PREDIMED and PREDIMED-Plus randomized controlled trials on adiposity variables in high-risk populations are discussed.

     

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.     

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.     

Safety of low-carbohydrate diets

Low-carbohydrate diets have re-emerged into the public spotlight and are enjoying a high degree of popularity as people search for a solution to the population's ever-expanding waistline. The current evidence though indicates that low-carbohydrate diets present no significant advantage over more traditional energy-restricted diets on long-term weight loss and maintenance. Furthermore, a higher rate of adverse side-effects can be attributed to low-carbohydrate dieting approaches. Short-term efficacy of low-carbohydrate diets has been demonstrated for some lipid parameters of cardiovascular risk and measures of glucose control and insulin sensitivity, but no studies have ascertained if these effects represent a change in primary outcome measures. Low-carbohydrate diets are likely effective and not harmful in the short term and may have therapeutic benefits for weight-related chronic diseases although weight loss on such a program should be undertaken under medical supervision. While new commercial incarnations of the low-carbohydrate diet are now addressing overall dietary adequacy by encouraging plenty of high-fibre vegetables, fruit, low-glycaemic-index carbohydrates and healthier fat sources, this is not the message that reaches the entire public nor is it the type of diet adopted by many people outside of the world of a well-designed clinical trial. Health effects of long-term ad hoc restriction of inherently beneficial food groups without a concomitant reduction in body weight remains unanswered.     

High‐protein low‐carbohydrate diets: what is the rationale?

Dietary recommendations released by the large majority of international scientific committees and organizations suggest that the diet for the prevention and treatment of the most compelling health problems of our societies (obesity, diabetes, atherosclerosis, cardiovascular diseases and cancer) should be a diet moderately low in caloric content, low in fat content (particularly in saturated fat), poor in total cholesterol and rich in fibre. Despite this uniformity in official recommendations, alternative diets with low carbohydrate and high protein content continue to be extremely popular within consumers and patients. Recently, new studies seem to suggest that high‐protein low‐carbohydrate diets may have particularly positive effects on reducing body weight and other risk factors for heart disease 1 . Gannon and Nuttall 2 - 4 conducted direct comparisons of high‐protein low‐carbohydrate diets compared with high‐carbohydrate low‐protein diets in subjects with type 2 diabetes. They found that high‐protein low‐carbohydrate diets reduced fasting plasma glucose, 24‐h glucose area under the curve and haemoglobin A . On the basis of these results, a joint committee of the American Diabetes Association, North American Society for the Study of Obesity and the American Society for Clinical Nutrition suggested that a low‐carbohydrate diet may be preferred to a low‐fat diet for the induction of weight loss and glycaemic control in subjects with type 2 diabetes 5 . What is the rationale? What mechanisms are involved? Copyright © 2011 John Wiley & Sons, Ltd.     

The role of dietary fat in obesity

Current scientific evidence indicates that dietary fat plays a role in weight loss and maintenance. Meta-analyses of intervention trials find that fat-reduced diets cause a 3-4-kg larger weight loss than normal-fat diets. A 10% reduction in dietary fat can cause a 4-5-kg weight loss in individuals with initial body mass index of 30 kg m (-2). Short-term trials show that nonfat dietary components are equally important. Sugar-sweetened beverages promote weight gain, and replacement of energy from fat by sugar-sweetened beverages is counterproductive in diets aimed at weight loss. Protein has been shown to be more satiating than carbohydrate, and fat-reduced diets with a high protein content (20-25% of energy) may increase weight loss significantly. There is little evidence that low-glycemic index foods facilitate weight control. Evidence linking certain fatty acids to body fatness is weak. Monounsaturated fatty acids may even be more fattening than polyunsaturated and saturated fats. No ad libitum dietary intervention study has shown that a normal-fat, high-monounsaturated fatty acid diet is comparable to a low-fat diet in preventing weight gain. Current evidence indicates that the best diet for prevention of weight gain, obesity, type 2 diabetes, and cardiovascular disease is low in fat and sugar-rich beverages and high in carbohydrates, fiber, grains, and protein.     

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.     

High-protein diets and obesity

An increased protein intake is one of the most common approaches to the dietary management of obesity. The authors analyze the issues related to protein requirement in normal-weight and obese subjects, to the use and to the usefulness of high-protein diets in the treatment of obesity. Caution with these diets is recommended in view of their only slight effect on weight and fat loss and owing to the scarce evidence of significant effects on satiety and energy intake. Furthermore, the risks of harmful outcomes may be correlated to an excessive protein intake. Moreover, these diets do not allow patients to adopt those nutritional behavior rules which are essential to maintain the weight and fat loss and, consequently, significantly reduce the cardiovascular and metabolic risks related to obesity.     

Fad Diets in the Treatment of Diabetes

Use of the term “fad diet” reflects the contentious nature of the debate in the treatment of diabetes and generally targets diets based on carbohydrate restriction, the major challenge to traditional dietary therapy. Although standard low-fat diets more accurately conform to the idea of a practice supported by social pressure rather than scientific data, it is suggested that we might want to give up altogether unscientific terms like “fad” and “healthy.” Far from faddish, diets based on carbohydrate restriction have been the historical treatment for diabetes and are still supported by basic biochemistry, and it is argued that they should be considered the “default” diet, the one to try first, in diseases of carbohydrate intolerance or insulin resistance. The barrier to acceptance of low-carbohydrate diets in the past has been concern about saturated fat, which might be substituted for the carbohydrate that is removed. However, recent re-analysis of much old data shows that replacing carbohydrate with saturated fat is, if anything, beneficial. The dialectic of impact of continued hemoglobin A_1c versus effect of dietary saturated fat in the risk of cardiovascular disease is resolved in direction of glycemic control. Putting biased language behind us and facing the impact of recent results that point to the value of low-carbohydrate diets would offer patients the maximum number of options.     

Low-carbohydrate diets: nutritional and physiological aspects

Recently, diets low in carbohydrate content have become a matter of international attention because of the WHO recommendations to reduce the overall consumption of sugars and rapidly digestible starches. One of the common metabolic changes assumed to take place when a person follows a low-carbohydrate diet is ketosis. Low-carbohydrate intakes result in a reduction of the circulating insulin level, which promotes high level of circulating fatty acids, used for oxidation and production of ketone bodies. It is assumed that when carbohydrate availability is reduced in short term to a significant amount, the body will be stimulated to maximize fat oxidation for energy needs. The currently available scientific literature shows that low-carbohydrate diets acutely induce a number of favourable effects, such as a rapid weight loss, decrease of fasting glucose and insulin levels, reduction of circulating triglyceride levels and improvement of blood pressure. On the other hand some less desirable immediate effects such as enhanced lean body mass loss, increased urinary calcium loss, increased plasma homocysteine levels, increased low-density lipoprotein-cholesterol have been reported. The long-term effect of the combination of these changes is at present not known. The role of prolonged elevated fat consumption along with low-carbohydrate diets should be addressed. However, these undesirable effects may be counteracted with consumption of a low-carbohydrate, high-protein, low-fat diet, because this type of diet has been shown to induce favourable effects on feelings of satiety and hunger, help preserve lean body mass, effectively reduce fat mass and beneficially impact on insulin sensitivity and on blood lipid status while supplying sufficient calcium for bone mass maintenance. The latter findings support the need to do more research on this type of hypocaloric low-carbohydrate diet.     

Low-carbohydrate diets,obesity, and metabolic risk factors for cardiovascular disease

Given the increased prevalence of obesity in the United States (and its associated cardiovascular risk) despite reduced fat intake, there has been increasing interest in the effect of low-carbohydrate diets on obesity. Recent prospective trials have demonstrated equivalent weight loss on low-carbohydrate versus low-fat diets, but with significantly different effects on metabolic risk factors for cardiovascular disease. Low-carbohydrate diets have more favorable effects on metabolic abnormalities found in insulin resistance syndromes, including serum triglyceride levels, high-density lipoprotein cholesterol levels, and small, dense low-density lipoprotein particles. The translation of these different metabolic effects on cardiovascular disease and events requires future studies. These studies should take into consideration that patients with insulin resistance syndromes would be the most likely group to benefit from carbohydrate restriction.     

Protective effect of dietary monounsaturated fat on arteriosclerosis: beyond cholesterol

The consumption of diets enriched in monounsaturated fat has been related to a lower rate of coronary heart disease. It is well known that this dietary model decreases LDL-cholesterol plasma levels when replacing a saturated fat enriched diet. For this reason, a high monounsaturated fat diet is now being advocated to prevent cardiovascular disease, especially in Mediterranean countries. However, some expert panels-the Joint Task Force of European and other Societies on Coronary Prevention and the International Task Force for Prevention of Coronary Heart Disease-recommend replacing dietary saturated fat by complex carbohydrates, limiting the intake of total fat to <30% of the energy and monounsaturated fat to no more than 10-15% of total calories, reaching a similar effect on LDL-cholesterol plasma levels to a high monounsaturated fat diet. The most appropriate nutritional model to prevent arteriosclerosis should be supported by research into other biological effects of both diets. Therefore, it is interesting to review the non-lipid effect of monounsaturated fat, starting with its influence on other cardiovascular risk factors, such as carbohydrate metabolism and blood pressure. Moreover, substantial evidence of the effect of dietary monounsaturated fat on a wide range of healthy benefits beyond cholesterol, which have been investigated in recent years, such as lipoprotein oxidation, coagulation, fibrinolysis and endothelium, will be discussed. Furthermore, many observational epidemiological studies suggest that a high intake of monounsaturated fat is associated with reduced coronary risk and this will be analyzed in accordance with the clinical evidence to discuss the best dietary model to prevent coronary artery disease.     

Beneficial effect of low carbohydrate in low calorie diets on visceral fat reduction in type 2 diabetic patients with obesity

The adequate composition of carbohydrate and fat in low calorie diets for type 2 diabetes mellitus patients with obesity is not fully established. The aim of this study was to investigate the effects of low carbohydrate diet on glucose and lipid metabolism, especially on visceral fat accumulation, and comparing that of a high carbohydrate diet. Obese subjects with type 2 diabetes mellitus were randomly assigned to take a low calorie and low carbohydrate diet (n = 11, 1000 kcal per day, protein:carbohydrate:fat = 25:40:35) or a low calorie and high carbohydrate diet (n = 11, 1000 kcal per day, protein:carbohydrate:fat = 25:65:10) for 4 weeks. Similar decreases in body weight and serum glucose levels were observed in both groups. Fasting serum insulin levels were reduced in the low carbohydrate diet group compared to the high carbohydrate diet group (-30% versus -10%, P < 0.05). Total serum cholesterol and triglyceride levels decreased in both groups, but were not significantly different from each other. High-density lipoprotein-cholesterol (HDL-C) increased in the low carbohydrate diet group but not in the high carbohydrate diet group (+15% versus 0%, P < 0.01). There was a larger decrease in visceral fat area measured by computed tomography in the low carbohydrate diet group compared to the high carbohydrate diet group (-40 cm(2) versus -10 cm(2), P < 0.05). The ratio of visceral fat area to subcutaneous fat area did not change in the high carbohydrate diet group (from 0.70 to 0.68), but it decreased significantly in the low carbohydrate diet group (from 0.69 to 0.47, P < 0.005). These results suggest that, when restrict diet was made isocaloric, a low calorie/low carbohydrate diet might be more effective treatment for a reduction of visceral fat, improved insulin sensitivity and increased in HDL-C levels than low calorie/high carbohydrate diet in obese subjects with type 2 diabetes mellitus.     

Diabetes und Ernährung

Nutrition therapy is an important part of the therapy of diabetes mellitus because diet is directly related to cardiovascular diseases and complications. Older dietary recommendations give exact percentages for nutrients but these have since been withdrawn and the emphasis is now placed on healthy dietary patterns. The recommendations for carbohydrate intake are currently 30–60?% of the total energy; however, these figures do not have a strong evidential basis. Carbohydrates with a low glycemic index show a moderate improvement in HbA_1c by 0.1–0.5?%. Fructose is not useful as a sugar substitute for diabetics but according to current knowledge moderate amounts are harmless. The fat intake is currently recommended at 20 % to over 40?% of daily total energy. The Mediterranean diet with a fat content of 40?% containing mainly olive oil and plant foods reduces the risk of cardiovascular disease and lowers the HbA_1c by 0.3?%. In accordance with current knowledge, a high protein diet is recommended by some experts but scientific evidence is still lacking. Studies have shown a reduction of HbA_1c by 0.52?%, a moderate reduction of weight and blood pressure under a high protein diet without evidence of an accelerated drop in kidney function but further investigations are necessary.     

A randomized trial comparing a very low carbohydrate diet and a calorie-restricted low fat diet on body weight and cardiovascular risk factors in healthy women

Untested alternative weight loss diets, such as very low carbohydrate diets, have unsubstantiated efficacy and the potential to adversely affect cardiovascular risk factors. Therefore, we designed a randomized, controlled trial to determine the effects of a very low carbohydrate diet on body composition and cardiovascular risk factors. Subjects were randomized to 6 months of either an ad libitum very low carbohydrate diet or a calorie-restricted diet with 30% of the calories as fat. Anthropometric and metabolic measures were assessed at baseline, 3 months, and 6 months. Fifty-three healthy, obese female volunteers (mean body mass index, 33.6 +/- 0.3 kg/m(2)) were randomized; 42 (79%) completed the trial. Women on both diets reduced calorie consumption by comparable amounts at 3 and 6 months. The very low carbohydrate diet group lost more weight (8.5 +/- 1.0 vs. 3.9 +/- 1.0 kg; P < 0.001) and more body fat (4.8 +/- 0.67 vs. 2.0 +/- 0.75 kg; P < 0.01) than the low fat diet group. Mean levels of blood pressure, lipids, fasting glucose, and insulin were within normal ranges in both groups at baseline. Although all of these parameters improved over the course of the study, there were no differences observed between the two diet groups at 3 or 6 months. beta- Hydroxybutyrate increased significantly in the very low carbohydrate group at 3 months (P = 0.001). Based on these data, a very low carbohydrate diet is more effective than a low fat diet for short-term weight loss and, over 6 months, is not associated with deleterious effects on important cardiovascular risk factors in healthy women.     

Effect of very high-fat diets on body weight,lipoproteins, and glycemic status in the obese

Given the increased prevalence of obesity in the United States, despite reduced fat intake, there has been increasing interest in the effect of dietary fat on body weight, lipoproteins, and glycemic status. Despite predictions from epidemiologic and physiologic studies, recent prospective trials have demonstrated equivalent weight loss on high-fat versus low-fat diets. Nevertheless, the type of dietary fat consumed has substantially different effects on lipoproteins. Saturated fat raises high-density lipoprotein cholesterol but has unfavorable effects on total cholesterol, and has been associated with increased cardiovascular events. In contrast, unsaturated fats, and particularly omega-3 fatty acids, have the combined benefits of lowering serum cholesterol and raising high-density lipoprotein, as well as favorable effects on insulin resistance and inflammation; they also lower cardiovascular events in high-risk patients. Although current national guidelines modestly liberalize unsaturated fat consumption, important questions still remain about the optimal percentage of unsaturated fats in the diet.     

Clinical approaches to healthier diet modifications.

There is a strong relation between diet and risk factors for cardiovascular diseases. Generally a high fat consumption will increase the serum cholesterol level. Different fatty acids have different effects on total cholesterol, LDL and HDL lipoproteins and therefore on the risk for cardiovascular diseases. This knowledge was translated into guidelines for a healthy diet and in advices to use less fat. In reality it was not easy for physicians to reach a substantial reduction in total cholesterol by limiting the fat consumption. A recent investigation in Belgium showed that the median reduction obtained by dietary changes through general practitioners was only 3.5%. In medical education not much attention is paid to the communication of the "fat message". In addition, individual patients have frequently multiple risk factors e.g. abdominal obesity, insulin resistance, diabetes, hypertension and a sedentary lifestyle. Therefore in the clinical setting an integrated approach is needed, in which dietary advice is supported by behavioural therapy and suggestions to increase physical activity. Currently there is a debate on the preference of a low fat or a modified fat diet. A low fat diet is rich in carbohydrates and a modified fat diet is rich in mono-unsaturated fatty acids. Recent investigations in diabetic patients are in favour of a modified fat diet. This has the advantage that in practice the possibilities for realizing a low saturated fat diet are increased.     

Randomized comparison of reduced fat and reduced carbohydrate hypocaloric diets on intrahepatic fat in overweight and obese human subjects

Obesity-related hepatic steatosis is a major risk factor for metabolic and cardiovascular disease. Fat reduced hypocaloric diets are able to relieve the liver from ectopically stored lipids. We hypothesized that the widely used low carbohydrate hypocaloric diets are similarly effective in this regard. A total of 170 overweight and obese, otherwise healthy subjects were randomized to either reduced carbohydrate (n = 84) or reduced fat (n = 86), total energy restricted diet (-30% of energy intake before diet) for 6 months. Body composition was estimated by bioimpedance analyses and abdominal fat distribution by magnetic resonance tomography. Subjects were also submitted to fat spectroscopy of liver and oral glucose tolerance testing. In all, 102 subjects completed the diet intervention with measurements of intrahepatic lipid content. Both hypocaloric diets decreased body weight, total body fat, visceral fat, and intrahepatic lipid content. Subjects with high baseline intrahepatic lipids (>5.56%) lost ≈7-fold more intrahepatic lipids compared with those with low baseline values (<5.56%) irrespective of diet composition. In contrast, changes in visceral fat mass and insulin sensitivity were similar between subgroups, with low and high baseline intrahepatic lipids. CONCLUSION: A prolonged hypocaloric diet low in carbohydrates and high in fat has the same beneficial effects on intrahepatic lipid accumulation as the traditional low-fat hypocaloric diet. The decrease in intrahepatic lipids appears to be independent of visceral fat loss and is not tightly coupled with changes in whole body insulin sensitivity during 6 months of an energy restricted diet.