Dietary protein and weight reduction: a statement for healthcare professionals from the Nutrition Committee of the Council on Nutrition, Physical Activity, and Metabolism of the American Heart Association

High-protein diets have recently been proposed as a "new" strategy for successful weight loss. However, variations of these diets have been popular since the 1960s. High-protein diets typically offer wide latitude in protein food choices, are restrictive in other food choices (mainly carbohydrates), and provide structured eating plans. They also often promote misconceptions about carbohydrates, insulin resistance, ketosis, and fat burning as mechanisms of action for weight loss. Although these diets may not be harmful for most healthy people for a short period of time, there are no long-term scientific studies to support their overall efficacy and safety. These diets are generally associated with higher intakes of total fat, saturated fat, and cholesterol because the protein is provided mainly by animal sources. In high-protein diets, weight loss is initially high due to fluid loss related to reduced carbohydrate intake, overall caloric restriction, and ketosis-induced appetite suppression. Beneficial effects on blood lipids and insulin resistance are due to the weight loss, not to the change in caloric composition. Promoters of high-protein diets promise successful results by encouraging high-protein food choices that are usually restricted in other diets, thus providing initial palatability, an attractive alternative to other weight-reduction diets that have not worked for a variety of reasons for most individuals. High-protein diets are not recommended because they restrict healthful foods that provide essential nutrients and do not provide the variety of foods needed to adequately meet nutritional needs. Individuals who follow these diets are therefore at risk for compromised vitamin and mineral intake, as well as potential cardiac, renal, bone, and liver abnormalities overall.     

Effect of low fat-high carbohydrate diets in hypertensive patients with non-insulin-dependent diabetes mellitus

Effects of variations in dietary fat and carbohydrate content on various aspects of glucose, insulin, and lipoprotein metabolism were evaluated in 11 patients with hypertension, who also had non-insulin-dependent diabetes mellitus (NIDDM). All of these patients were being treated with sulfonylureas, thiazides, and beta-adrenergic receptor antagonists. The comparison diets contained either 40 or 60% of total calories as carbohydrate, with reciprocal changes in fat content from 40 to 20%. The diets were consumed in a random order for 15 days in a crossover experimental design. The ratio of polyunsaturated to saturated fat and total cholesterol intake were held constant in the two diets. Plasma glucose and insulin concentrations were significantly (P less than .001) elevated throughout the day when patients consumed the 60% carbohydrate diet. Fasting plasma total and very-low-density lipoprotein (VLDL) and triglyceride (TG) concentrations increased by 30% (P less than .001) after 15 days on the 60% carbohydrate diet. Total plasma cholesterol concentrations were similar on both diets, as were low-density lipoprotein (LDL) and high-density lipoprotein (HDL) cholesterol concentrations.     

Diet, insulin resistance, and obesity: zoning in on data for Atkins dieters living in South Beach

Insulin resistance is a central pathogenic factor for the metabolic syndrome and is associated with both generalized obesity and the accumulation of fat in the omental and intramyocellular compartments. In the context of the current obesity epidemic, it is imperative to consider diets in terms of their ability to both promote weight loss and ameliorate insulin resistance. Weight loss under any dietary formulation depends on hypocaloric intake, and only moderate weight loss (5-10%) is sufficient to augment insulin sensitivity. However, increments in insulin sensitivity may be more directly related to loss of intramyocellular or omental fat rather than loss of total body weight per se. The widespread acceptance of popular low-carbohydrate high-fat diets (e.g. Atkins Diet, Zone Diet, South Beach diet) further underscores the need to evaluate dietary interventions regarding their safety and metabolic effects. These high-fat diets have been shown to be safe in the short term; however, their long-term safety has not been established. With respect to insulin sensitivity, diets enriched in saturated fats can induce insulin resistance, whereas fat substitution with monounsaturated fats can enhance insulin sensitivity. On the other hand, high-fiber, high-carbohydrate diets comprised of foods with low caloric density can similarly be used for effective weight reduction and to ameliorate insulin resistance. Although some data suggest that low-glycemic index diets are most advantageous in this regard, these effects may have more to do with increments in dietary fiber than differences in available carbohydrates. Popular low-carbohydrate, high-fat diets are being fervently embraced as an alternative to challenging modifications in lifestyle and intentional calorie reduction. Current data do not support such unbridled enthusiasm for these diets, particularly in relationship to high-fiber, high-carbohydrate diets emphasizing intake of fresh vegetables and fruits. Long-term studies to determine the efficacy and safety of both popular and experimental diets are warranted.     

Comparison of the effects on insulin sensitivity of high carbohydrate and high fat diets in normal subjects.

To examine whether achievable dietary changes influence insulin sensitivity, we performed euglycemic hyperinsulinemic glucose clamps in eight normal subjects who were prescribed high carbohydrate and high fat diets. The high carbohydrate diet was more than 50% (of energy intake) carbohydrate and less than 30% fat; the high fat diet was more than 45% fat (predominantly saturated) and less than 40% carbohydrate. The diets were consumed over consecutive 3-week periods in random sequence. The mean whole body glucose uptake during the glucose clamps was similar after the high carbohydrate (48.3 mumol/kg.min) and high fat diets (47.0 mumol/kg.min; P = 0.5; 95% confidence interval for the difference, -3.4 to 5.9 mumol/kg.min). Fasting blood glucose and serum insulin concentrations were also unchanged. In contrast, there were substantial effects on lipoprotein metabolism. During the high carbohydrate diet, fasting serum cholesterol decreased by 17% (P = 0.06), low density lipoprotein cholesterol decreased by 20% (P = 0.05), high density lipoprotein cholesterol decreased by 24% (P less than 0.005), and triglyceride increased by 33% (P = 0.06) compared with levels during the high fat diet. These results suggest that practically achievable high carbohydrate diets do not enhance insulin sensitivity in nondiabetic subjects and have net effects on lipoprotein metabolism that may be unfavorable.     

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.     

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.     

Perilipin polymorphism interacts with saturated fat and carbohydrates to modulate insulin resistance

Background and aimsMacronutrient intakes and genetic variants have been shown to interact to alter insulin resistance, but replications of gene–nutrient interactions across independent populations are rare, despite their critical importance in establishing credibility. We aimed to investigate a previously demonstrated saturated fat and carbohydrate interaction for insulin resistance for perilipin (PLIN1), a regulator of adipocyte metabolism.Methods and resultsWe investigated the previously shown interaction for PLIN1 11482G > A (rs894160) on insulin resistance in US men (n = 462) and women (n = 508) (mean ± SD, 49 ± 16 years). In multivariable linear regression models, we found an interaction (P < 0.05) between the ratio of saturated fat to carbohydrate intake as a continuous variable and PLIN1 11482G > A for HOMA-IR (homeostasis model assessment of insulin resistance) in women. For carriers of the minor allele but not for non-carriers, as the ratio of saturated fat to carbohydrate intake increased, predicted HOMA-IR increased (P = 0.002). By dichotomizing the ratio of saturated fat to carbohydrate intake into high and low, we found significant interaction terms for insulin and HOMA-IR (P < 0.05). When the ratio of saturated fat to carbohydrate was high, insulin and HOMA-IR were higher in minor allele carriers (P = 0.004 and P = 0.003, respectively), but did not differ when the ratio was low. Similar patterns or trends were observed when saturated fat and carbohydrate were dichotomized into high and low as individual macronutrients.ConclusionsReplication of the previously reported interaction between macronutrient intakes and PLIN1 genotype for insulin resistance reinforces the potential usefulness of applying genotype information in the dietary management of insulin resistance.     

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.     

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.     

A randomised controlled trial of the effect of low fat diet advice on dietary response in insulin independent diabetic women

Summary Type 2 (insulin independent) diabetic women were randomly allocated to receive advice for low fat diets or low carbohydrate diets. By 24 h weighed dietary intakes before and after a mean interval of six months, patients in the low fat group had reduced their fat intake from 41% to 31% of total energy, while carbohydrate percentage of total energy intake increased from 38% to 46%. Percentage energy intake from fat and carbohydrate in the control group remained unchanged. Body weight fell in both groups, especially for patients in the low fat group who were obese (weight/height2 28 kg/m²). Mean plasma glucose, HbA₁, and triglycerides were unchanged. Mean plasma total cholesterol fell significantly in the low fat group compared with the controls (p < 0.001), but there was no significant difference in the small reduction of high density lipoprotein cholesterol observed in both groups. Thus, adherence to low fat diets occurred without deterioration of diabetes and with benefit for weight and total cholesterol.     

Effects of dietary carbohydrates on metabolism of calcium and other minerals in normal subjects and patients with noninsulin-dependent diabetes mellitus

Transient hypercalciuria has been noted after high carbohydrate meals which is independent of dietary calcium and is probably due to impaired renal calcium reabsorption mediated by an increase in plasma insulin levels. Based on these observations, some investigators believe that long term intake of high carbohydrate diets may increase the risk of nephrolithiasis and possibly osteoporosis. Using a randomized cross-over design, we compared high carbohydrate diets (60% carbohydrate and 25% fat) with high fat diets (50% fat and 35% carbohydrate) for effects on metabolism of calcium and other minerals in eight normal subjects and eight euglycemic patients with noninsulin-dependent diabetes mellitus. All other dietary constituents, such as protein, fiber, fluid, minerals (including Ca, Mg, Na, K, and P), and caffeine intake, were kept constant. Despite higher daylong levels of plasma insulin on the high carbohydrate diets compared to the high fat diet in both normal and noninsulin-dependent diabetic subjects, no changes in daily urinary excretion of calcium or other constituents, associated with renal stone risk, were observed. Furthermore, there was no change in fractional intestinal 47Ca absorption. Although hypercalciuria may ensue transiently after high carbohydrate meals, we conclude that substitution of simple or complex carbohydrates for fats in an isocaloric manner for a longer duration does not result in significant urinary calcium loss, and therefore, high intakes of digestible carbohydrates may not increase the risk of nephrolithiasis or osteoporosis via this mechanism.     

Medium‐chain fatty acids ameliorate insulin resistance caused by high‐fat diets in rats

Background High dietary intake of saturated fat impairs insulin sensitivity and lipid metabolism. The influence of fatty acid chain length, however, is not yet fully understood, but evidence exists for different effects of saturated long‐chain (LC) versus saturated medium‐chain (MC) fatty acids (FA). Methods To investigate the effects of the FA chain length, male Wistar rats were fed high‐fat diets containing triacylglycerols composed of either MC‐ or LCFA for 4 weeks; rats fed maintenance diet served as a control. The animals underwent euglycemic hyperinsulinemic clamping or oral metabolic tolerance testing respectively; enzyme activities of mitochondrial (EC2.3.1.21 carnitine palmitoyl transferase) and peroxisomal (EC1.3.3.6 acyl‐CoA oxidase) FA oxidation were measured in liver and muscle. Results LCFA consumption resulted in higher fasted serum insulin and glucose concentrations compared to controls, while MCFA‐fed animals did not differ from controls. Insulin sensitivity was reduced by 30% in the LCFA group while the MCFA group did not differ from controls. Feeding MCFA resulted in the controls' lowered fasted and post‐prandial triacylglycerol concentration compared to LCFA, while triacylglycerol concentrations in muscle were higher in both high‐fat groups compared to controls. No diet‐induced changes were found in acyl‐CoA oxidase (ACO) activity (liver and muscle), while LCFA feeding significantly raised carnitine palmitoyltransferase activity. Conclusions The chain length of saturated fatty acids in isocaloric diets affects insulin sensitivity, lipid metabolism and mitochondrial fatty acid oxidation without influencing body weight. While dietary LCFA clearly impair insulin sensitivity and lipid metabolism, MCFA seem to protect from lipotoxicity and subsequent insulin resistance without caloric restriction. Copyright © 2009 John Wiley & Sons, Ltd.     

Diet and risk of Type II diabetes: the role of types of fat and carbohydrate

Although diet and nutrition are widely believed to play an important part in the development of Type II (non-insulin-dependent) diabetes mellitus, specific dietary factors have not been clearly defined. Much controversy exists about the relations between the amount and types of dietary fat and carbohydrate and the risk of diabetes. In this article, we review in detail the current evidence regarding the associations between different types of fats and carbohydrates and insulin resistance and Type II diabetes. Our findings indicate that a higher intake of polyunsaturated fat and possibly long-chain n-3 fatty acids could be beneficial, whereas a higher intake of saturated fat and trans-fat could adversely affect glucose metabolism and insulin resistance. In dietary practice, exchanging nonhydrogenated polyunsaturated fat for saturated and trans-fatty acids could appreciably reduce risk of Type II diabetes. In addition, a low-glycaemic index diet with a higher amount of fiber and minimally processed whole grain products reduces glycaemic and insulinaemic responses and lowers the risk of Type II diabetes. Dietary recommendations to prevent Type II diabetes should focus more on the quality of fat and carbohydrate in the diet than quantity alone, in addition to balancing total energy intake with expenditure to avoid overweight and obesity. [Diabetologia (2001) 44: 805–817]     

The long-term effect of low-carbohydrate/high-fat diet on the development of diabetes mellitus in spontaneously diabetic rats

The long-term effect of low-carbohydrate/high-fat diets on the development of diabetes mellitus was studied in Otsuka Long-Evans Tokushima Fatty strain (OLETF) rats. Four groups of spontaneously diabetic (type 2) male rats at 10 weeks of age were pair-fed semi-purified powder diets containing different amounts of carbohydrate (80%, 60%, 40%, 20% of total calories) for 30 weeks. The carbohydrate content was isocalorically substituted for the fat content in the diet. At the onset of experimental feeding (10 weeks of age), an oral glucose tolerance test (OGTT) was normal in each group. After 15 weeks of the test diet feeding there was no significant difference in the glucose tolerance among the 4 groups, although most of the rats were diabetic. The body weight increased with the decrease of the carbohydrate intake and increase of the fat intake (p <0.05), and the difference increased in proportion to age (p<0.05). The severity of diabetes mellitus was also increased along with the lower carbohydrate intake and higher fat intake, when the carbohydrate intake was less than 60% (in energy). On the other hand, there was a significant increase in the 20% group in the postload plasma insulin levels as compared with the other 3 groups at 40 weeks of age. Fasting plasma free fatty acid levels were increased in the lower carbohydrate content groups (20% and 40%) as compared with the higher carbohydrate content groups (60% and 80%) at the end of the experiment. Impairment of insulin secretion may be the cause of glucose intolerance induced by low carbohydrate intake rather than insulin resistance. These findings suggest that low-carbohydrate/high-fat diet aggravates diabetes mellitus in genetically diabetic rats, and that the development of diabetes mellitus is associated with the activation of the glucose-fatty acid cycle.     

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.     

Relationship of dietary fat and serum cholesterol ester and phospholipid fatty acids to markers of insulin resistance in men and women with a range of glucose tolerance

High-fat diets are associated with insulin resistance, however, this effect may vary depending on the type of fat consumed. The purpose of this study was to determine the relationship between intakes of specific dietary fatty acids (assessed by 3-day diet records and fatty acid composition of serum cholesterol esters [CEs] and phospholipids [PLs]) and glucose and insulin concentrations during an oral glucose tolerance test (OGTT). Nineteen men and 19 women completed the study. Nine subjects had type 2 diabetes or impaired glucose tolerance. Fasting insulin correlated with reported intakes of total fat (r = .50, P < .01), monounsaturated fat (r = .44, P < .01), and saturated fat (r = .49, P < .01), but not with trans fatty acid intake (r = .11, not significant [NS]). Fasting glucose also correlated with total (r = .39, P < .05) and monounsaturated fat intakes (r = .37, P < .05). In multivariate analysis, both total and saturated fat intake were strong single predictors of fasting insulin (R2 approximately .25), and a model combining dietary and anthropometric measures accounted for 47% of the variance in fasting insulin. Significant relationships were observed between fasting insulin and the serum CE enrichments of myristic (C14:0), palmitoleic (C16:1), and dihomo-gamma-linolenic (C20:3n-6) acids. In multivariate analysis, a model containing CE 14:0 and percent body fat explained 45% of the variance in fasting insulin, and C14:0 and age explained 30% of the variance in fasting glucose. PL C20:3n-6 explained 30% of the variance in fasting insulin, and a model including PL C18:1n-11 cis, C20:3n-6, age and body fat had an R2 of .58. In conclusion, self-reported intake of saturated and monounsaturated fats, but not trans fatty acids, are associated with markers of insulin resistance. Furthermore, enhancement of dihomo-gamma-linolenic and myristic acids in serum CE and PL, presumably markers for dietary intake, predicted insulin resistance.     

The pediatric obesity epidemic: causes and controversies

Obesity in children and adolescents has reached alarming proportions in the United States. Nutritional surveys do not indicate a significant increase in caloric intake in children and adolescents over the last 3 decades, although caloric intake has increased recently in adolescent females. Dietary fat has also been falling. There is no conclusive evidence linking physical inactivity to the obesity epidemic, and longitudinal studies indicate that physical inactivity may be the result of obesity rather than its cause. Hence, attention should be focused on dietary carbohydrate. Carbohydrate intake has increased as a result of the decrease in dietary fat. Indirect evidence also indicates that the quality of carbohydrate has been changing, so that American children are eating more carbohydrates with a higher glycemic index. It is proposed that high-glycemic-index diets lead to excessive weight gain as a consequence of postprandial hyperinsulinemia. Low-glycemic-index diets lower postprandial insulin levels and insulin resistance. It seems likely that diets restricted in sweetened sodas and noncitrus juices and containing ample whole grains, vegetables, and fruit could have a major impact on the prevalence of pediatric obesity.     

Effect of a low fat diet on carbohydrate metabolism in patients with hypertension

Plasma glucose and insulin responses to both a 75-g oral glucose challenge and to conventional meals were determined in eight patients with hypertension and compared with values of a control population. The results indicated that patients with hypertension had significantly higher than normal plasma glucose and insulin concentrations in both situations. Furthermore, when dietary carbohydrate was increased by 16% of total calories (with a reciprocal reduction in dietary fat), the hyperglycemia and hyperinsulinemia present in patients with hypertension were accentuated. Since low fat-high carbohydrate diets are usually recommended for patients with hypertension, these data suggest that abnormalities of glucose and insulin metabolism associated with hypertension would be increased if patients with high blood pressure followed conventional dietary advice. Since hyperglycemia and hyperinsulinemia have been shown to be associated with an increased risk of developing coronary artery disease, it may be appropriate to reevaluate the clinical utility of low fat-high carbohydrate diets in the treatment of hypertension.     

Effect of dietary fats and carbohydrate on blood pressure of mildly hypertensive patients

The effect on blood pressure (BP) of replacing dietary saturated fat with either polyunsaturated fat (linoleic acid) or carbohydrate was studied in 21 untreated mildly hypertensive patients. In a randomized, double-blind, crossover protocol, all subjects received dietary supplements of cream, safflower oil, and carbohydrate in random sequence, each prepared in flavored yogurt or milk. Each supplement was administered for 6 weeks and followed by a 4-week washout period of no supplementation. Dietary linoleic acid increased from 4.6 to 13% of energy intake when the safflower oil replaced cream, while saturated fat decreased from 16 to 10%. Total fat intake was 37 to 38% during the cream and safflower oil periods but was 28% during the carbohydrate period. Compliance with the diets was demonstrated by significant changes in fasting plasma fatty acid measurements. Mean clinic BP was 135 +/- 9/93 +/- 6 mm Hg at baseline. There were no significant differences in BP measured in the clinic or at home among the three dietary periods. The protocol had more than 80% power to detect a mean effect of diet of 3 mm Hg systolic or 2 mm Hg diastolic BP. Therefore, replacing dietary saturated fat with carbohydrate or with linoleic acid does not affect BP in subjects with mild hypertension.