A randomized trial comparing low-fat and low-carbohydrate diets matched for energy and protein

Several recent studies have found greater weight loss at 6 months among participants on a very-low-carbohydrate (VLC) weight-loss diet compared with a low-fat (LF) weight-loss diet. Because most of these studies were not matched for calories, it is not clear whether these results are caused by decreased energy intake or increased energy expenditure. It is hypothesized that several energy-consuming metabolic pathways are up-regulated during a VLC diet, leading to increased energy expenditure. The focus of this study was to investigate whether, when protein and energy are held constant, there is a significant difference in fat and weight loss when fat and carbohydrate are dramatically varied in the diet. The preliminary results presented in this paper are for the first four of six postmenopausal overweight or obese participants who followed, in random order, both a VLC and an LF diet for 6 weeks. Other outcome measures were serum lipids, glucose, and insulin, as well as dietary compliance and side effects. Our results showed no significant weight loss, lipid, serum insulin, or glucose differences between the two diets. Lipids were dramatically reduced on both diets, with a trend for greater triglyceride reduction on the VLC diet. Glucose levels were also reduced on both diets, with a trend for insulin reduction on the VLC diet. Compliance was excellent with both diets, and side effects were mild, although participants reported more food cravings and bad breath on the VLC diet and more burping and flatulence on the LF diet.     

Ketogenic low-carbohydrate diets have no metabolic advantage over nonketogenic low-carbohydrate diets

BACKGROUND: Low-carbohydrate diets may promote greater weight loss than does the conventional low-fat, high-carbohydrate diet. OBJECTIVE: We compared weight loss and biomarker change in adults adhering to a ketogenic low-carbohydrate (KLC) diet or a nonketogenic low-carbohydrate (NLC) diet. DESIGN: Twenty adults [body mass index (in kg/m(2)): 34.4 +/- 1.0] were randomly assigned to the KLC (60% of energy as fat, beginning with approximately 5% of energy as carbohydrate) or NLC (30% of energy as fat; approximately 40% of energy as carbohydrate) diet. During the 6-wk trial, participants were sedentary, and 24-h intakes were strictly controlled. RESULTS: Mean (+/-SE) weight losses (6.3 +/- 0.6 and 7.2 +/- 0.8 kg in KLC and NLC dieters, respectively; P = 0.324) and fat losses (3.4 and 5.5 kg in KLC and NLC dieters, respectively; P = 0.111) did not differ significantly by group after 6 wk. Blood beta-hydroxybutyrate in the KLC dieters was 3.6 times that in the NLC dieters at week 2 (P = 0.018), and LDL cholesterol was directly correlated with blood beta-hydroxybutyrate (r = 0.297, P = 0.025). Overall, insulin sensitivity and resting energy expenditure increased and serum gamma-glutamyltransferase concentrations decreased in both diet groups during the 6-wk trial (P < 0.05). However, inflammatory risk (arachidonic acid:eicosapentaenoic acid ratios in plasma phospholipids) and perceptions of vigor were more adversely affected by the KLC than by the NLC diet. CONCLUSIONS: KLC and NLC diets were equally effective in reducing body weight and insulin resistance, but the KLC diet was associated with several adverse metabolic and emotional effects. The use of ketogenic diets for weight loss is not warranted.     

Effects of a high-protein ketogenic diet on hunger, appetite, and weight loss in obese men feeding ad libitum

BACKGROUND: Altering the macronutrient composition of the diet influences hunger and satiety. Studies have compared high- and low-protein diets, but there are few data on carbohydrate content and ketosis on motivation to eat and ad libitum intake. OBJECTIVE: We aimed to compare the hunger, appetite, and weight-loss responses to a high-protein, low-carbohydrate [(LC) ketogenic] and those to a high-protein, medium-carbohydrate [(MC) nonketogenic] diet in obese men feeding ad libitum. DESIGN: Seventeen obese men were studied in a residential trial; food was provided daily. Subjects were offered 2 high-protein (30% of energy) ad libitum diets, each for a 4-wk period-an LC (4% carbohydrate) ketogenic diet and an MC (35% carbohydrate) diet-randomized in a crossover design. Body weight was measured daily, and ketosis was monitored by analysis of plasma and urine samples. Hunger was assessed by using a computerized visual analogue system. RESULTS: Ad libitum energy intakes were lower with the LC diet than with the MC diet [P=0.02; SE of the difference (SED): 0.27] at 7.25 and 7.95 MJ/d, respectively. Over the 4-wk period, hunger was significantly lower (P=0.014; SED: 1.76) and weight loss was significantly greater (P=0.006; SED: 0.62) with the LC diet (6.34 kg) than with the MC diet (4.35 kg). The LC diet induced ketosis with mean 3-hydroxybutyrate concentrations of 1.52 mmol/L in plasma (P=0.036 from baseline; SED: 0.62) and 2.99 mmol/L in urine (P<0.001 from baseline; SED: 0.36). CONCLUSION: In the short term, high-protein, low-carbohydrate ketogenic diets reduce hunger and lower food intake significantly more than do high-protein, medium-carbohydrate nonketogenic diets.     

Is a calorie a calorie?

The aim of this review was to evaluate data regarding potential thermodynamic mechanisms for increased rates of weight loss in subjects consuming diets high in protein and/or low in carbohydrate. Studies that compared weight loss and energy expenditure in adults consuming diets high in protein and/or low in carbohydrate with those in adults consuming diets low in fat were reviewed. In addition, studies that measured the metabolizable energy of proteins, fats, and carbohydrates were reviewed. Diets high in protein and/or low in carbohydrate produced an approximately equal to 2.5-kg greater weight loss after 12 wk of treatment. Neither macronutrient-specific differences in the availability of dietary energy nor changes in energy expenditure could explain these differences in weight loss. Thermodynamics dictate that a calorie is a calorie regardless of the macronutrient composition of the diet. Further research on differences in the composition of weight loss and on the influence of satiety on compliance with energy-restricted diets is needed to explain the observed increase in weight loss with diets high in protein and/or low in carbohydrate.     

Glycemic Carbohydrate and Body Weight Regulation

The purpose of this review is to examine the relationship between glycemic carbohydrate and its effect on body weight regulation. By contrast to fat, carbohydrate has a positive impact on energy intake, energy expenditure, and body weight control. Despite some debate about the role of the carbohydrate-to-fat ratio in the diet and the prevalence of obesity, metabolic studies show that diets high in fat are more likely to result in body weight gain than diets high in carbohydrate. So far there are no indications that carbohydrate classes differ greatly with respect to energy expenditure and energy balance. However, the impact of carbohydrate source and class, as well as the form in which carbohydrate is consumed (i.e., solid or liquid) on body weight control requires further consideration.     

The cafeteria diet--an inappropriate tool for studies of thermogenesis

Diet-induced thermogenesis (DIT) is defined as a regulatory, facultative component of energy expenditure, stimulated by overeating, which helps maintain energy balance. DIT may play a central role in the regulation of energy expenditure and in the etiology of certain types of obesity. Most experiments testing the existence or the mechanisms of DIT have used the cafeteria diet for the purposes of stimulating hyperphagia, a requisite for studies of DIT. Yet such a diet is inappropriate for studies of thermogenesis because its use prevents researchers from obtaining an experimental outcome that can be clearly interpreted. The primary limitation of the cafeteria diet is that its nutritional composition is uncontrolled. The diet is self-selected from a variety of supermarket foods that tend to be high in fat and/or carbohydrate and low in protein, vitamins and minerals. Hence, the diets consumed by the animals are likely to be deficient in protein, vitamins or minerals. There is evidence that dietary deficiency of protein, vitamins and minerals can increase thermogenesis and in protein-adequate diets, the balance of fat and carbohydrate in the diet can also influence thermogenesis with high carbohydrate diets increasing thermogenesis more than isoenergetic high fat diets. Hence, an observed increase in thermogenesis in cafeteria fed animals might be interpreted incorrectly to be the result of increased energy consumption when it is attributable to dietary imbalance or deficiency. Because the diet is self-selected, it is possible for each animal to choose a diet that varies in nutritional composition from that selected by every other animal, so control of dietary intake is compromised.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of fat-reduced diets on 24-h energy expenditure: comparisons between animal protein, vegetable protein, and carbohydrate

BACKGROUND: Single-meal tests have shown that protein has greater thermogenic and satiating effects than does carbohydrate, which may be relevant for the prevention and treatment of obesity if these effects can be maintained over 24 h. OBJECTIVE: The effects of pork-meat protein, soy protein, and carbohydrate on 24-h energy expenditure were compared. DESIGN: Twelve young, healthy, overweight and mildly obese [body mass index (in kg/m(2)): 26-32] nonsmoking men participated in a randomized, single-blind, 3-way crossover study lasting 4 d. The intervention had a 1-10-wk washout period. The 3 isoenergetic intervention diets were as follows: pork diet (29% of energy as fat and 29% as protein, mainly from pork meat), soy diet (29% of energy as fat and 28% as protein, mainly from soy), and carbohydrate diet (28% of energy as fat and 11% as protein). Twenty-four-hour energy expenditure was measured in a respiratory chamber at baseline and on day 4 of each intervention period. RESULTS: Twenty-four-hour energy expenditure was higher with the pork than with the soy (248 kJ/d, 1.9%; P: = 0.05) or carbohydrate (492 kJ/d, 3.9%; P: < 0.0001) diet and higher with the soy than with the carbohydrate (244 kJ/d, 1.9%; P: < 0.05) diet. However, because of a higher satiating effect, energy intake was 10-15% lower during the chamber stay than at baseline (P: > 0.05) with all 3 diets. The differences in energy expenditure remained unchanged after adjustment for differences in 24-h energy balance. CONCLUSIONS: Substitution of carbohydrate with 17-18% of energy as either pork-meat or soy protein produced a 3% higher 24-h energy expenditure. The animal protein in pork meat produced a 2% higher 24-h energy expenditure than did the vegetable protein in soy.     

A ketogenic diet increases protein phosphorylation in brain slices of rats

Ketogenic diets have been used to treat seizure disorders of children and recently it was shown to increase the drug-induced seizure threshold in rats. Protein phosphorylation is a major regulatory mechanism of signal transduction that has been implicated in modulating neuronal excitability. We investigated the basal protein phosphorylation in microslices from different brain regions (hippocampus, cerebral cortex and cerebellum) of young rats fed a ketogenic diet, and we evaluated the effect of this diet on weight development and health of these rats based on serum biochemistry. Thirty-day-old rats consumed ad libitum ketogenic (high fat) or control diets for 8 wk. Rats consuming the high fat diet had ketonemia without signs of undernutrition or illness. Microslices were incubated in media containing (32)P-phosphate, and (32)P-phosphoprotein content was analyzed by one- or two-dimensional electrophoresis followed by autoradiography. Basal protein phosphorylation was greater in brain slices from ketogenic rats. Different increments of synapsin I, GAP-43 and GFAP phosphorylation were observed in two-dimensional autoradiography. A ketogenic diet induced metabolic changes affecting the basal status of protein phosphorylation. This change could affect the mechanisms of signal transduction in neural cells involved in the increase in the seizure threshold.     

Overweight and diabetes prevention: is a low-carbohydrate–high-fat diet recommendable?

In the past, different types of diet with a generally low-carbohydrate content (<?50–< 20 g/day) have been promoted, for weight loss and diabetes, and the effectiveness of a very low dietary carbohydrate content has always been a matter of debate. A significant reduction in the amount of carbohydrates in the diet is usually accompanied by an increase in the amount of fat and to a lesser extent, also protein. Accordingly, using the term “low carb–high fat” (LCHF) diet is most appropriate. Low/very low intakes of carbohydrate food sources may impact on overall diet quality and long-term effects of such drastic diet changes remain at present unknown. This narrative review highlights recent metabolic and clinical outcomes of studies as well as practical feasibility of low LCHF diets. A few relevant observations are as follows: (1) any diet type resulting in reduced energy intake will result in weight loss and related favorable metabolic and functional changes; (2) short-term LCHF studies show both favorable and less desirable effects; (3) sustained adherence to a ketogenic LCHF diet appears to be difficult. A non-ketogenic diet supplying 100–150 g carbohydrate/day, under good control, may be more practical. (4) There is lack of data supporting long-term efficacy, safety and health benefits of LCHF diets. Any recommendation should be judged in this light. (5) Lifestyle intervention in people at high risk of developing type 2 diabetes, while maintaining a relative carbohydrate-rich diet, results in long-term prevention of progression to type 2 diabetes and is generally seen as safe.     

Blood ketones are directly related to fatigue and perceived effort during exercise in overweight adults adhering to low-carbohydrate diets for weight loss: a pilot study

Ketogenic diets have been associated with reductions in free-living physical activity, a response that can be counterproductive in individuals trying to lose weight. To explore whether popular low-carbohydrate diets might impact the desire to exercise by raising blood ketone concentrations, fatigue and perceived effort during exercise were compared in untrained, overweight adults adhering to a ketogenic low-carbohydrate diet or to a control diet low in carbohydrate, but not ketogenic (5%, 65%, and 30% or 40%, 30%, and 30% of energy from carbohydrate, fat, and protein, respectively). In this prospective, randomized, 2-week pilot study, all meals and snacks were provided to subjects, and energy intake was strictly controlled to provide approximately 70% of that needed for weight maintenance. At baseline and at the end of week 2, exercise testing was conducted in fasting participants. Weight loss and the reductions in fat mass did not differ by group during the trial. At week 2, blood beta-hydroxybutyrate concentrations were 3.6-fold greater for the ketogenic vs nonketogenic group (P=0.018) and correlated significantly with perceived exercise effort (r2=0.22, P=0.049). Blood beta-hydroxybutyrate was also significantly correlated to feelings of "fatigue" (r=0.458, P=0.049) and to "total mood disturbance" (r=0.551, P=0.015) while exercising. These pilot data indicate that ketogenic, low-carbohydrate diets enhance fatigability and can reduce the desire to exercise in free-living individuals.     

Metabolic adaptation to high-fat and high-carbohydrate diets in children and adolescents

BACKGROUND: Difficulty adapting to high-fat (HF) and high carbohydrate (HC) diets may predispose children to obesity and diabetes. OBJECTIVE: We tested the hypothesis that children have metabolic flexibility to adapt to HF and HC diets. DESIGN: In protocol 1, 12 children aged 6-9 y and 12 adolescents aged 13-16 y were randomly assigned in a crossover design to consume low-fat (LF), HC (25% and 60% of energy, respectively) or HF, low-carbohydrate (LC) (55% and 30% of energy, respectively) diets. In protocol 2, 12 adolescents aged 13-16 y were randomly assigned in a crossover design to consume an LF-HC diet with 11% or 40% of carbohydrate as fructose. Total energy expenditure, nonprotein respiratory quotients (NPRQs), and substrate utilization were measured by using 24-h calorimetry. Effects of sex, puberty, body fat (dual-energy X-ray absorptiometry), intraabdominal fat (magnetic resonance imaging), and fitness on substrate utilization were tested. RESULTS: Substrate utilization was not affected by puberty, body fat, intraabdominal fat, or fitness. Total energy expenditure was not affected by diet. In protocol 1, NPRQs and carbohydrate and fat utilization were significantly affected by diet (P = 0.001) and sex (P = 0.005). NPRQs and carbohydrate utilization increased with the LF-HC diet. NPRQs decreased and fat utilization increased with the HF-LC diet; changes in substrate utilization were less pronounced in females than in males. In protocol 2, 24-h NPRQs and 24-h substrate utilization were not significantly affected by fructose, although net carbohydrate and fat utilization were significantly lower and higher, respectively, with the high-fructose diet during fasting (P = 0.01) and in the subsequent feeding period (P = 0.05). CONCLUSION: Healthy, nonobese children and adolescents adapt appropriately to HF and HC diets.     

Sugars and fats: the neurobiology of preference

The appetite for specific foods and nutrients may be under neuroregulatory control. In animal studies, fat intake is increased by both opioids and galanin and reduced by enterostatin, whereas carbohydrate intake is increased by neuropeptide Y (NPY). However, what may be affected is the consumption of preferred foods rather than macronutrients. Fat and sugars are highly preferred whether consumed separately or as mixtures in foods. Studies suggest that sustained consumption of sugars and fats may have additional metabolic consequences; among these are neurochemical changes in brain sites involved in feeding and reward, some of which are also affected by drugs of abuse. Furthermore, the consumption of fats and sugars alters tissue expression of uncoupling proteins, which are also influenced by neuroregulatory peptides and may be markers of energy expenditure. These data suggest that these palatable nutrients may influence energy expenditure through changes in central neuropeptide activity. Fats and sugars could affect central reward systems, thereby increasing food intake, and might have an additional effect on energy expenditure. Such palatable substances may contribute to the observed increase in the body weight of populations from affluent societies during the past few decades.     

Thermogenesis from the breakdown of a ketogenic diet in an experimental model using swine

A respiration calorimetry experiment with 10 matures sows was conducted to study the effect of a ketogenic diet on heat production and energy utilization. The ketogenic diet contained no carbohydrates and 85% of its energy as fat. The control diet was a mixed diet with most of its energy as carbohydrates. Both diets provided the same daily protein intake, which was slightly below the calculated protein requirement. The level of energy intake was adjusted to a moderate positive energy balance. Both diets were allocated to the same animal for three weeks each according to a crossover design. Seven-day collection periods and 48-hours measurements of the gaseous exchange (carbon-nitrogen balance method) were conducted on individual sows per diet. The ketogenic diet substantially increased the energy losses in faeces, which resulted in a energy digestibility of 75% compared to 93% with the mixed diet. The methane production was also significantly depressed. In contrast, the urine energy excretion was not influenced by the extremely high fat diet. The nitrogen balance showed lower faecal nitrogen losses and a higher urine nitrogen excretion in the animals with the high fat diet. Because of this compensative response no difference in nitrogen retention was observed between the two dietary treatments. The ketogenic diet caused no detrimental effects on thermogenesis or energy utilization. The results of both criteria could be fully explained by the well accepted ideas of the efficiency of utilization of the energy from fat under balanced nutrition conditions. Overall the results demonstrate that a high fat diet has no regulatory effect on the usually diet-induced thermogenesis.     

Mild experimental ketosis increases brain uptake of 11C-acetoacetate and 18F-fluorodeoxyglucose: a dual-tracer PET imaging study in rats

Brain glucose and ketone uptake was investigated in Fisher rats subjected to mild experimental ketonemia induced by a ketogenic diet (KD) or by 48 hours fasting (F). Two tracers were used, (11)C-acetoacetate ((11)C-AcAc) for ketones and (18)F-fluorodeoxyglucose for glucose, in a dual-tracer format for each animal. Thus, each animal was its own control, starting first on the normal diet, then undergoing 48 hours F, followed by 2 weeks on the KD. In separate rats on the same diet conditions, expression of the transporters of glucose and ketones (glucose transporter 1 (GLUT1) and monocarboxylic acid transporter (MCT1)) was measured in brain microvessel preparations. Compared to controls, uptake of (11)C-AcAc increased more than 2-fold while on the KD or after 48 hours F (P < 0.05). Similar trends were observed for (18)FDG uptake with a 1.9-2.6 times increase on the KD and F, respectively (P < 0.05). Compared to controls, MCT1 expression increased 2-fold on the KD (P < 0.05) but did not change during F. No significant difference was observed across groups for GLUT1 expression. Significant differences across the three groups were observed for plasma beta-hydroxybutyrate (beta-HB), AcAc, glucose, triglycerides, glycerol, and cholesterol (P < 0.05), but no significant differences were observed for free fatty acids, insulin, or lactate. Although the mechanism by which mild ketonemia increases brain glucose uptake remains unclear, the KD clearly increased both the blood-brain barrier expression of MCT1 and stimulated brain (11)C-AcAc uptake. The present dual-tracer positron emission tomography approach may be particularly interesting in neurodegenerative pathologies such as Alzheimer's disease where brain energy supply appears to decline critically.     

Prior carbohydrate consumption affects the amount of carbohydrate that rats choose to eat

Consumption of protein-rich, carbohydrate-restricted reducing diets has been associated anecdotally with an increased appetite for carbohydrate. We have tested the effect of such a diet on carbohydrate intake by rats. Rats were given either a calorie-restricted ketogenic diet containing protein and fat or a control diet containing carbohydrate along with the protein and fat. When allowed to choose from a pair of isocaloric, isonitrogenous diets containing 25 or 75% dextrin, ketotic rats ate significantly more carbohydrate and total food than control animals during the first 30 minutes of feeding, apparently requiring more of the carbohydrate to obtain an increase in brain tryptophan similar to controls. Ketotic rats ate a significantly higher proportion of total calories as carbohydrate. Similar results were obtained when sucrose replaced dextrin. When ketotic and control rats chose between two diets differing in proportions of fat or protein, no differences were observed between the groups in total food intake nor in the amounts or proportions of fat or protein eaten. We also compared the effects of a small, isocaloric premeal containing only carbohydrate (1.4 g dextrose) or mixed nutrients on subsequent carbohydrate consumption in otherwise untreated rats allowed to choose from 25 and 75% dextrin diets. Rats eating the carbohydrate premeal subsequently ate as much total food as the mixed-nutrient controls, but significantly less carbohydrate. These observations suggest that carbohydrate intake is influenced by prior nutrient consumption and that prolonged deprivation of carbohydrate can lead to overconsumption of this nutrient when it is reintroduced into the diet.     

Dietary-Induced Ketogenesis: Adults Are Not Children

There is increasing interest in the use of a ketogenic diet for various adult disorders; however, the ability of adults to generate ketones is unknown. Our goal was to challenge the hypothesis that there would be no difference between adults and children regarding their ability to enter ketosis. Methods: Two populations were studied, both treated with identical very low-carbohydrate high-fat diets: a retrospective series of children with epilepsy or/and metabolic disorders (2009–2016) and a prospective clinical trial of adults with glioblastoma. Dietary intake was assessed based upon written food diaries and 24-h dietary recall. Ketogenic ratio was calculated according to [grams of fat consumed]/[grams of carbohydrate and protein consumed]. Ketone levels (β-hydroxybutyrate) were measured in blood and/or urine. Results: A total of 168 encounters amongst 28 individuals were analyzed. Amongst both children and adults, ketone levels correlated with nutritional ketogenic ratio; however, the absolute ketone levels in adults were approximately one quarter of those seen in children. This difference was highly significant in a multivariate linear regression model, p < 0.0001. Conclusions: For diets with comparable ketogenic ratios, adults have lower blood ketone levels than children; consequently, high levels of nutritional ketosis are unobtainable in adults.     

The Effects of High Protein Diets on Thermogenesis,Satiety and Weight Loss: A Critical Review

For years, proponents of some fad diets have claimed that higher amounts of protein facilitate weight loss. Only in recent years have studies begun to examine the effects of high protein diets on energy expenditure, subsequent energy intake and weight loss as compared to lower protein diets. In this study, we conducted a systematic review of randomized investigations on the effects of high protein diets on dietary thermogenesis, satiety, body weight and fat loss. There is convincing evidence that a higher protein intake increases thermogenesis and satiety compared to diets of lower protein content. The weight of evidence also suggests that high protein meals lead to a reduced subsequent energy intake. Some evidence suggests that diets higher in protein result in an increased weight loss and fat loss as compared to diets lower in protein, but findings have not been consistent. In dietary practice, it may be beneficial to partially replace refined carbohydrate with protein sources that are low in saturated fat. Although recent evidence supports potential benefit, rigorous longer-term studies are needed to investigate the effects of high protein diets on weight loss and weight maintenance.     

Metabolic status in growing rats fed isocaloric diets with increased carbohydrate-to-fat ratio

OBJECTIVE: A low-fat diet is hypothesized to be associated with significant weight loss. However, most previous studies have been limited to low-fat, low-calorie restrictive diets. This study evaluated the effect of isocaloric diets given "ad libitum" but different in relative amounts of fat and carbohydrate on body size, energy metabolism, body composition, insulin-like growth factor-1, and leptin serum levels in growing Wistar rats. METHODS: Weanling male rats were fed with one of three diets that contained a ratio of carbohydrate to fat of 1:1, 2:1, or 3:1. Food intake, body weight, body length, oxygen consumption, and body composition were measured at ages 21 to 50 d. Serum levels of insulin-like growth factor-1 and leptin were also determined. RESULTS: Energy intake was similar across groups. The ratio of body weight to body length remained adequate throughout the experimental period. However, groups that received 3:1 and 2:1 showed increased weight and progressive decreases in energy expenditure, body fat composition, and serum level of leptin, but the ratio of insulin-like growth factor-1 to body length was not affected. CONCLUSIONS: Dietary substitution of fat with carbohydrates contributes to weight gain by decreasing energy expenditure and possibly by decreasing leptin secretion.     

The effects of high protein diets on thermogenesis, satiety and weight loss: a critical review

For years, proponents of some fad diets have claimed that higher amounts of protein facilitate weight loss. Only in recent years have studies begun to examine the effects of high protein diets on energy expenditure, subsequent energy intake and weight loss as compared to lower protein diets. In this study, we conducted a systematic review of randomized investigations on the effects of high protein diets on dietary thermogenesis, satiety, body weight and fat loss. There is convincing evidence that a higher protein intake increases thermogenesis and satiety compared to diets of lower protein content. The weight of evidence also suggests that high protein meals lead to a reduced subsequent energy intake. Some evidence suggests that diets higher in protein result in an increased weight loss and fat loss as compared to diets lower in protein, but findings have not been consistent. In dietary practice, it may be beneficial to partially replace refined carbohydrate with protein sources that are low in saturated fat. Although recent evidence supports potential benefit, rigorous longer-term studies are needed to investigate the effects of high protein diets on weight loss and weight maintenance.     

A high-protein diet induces sustained reductions in appetite, ad libitum caloric intake, and body weight despite compensatory changes in diurnal plasma leptin and ghrelin concentrations

BACKGROUND: Ad libitum, low-carbohydrate diets decrease caloric intake and cause weight loss. It is unclear whether these effects are due to the reduced carbohydrate content of such diets or to their associated increase in protein intake. OBJECTIVE: We tested the hypothesis that increasing the protein content while maintaining the carbohydrate content of the diet lowers body weight by decreasing appetite and spontaneous caloric intake. DESIGN: Appetite, caloric intake, body weight, and fat mass were measured in 19 subjects placed sequentially on the following diets: a weight-maintaining diet (15% protein, 35% fat, and 50% carbohydrate) for 2 wk, an isocaloric diet (30% protein, 20% fat, and 50% carbohydrate) for 2 wk, and an ad libitum diet (30% protein, 20% fat, and 50% carbohydrate) for 12 wk. Blood was sampled frequently at the end of each diet phase to measure the area under the plasma concentration versus time curve (AUC) for insulin, leptin, and ghrelin. RESULTS: Satiety was markedly increased with the isocaloric high-protein diet despite an unchanged leptin AUC. Mean (+/-SE) spontaneous energy intake decreased by 441 +/- 63 kcal/d, body weight decreased by 4.9 +/- 0.5 kg, and fat mass decreased by 3.7 +/- 0.4 kg with the ad libitum, high-protein diet, despite a significantly decreased leptin AUC and increased ghrelin AUC. CONCLUSIONS: An increase in dietary protein from 15% to 30% of energy at a constant carbohydrate intake produces a sustained decrease in ad libitum caloric intake that may be mediated by increased central nervous system leptin sensitivity and results in significant weight loss. This anorexic effect of protein may contribute to the weight loss produced by low-carbohydrate diets.