Carbohydrate-restricted diets tend to increase HDL
Diets high in protein tend to be more filling
The appearance and sudden popularity of the Atkins diet in the 1990s had dieters running to the meat department, leaving carbs in the dust. The apparent success of this diet, mostly ascertained from anecdotal evidence, had the overweight population excited and health experts worried. A diet characterized by high amounts of meat and fat was deemed impossible to be effective and a serious health risk.
At the time, few clinical trials had been done analyzing the efficacy and safety of such a diet, which understandably led to extreme skepticism among dietitians and doctors. Recent years have seen numerous such studies comparing a calorie unlimited, low carbohydrate diet to other popular diets, such as the ultra low-fat Ornish diet, or the calorie restricted generally accepted healthy diet promoted by the government, with a majority of calories coming from carbs. In other words, eat until you are full and limit carbs, or eat until you reach a calorie limit and restrict fat.
To the surprise of many, when compared to other diets, the calorie unrestricted, lowest carbohydrate diet group generally — but not always — loses more weight. With few exceptions, their HDL increases and their blood triglyceride levels decrease without having any significant effect on LDL (bad cholesterol). When subjects keep their carbohydrate intake lower than 50 grams per day, they seem to be most successful.
Often times the various groups fare the same, both losing approximately the same amount of weight. But never, in dietary clinical trial history, has a the low-fat, low-calorie diet resulted in more weight loss than a low-carb diet (If you can find one, please e-mail it to me and I will post it).
The High-Fat Paradox
The very idea that a diet characterized by high-fat foods and unlimited calories can do as well, or better, than a low-fat, calorie-restricted diet poses a challenge to the current weight-loss recommendations. Since fat has 9 calories per gram and protein or carbs have 4 calories per gram, a high fat diet seems destined to fail.
Two scenarios could potentially explain this paradoxical phenomenon, both of which seem to shed positive light on carbohydrate-restricted diets.It is possible that the simple act of eating high-protein, high-fat foods causes people to spontaneously eat less total calories. It has been tested and proven many times, that subjects who eat a high-protein meal report being more satisfied and often eat less in the following meal (see satiety). With this explanation, it seems that the regulation of calorie intake does not happen consciously, but rather at the cellular level, sending signals perceived consciously as fullness.
Another explanation, championed by many low-carb enthusiasts, is that carbohydrates, especially refined ones, cause weight gain via their stimulatory effects on insulin, the main hormone required for fat storage. Many foods spike insulin to varying degrees, but breads, pastas, sugars and refined flours are particularly potent. The USDA-promoted diet, consisting of 65% of one's daily calories deriving from carbohydrates, stimulates net insulin secretion to a greater degree than a low carbohydrate diet. As the theory goes, this excess insulin release may be chronically directing more calories into your fat cells as opposed to your body to be burnt for fuel. This, in turn, will cause the person to remain hungry since a certain amount of necessary energy did not reach the cells but rather was stored away as fat, perpetuating a vicious cycle of hunger co-existing with adipose tissue growth.
This hypothesis implies that calories are secondary in relation to how many carbs you eat. Many of the studies listed below which measured calorie intake seem to support this theory, since the subjects consuming the carbohydrate-restricted diet did not report eating less calories, and often lost more weight. However this is still unclear.
The mechanism by which this extra weight loss occurs remains controversial, but the positive effects of losing the weight is not. In general, the weight loss seems to be most dramatic during the first six months and sometimes levels off after a year or two. The subjects become less compliant to the diet as time goes on, making it impossible to tell if the diet doesn’t work after six months, or the subjects are just not following it properly.
Regardless of the mechanism, carbohydrate-restricted diets seem to be the most effective way to lose weight based on the clinical data. From a practical standpoint it seems logical that the majority of calories consumed on a successful diet should come from the most satiating nutrients. The clinical trials suggest that the simple act of placing someone on a carbohydrate-restricted diet is the only intervention and education required for the patients to lose weight. Their own internal hunger and satiety mechanisms regulate their food intake, making calorie counting unnecessary.
Critics of low carb diets point to the fact that protein spikes insulin as well as carbohydrates. Yet fat does not, so the hypothesis predicts that insulin would be elevated to a lesser degree in a diet made up mostly of non-starchy vegetables, meats and fats. Since meats consist of protein (which does spike insulin) and fat (which does not), and non-starchy vegetables have minimal insulin-effects, there is likely less total insulin release. If there is less net insulin circulating in your blood throughout a day or week or month, then one would expect less calories would be stored, and more calories would be released from your adipose tissue.
But perhaps the most common argument against a low-carbohydrate diet has been it’s potential long term negative effects on the heart. Since these diets are typically characterized by high amounts of saturated fats that raise total cholesterol (as well as HDL and LDL), they may be unhealthy when eaten for a long period of time. In the dietary clinical trials involving weight loss (as seen below), the cholesterol and LDL levels of the subjects eating the high saturated fat, low carb diets rarely increase much. However, the increased weight loss accompanying this diet may mask the effects that saturated fats have on blood cholesterol.
Since the 1960s there have been a relatively small number of clinical trials testing the dangers of a high saturated fat diet, usually in the absence of weight loss. Some were randomized, and some were not; some contained less than 100 subjects while others enlisted over 40,000; some show a large decrease in heart disease rates, and many do not. Despite 50 years of research, the results are inconclusive at best. The only two randomized, double blind studies ever done, lasting 4.5 years and 8 years respectively, found no decrease in heart disease. For a full list of all the clinical trials, and a more in depth analysis, see saturated fats and heart disease.
These results don’t necessarily imply that the such a diet is the magic bullet for everyone; or that anyone who follows one will suddenly become skinny and healthy. However, in clinical trials when a low-carb diet is compared to another diet, the subjects eating less carbs usually lose more weight and improve their HDL and triglyceride levels.
Outcome: After 10 months, the low carb (LC) dieters lost more weight than the low fat (LF) group (20 pounds vs. 10.9 pounds) and more fat (12 pounds vs. 5.7 pounds in the LF group), despite reportedly eating the same amount of calories. The adiponectin levels, an adipose-derived protein with beneficial metabolic effects, increased significantly in the LC group and not in the LF group. There was no correlation between weight loss and increase in adiponectin, suggesting the LC diet may have a beneficial effect on adiponectin.
Overview: 81 obese women randomized into 2 cohorts (one 6 month and one 4 month) to consume a low fat (LF) or low carb (LC) diet.
Intervention: a calorie-restricted low-fat (LF) diet, modeled after the American Heart Association Step 1 diet (LF), or an ad libitum (calorie unlimited), very low-carbohydrate (LC) diet, based on the guidelines of the Atkins diet (LC)
Comments: Subjects on LC diet reportedly ate slightly more calories, although the difference was not significant.
Outcome: After 4 weeks, the high protein, low carbohydrate diet resulted in more weight loss (13.9 pounds vs. 9.4 pounds in the medium carbohydrate group), significantly less hunger, while spontaneously consuming 167 less calories each day. Non-significant decreases in LDL and Triglycerides in both groups, and non-significant increase in HDL in the low carb group.
Overview: 17 obese men studied in a randomized crossover trial in a residential facility. Men were assigned to begin with a low carb, ketogenic diet or a medium carb diet. After 4 weeks of following each diet, the groups were switched to consume the other diet. Food was provided daily.
Intervention: Either a low carb, ad libitum ketogenic diet (4% calories from carbohydrates), or a medium carb, ad libitum diet (35% calories from carbohydrates).
Comments: A rare dietary trial in which the subjects actually lived in the research facility, and were provided with their food. Researchers paid meticulous attention to detail, weighing and measuring all intake before and after each meal. Subjects had no preferences for either diet. However they were significantly more full after consuming the low carb diet while consuming 167 less calories per day, suggesting you can feel more satisfied with less calories on a low carb diet.
Outcome: The low carb, ketogenic diet (LCKD) group had greater improvements in hemoglobin A1c (-1.5% vs. -0.5%), body weight (24.4 pounds vs. 13.8 pounds), and high density lipoprotein cholesterol (+5.6 mg/dL vs. 0 mg/dL) compared to the low glycemic, reduced calorie diet (LGID) group. Diabetes medications were reduced or eliminated in 95.2% of LCKD vs. 62% of LGID participants. The LCKD reported eating more calories (1550 calories vs. 1335 in the LGID group).
Overview: Eighty-four community volunteers with obesity and type 2 diabetes were randomized to either a low carb or low calorie diet for 24 weeks.
Intervention: Either a low-carbohydrate, ketogenic diet (<20 g of carbohydrate daily; LCKD) or a low-glycemic, reduced-calorie diet (500 kcal/day deficit from weight maintenance diet; LGID).
Comments: Only 58% of subjects completed the study. Benefits seen after 24 weeks despite low carb group reportedly eating more calories.
Outcome: Low carb group lost more weight (10.3 lbs) than the low fat group (6.4 lbs) and slightly more than the mediterranean diet group (9.68 lbs). Low-carb group had 20% reduction in total cholesterol:HDL ratio compared to 12% in low-fat group.
Overview: 2-year randomized trial involving 322 moderately obese subjects assigned to one of three diets
Intervention:1 of 3 diets: low-fat, restricted-calorie, Mediterranean, restricted-calorie, or low-carbohydrate, non–restricted-calorie.
Comments: Very high adherance rates (95.4% at 1 year and 84.6% at 2 years). Energy intake decreased equally in all groups. Subjects following calorie unrestricted diet lost most weight. Percent of calories from fat in low-fat group did not decrease as planned. Subjects in low-carbohydrate group consuming much more carbohydrates than assigned.
Outcome: Atkins group lost the most weight (13.2 pounds at 6 months, 10.3 pounds at 1 year); Systolic BP significantly lower at 12 months. At all times, HDL and triglyceride levels favored the Atkins group
Overview: 12-month randomized trial comparing 4 diets among 311 overweight women
Intervention: Randomly assigned to 1 of 4 diets: Atkins, Ornish, Zone, LEARN. Weekly instruction for 2 months, than a 10 month follow up
Comments: Total reported energy intake not different at any point. Mean weight loss at 6 months on Atkins was 6kg. After 6 months, diets were not followed properly. At all time points HDL and triglyceride levels favored Atkins. 80% retention rate.
Outcome: Very low carb group lost more weight (21.5 pounds vs. 13.4 pounds in low-fat group) and more body fat (13.6 lbs vs. 7 lbs on low-fat). No difference in energy intake between groups. Physical activity and Resting energy expenditure were similar in both groups.
Overview: 4-month randomized trial comparing very low carbohydrate, calorie unrestricted diet to calorie restricted low fat diet in 50 healthy, obese women. Resting Energy Expenditure (REE). Physical activity and Thermic Effect of Food (TEF) were assessed as well
Intervention: Randomly assigned to calorie unlimited very low carbohydrate diet or calorie-restricted, low-fat diet
Comments: Calorie unrestricted diet lost twice as much body fat as calorie-restricted, low-fat diet. REE, physical activity, or Thermic effect of food did not account for this difference. Sample size small.
Outcome: High fat and high protein groups had significantly greater reductions in weight, waist circumference, and triglycerides. Insulin decreased in all 3 diets. Energy intake did not differ between groups. 88% completed the study.
Overview: 6-month randomized trial in 96 obese, insulin-resistant women.
Intervention: Assigned to 1 of 3 diets: high carbohydrate, high fiber diet (HC), high-fat Atkins (HF), or high protein Zone diet (HP). No calorie restriction for any diet.
Comments: HP and HF groups performed better. Very high retention rate. LDL decreased in HP and HC diets. 25% of those on HF diet, 13% of HC diet, and 3% of HP diet showed >10% increase in LDL
Outcome: After 3 months, compared with the NCEP diet (fat (30%), carbs (55%), and protein (15%)), the low carb diet resulted in significantly more weight loss (13.6 lb) than in the NCEP group (7.5 lb). No differences between the groups for total, low density, and high-density lipoprotein cholesterol, triglycerides, or the proportion of small, dense low-density lipoprotein particles.
Overview: 60 participants ages 28-71 were randomized for 12 weeks to receive the National Cholesterol Education Program (NCEP) diet or a modified low carbohydrate diet (MLC), which emphasized the intake of monounsaturated fats.
Intervention: Either an NCEP diet: (fat (30%), carbs (55%), and protein (15%)), or a MLC diet consisting of 2 phases of 2 weeks’ durationand of a third or maintenance phase of 8 weeks. First phase: fat (62%), low intakeof carbohydrates (10%), and an intake of protein of 28%. Second phase: fat intake was decreased to 43% whereas carbohydrateintake was increased to 27% and protein intake to 30%. In thethird phase, the percentages of calories were 39% from fat,28% from carbohydrates, and 33% from protein. The percentagesof calories from monounsaturated fats were 13% in phase 1 and8% in phases 2 and 3.
Comments: Dietary intake assessed by 24 hour recall, however since the recall was used as feedback, the information was insufficient to perform any quantitative analysis
Outcome: Calorie unrestricted, low carbohydrate diet group had greater weight loss (12.9% of weight vs. 6.7% in low-fat), greater loss of fat mass (-20.68 pounds vs -10.3 pounds), greater decreases in triglycerides (-74.2 mg/dl vs -27.9 mg/dl), and greater increases in HDL (5.5 mg/dl vs. -1.6m mg/dl). Changes in LDL did not differ.
Overview: 6-month randomized trial in 120 overweight, hyperlipidemic subjects.
Intervention: Assigned to either calorie unlimited low carbohydrate diet with nutritional supplement or low-fat calorie restricted diet with 500-1000 kcal deficit/day. Both groups also received exercise recommendations and group meetings
Comments: More low carb completed study (76% vs. 57%). Energy intake differed slightly (1461 +/- 325.7 kcal in low carb vs. 1502 +/-162.1 in low fat). 79 completed the study
Outcome: Men on VLCK diet performed significantly better in weight loss (-17.6 pounds vs. -10 pounds), total fat loss, and trunk fat loss, despite consuming slightly more calories (1855 kcals vs 1562 for LF). Women on a VLCK diet also performed slightly better, but it was not significant.
Overview: Randomized, cross-over trial in overweight men and women. Men followed diet for 50 days and women followed diet for 30 days (to control for possible menstrual effects)
Intervention: Randomly assigned to either Very-low carbohydrate ketogenic diet (VLCK, less than 10% kcal from carbs) or low-fat (LF) diet. Both were designed to be hypoenergetic (-500kcal/day)
Comments: Small sample size, yet paid meticulous attention to detail. All subjects completed 21 days of weighed food records. To ensure the VLCK group restricted carbohydrates, urinary ketones were measured daily.
Outcome: Very low carb group lost more weight (18.7 lbs vs. 8.5 lbs) and more body fat (10.5 lbs vs. 4.4 lbs) than the low fat group. No difference between diets in blood pressure, lipids, fasting glucose and insulin.
Overview: 6-month randomized trial comparing very low carbohydrate, calorie unrestricted diet to calorie restricted low fat diet in 53 healthy, obese women
Intervention: Randomly assigned to calorie-unlimited very low carbohydrate diet or calorie-restricted diet with 30% calories from fat
Comments: Total reported energy intake not significantly different, despite low carbohydrate diet being calorie unrestricted. Sample size small.
Outcome: Subjects on calorie unrestricted, low-carb diet lost more body fat at 3 months (-6.8 % vs. -2.7%), and at 6 months (-7% vs. -3.2% in the low fat group). At 12 months, difference was not significant. HDL increase and triglyceride decrease were larger on the low carb diet. No differences between LDL. Both diets decreased diastolic BP and insulin response.
Overview: 1 year, multi-center randomized trial in 63 obese men and women.
Intervention: Randomly assigned to either calorie unrestricted low-carbohydrate diet or low-fat diet, calorie restricted diet.
Comments: Dietary intake not assessed. No mention of food records or 24-hr recalls in methods or discussion. Low carb subjects in ketosis through 3 months, suggesting a large increase in carbohydrate intake after 3 months.
Outcome: Subjects on calorie unrestricted, low-carb diet lost more weight on average (-12.76 lbs vs. -4.18 lbs), and greater decreases in triglyceride levels (-20% vs. -4%) as compared to low-fat diet. Insulin sensitivity (measured in non-diabetics only) also improved more on low carbohydrate diet (+6% vs. -3%)
Outcome: After 6 months the low carb group lost more weight ( 19.58 pounds vs. 11.2 pounds) and lost more fat (16.7 pounds vs. 9.46 pounds) than the high carbohydrate group. 35% of subjects in the low carb group lost more than 22 pounds compared to only 9% in the high carb group. Only the high protein, low carb group significantly decreased their triglyceride levels.
Low carbohydrate and low-fat groups lost similar amounts of weight:
Outcome: Weight loss was similar for the low carb and the Orlistat + low fat diet. The Low carb group had a more beneficial impact than Orlistat and low fat diet group on systolic (−5.9 vs 1.5mm Hg) and diastolic (−4.5 vs 0.4 mm Hg) blood pressures. High-density lipoprotein cholesterol and triglyceride levels improved similarly within both groups. Low-density lipoprotein cholesterol levels improved within the O + LFD group only, whereas glucose, insulin, and hemoglobin A1c levels improved within thelow carb group only.
Overview: 11 month randomized clinical trial in 146 subjects comparing a very low carbohydrate diet with a low fat diet plus orlistat
Intervention: Randomly assigned to either a calorie unrestricted low carbohydrate, ketogenic diet (LCKD) (initially < 20% carbs) or a calorie restricted low-fat diet (<30% fat, 500-1000 kcal deficit) combined with orlistat (O + LFD), 120 mg orally 3 times daily
Comments: Very high retention rate. 57 of the LCKD group (79%) and 65 of the O + LFD group (88%) completed the study. LCKD group reported consuming slightly more calories at each time point, and maintained their low carbohydrate diet throughout the study to at or below 15% of total calories. The LCKD faired as well as a low fat diet combined with a drug.
Outcome: Weight loss was approximately 11 kg (11%) at 1 year and 7 kg (7%) at 2 years. There were no differences in weight, body composition, or bone mineral density between the groups at any time point. During the first 6 months, the low-carbohydrate diet group had greater reductions in diastolic blood pressure, triglyceride levels, and very-low-density lipoprotein cholesterol levels, lesser reductions in low-density lipoprotein cholesterol levels, and more adverse symptoms than did the low-fat diet group. The low- carbohydrate diet group had greater increases in high-density lipoprotein cholesterol levels at all time points, approximating a 23% increase at 2 years.
Overview: 2 year Randomized control trial in 307 participants with a mean age of 45.5 years and mean body mass index of 36.1
Intervention: Randomly assigned to either calorie unrestricted low-carbohydrate diet (20 g/day for 1st 3 months) or low-fat diet, calorie restricted diet (deficit of 500 kcal/day). Both diets had behavioral treatment as well.
Comments: Dietary intake not assessed. No mention of food records or recalls. No significant difference in urinary ketone analysis after 6 months, suggesting a larger carbohydrate increase in the low-carbohydrate group.
Outcome: Low carb group lost slightly more weight, but was not significant (31.9 pounds vs. 35.3 pounds in the low fat group). Low carb group also had greater decreases in triglycerides, and increases in HDL and LDL cholesterol.
Overview: 1 year randomized trial in 118 men and women comparing effects of iso-caloric low-carbohydrate (LC) or low-fat diets (LF).
Intervention: Randomly assigned to either isocaloric LC diet (4% carb, 35% protein, 61% fat) or a LF diet (46% carb, 24% protein, 30% fat). Weight, body composition and metabolic risk factors assessed.
Comments:Very high attrition rate (only 59% completed study). Subjects who completed the study (n=69) were very compliant to their respective diets.
Outcome: At 6 months, subjects lost average of 6kg, which they began to regain after 12 months. At 2 years, weight-loss was similar among all groups. Group session attendance strongly associated with weight loss.
Overview: 2 year randomized trial in 811 overweight adults randomly assigned to 4 diets differing in macronutrient composition
Intervention: Randomly assigned to 1 of 4 diets; the targeted percentages of calories from fat, protein, and carbs were: 20, 15, 65%; 20, 25, 55%; 40, 15, 45%; and 40, 25, 35%.
Comments: Assigned diets were very similar in macronutrient composition. Researchers said “blinding was maintained by the use of similar foods in each diet”, which seems impossible when comparing a low carbohydrate diet to a low-fat diet. The lowest carbohydrate group was consuming 175 g of carbohydrates (or equivalent of ~15 slices of bread) at 6 months and 215 grams of carbs at 2 years. The researchers also say HDL is a “biomarker for dietary carbohydrate.” The difference in HDL between highest and lowest carbohydrate groups was very small, indicating similar amounts of carbohydrate intake.
Outcome: Mean weight loss was not significantly different between the high protein and high carb groups. Protein intake at 64 wk was directly related to weight loss (P < 0.0001), accounting for 15% of the variance. Highest protein intake associated with most weight loss.
Overview: Assessing efficacy of a higher protein diet on maintenance of weight loss after 64 week follow-up in 79 healthy women. 12 week weight loss program and 52 week follow up.
Intervention: Either a HP diet (high in protein, low in saturated fat; 34% of energy from protein, 20% of energy from fat, and 46% of energy from carbohydrate) or an HC diet (low in saturated fat, 17% of energy from protein, 20% of energy from fat, and 64% of energy from carbohydrate); both diets had < 10% of energy from saturated fat. During the initial 12-wk weight-loss phase, the subjects attended individual consultations with 2 dietitians every 4 wk.
Comments: The two were essentially the same. There was only a 3.6% difference in energy as protein, which according to the authors “was not large enough to be of biological significance.” Carbohydrate intake increased by 41 g in the HP group and did not change in the HC group. At 64 wk, the carbohydrate intake in grams was the same in both groups.
Outcome: Weight loss was similar in both groups (11.9 kg in high fat, low carb group vs 10.1 kgs lost in the high carb group). Blood pressure, CRP, fasting glucose, and insulin reduced similarly in both diets. VLCHF diet produced greater decreases in triglycerides ( -56.96 mg/dl vs -31.15 in the high carb group) and increases in HDL (9.75 mg/dl vs HCLF 3.12 in the high carb group).
Overview: Comparing the effects of an energy-reduced, isocaloric very-low-carbohydrate, high-fat (VLCHF) diet and a high-carbohydrate, low-fat (HCLF) diet on weight loss and cardiovascular disease (CVD) risk outcomes in 88 abdominally obese adults.
Intervention: The VLCHF diet = 4% of total energy as carbohydrate, 35% as protein, 61% as total fat (20% saturated fat); HCLF diet = 46% of total energy as carbohydrate, 24% as protein, 30% as total fat (< 8% saturated fat). The diets were designed to be isocaloric, with a moderate energy restriction of ~30%. In the VLCHF diet, carbohydrate intake was restricted to <20 g/day during the first 8 weeks of the study, and then given the option to increase carbohydrate intake to <40 g/day. Subjects in the HCLF diet were asked to restrict saturated fat intake to <10 g/day.
Comments: Very high retention rate (~80%). Subjects in low-carb group dramatically reduced their carbohydrate intake. Food scales were given to subjects to help fill out food records, and urinary ketones were higher in the VLCHF group, suggesting compliance. For weight loss, both diets equally as effective for weight-loss. Triglyceride and HDL levels improved during the VLCHF diet, while LDL levels improved in the HCLF group, however the results were highly variable.
Outcome: Weight loss was similar for both groups (about 4 kg). Reductions in triglycerides, LDL-C concentration, total cholesterol-HDL-C and apolipoprotein B-apolopoprotein A1 ratios were greater for the low-carbohydrate group
Overview: 4 week, randomized clinical trial in 47 subjects comparing a plant based, low carbohydrate diet compared to a high-carbohydrate lacto-ovo vegetarian diet
Intervention: Randomly assigned to either a calorie restricted low carbohydrate (LC) (130 grams carbs/ day), eliminating starches, with protein and fats coming from nuts, gluten, and soy, and meats. The calorie restricted low-fat, lacto-ovo vegetarian diet (LF)
Comments:Small sample size, and very short term study. All foods were metabolically controlled and given to subjects. Subjects also given scale to weigh foods they consumed. LC group was consuming high amounts of carbohydrates compared to most low carb diet recommendations.
Outcome: No significant differences in weight loss among all groups. Weight loss was associated with reported dietary adherence (r=0.60, P<0.001). Each diet significantly reduced LDL/HDL ratio by about 10%
Overview: 1 year randomized trial assessing adherence rates and effectiveness of 4 popular diets for weight loss and cardiac risk factor reduction
Intervention: Randomly assigned 1 of 4 diets: Atkins (Calorie unrestricted, low carbohydrate), Weight Watchers (calorie restriction), Zone diet (40:30:30 ratio of carbohydrate:fat:protein), or the Ornish diet (<10% calories from total fat)
Comments: Adherence to diets was extremely poor. Subjects on Ornish diet eating 3X as much fat as supposed to. People in Atkins group eating 190 grams of carbs per day, well above their 20-50 limit. Attrition rate was extremely high (about 50% dropped out of the Ornish and Atkins group, and 35% dropped out of Zone and Weight Watchers). Only 25% of subjects sustained a self reported adherence to their diet of 6 out of 10 or greater.
Overview: Systematic critical review of the evidence that high protein diets are beneficial for satiety, thermogenesis and weight loss.
For thermogenesis - the authors stated there is “convincing evidence that a higher protein intake increases thermogenesis.” The increase is rather small, anywhere from 30-60 calories more burned within 7-9 hours after the meal. Based on the current formula to predict weight loss, this could result in increased weight loss over time, although this has not been proven.
For satiety, or hunger satisfaction, the researchers said “the evidence supports that meals high in protein tend to increase satiety when compared to meals lower in protein, at least in the short term.” Whether this has a long-term effect on energy intake remains unclear.
For Subsequent energy intake 8 of the 15 studies identified by the researchers showed a significant decrease in energy intake after the higher protein pre-load.
Comments: Authors concluded by stating that “there is convincing evidence that protein exerts an increased thermic effect when compared to fat and carbohydrate.” For satiety, the evidence is also convincing “that higher protein diets increase satiety when compared to lower protein diets. This may enhance a dieter’s ability to ‘stick with’ a hypocaloric diet over the long term.”
Outcome: Satiety increased with the isocaloric high-protein diet. With the calorie unlimited high protein diet, energy intake decreased by 441 calories per day; body weight decreased by 4.9 kg, fat mass decreased by 3.7 kg.
Overview: Examining the effects of increasing dietary protein on caloric intake and weight loss in 19 subjects given prepared food for 12 weeks. Then asked to answer subjective questions about their fullness.
Intervention: Subjects placed sequentially on following diets: weight maintaining diet (15% protein, 35% fat, 50% carb) for 2 wks, isocaloric diet (30% protein, 20% fat, 50% carb) for 2 wk, and then a calorie unlimited diet (30% protein, 20% fat, 50% carb)
Comments: Authors conclude that an increase in protein from 15% – 30% of energy at constant carbohydrate intake produces large decrease in ad libitum caloric intake, and stated that “the anorexic effect of protein may contribute to the weight loss produced by low-carbohydrate diets.”
Outcome: Voluntary energy intake after the high Glycemic index(GI) meal was 53% greater than after the medium-GI diet, and 81% greater than after the low-GI meal. High GI meal resulted in higher serum insulin levels, lower plasma glucagon levels, lower post-absorptive plasma glucose and serum fatty acids levels, and elevation in plasma epinephrine
Overview: Investigate the acute effects of dietary glycemic index (GI) on energy metabolism and voluntary good intake in 12 obese teenage boys
Intervention: Subjects consumed either low, medium, or high GI diets of equal caloric contents for breakfast and lunch. Then they were allowed to each as much as they wanted for the next 5 hours. Food intake was measured.
Comments: All the diets were essentially equal in calories. Researchers conclude, saying “consumption of high-GI foods incudes hormonal and metabolic changes that limit availability of metabolic fuels and lead to overeating in obese subjects