Low carbohydrate diets

1. In clinical trials, a low-carb, calorie unlimited diet often results in more weight-loss than a calorie restricted, low-fat diet

2. Low carbohydrate diets decrease triglyceride levels

3. Low carbohydrate diets tend to increase HDL (good cholesterol)

4. Diets high in protein tend to be more filling

Summary:

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 studies comparing a calorie unlimited, low carbohydrate diet to various others, like Dr. Dean Ornish’s extremely low-fat diet, or the calorie restricted generally accepted healthy diet promoted by the government, with a majority of calories coming from carbs. In other words, a battle between two notorious opponents:  Eat until you are full and limit carbs Vs. 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 50-75 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 the low-fat, low-calorie diet group lost more weight.

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 diets seem destined to fail.

Two scenarios could potentially explain this paradoxical phenomenon, both of which seem to shed positive light on high-protein diets. It is possible that the simple act of eating high-protein, high-fat foods causes people to 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).

Another explanation, which Dr. Atkins and other low-carb gurus suggest, is that carbohydrates, especially refined ones, cause weight gain via their stimulatory effects on insulin, the main hormone involved in fat storage. This theory suggests that calories are secondary in relation to how many carbs you eat. Most of the studies listed below which measured calorie intake seem to support this theory, since the subjects consuming the low carb diet did not report eating less calories, and often lost more weight.

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.

Criticisms

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 in a typical low carbohydrate diet, made up mostly of non-starchy vegetables and meats, insulin would be elevated to a lesser degree. Since meats consist of protein (which does spike insulin) and fat (which does not), and non-starchy vegetables have minimal insulin-effects, there may be less total insulin release.

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, one lasting over 1 year and the other lasting 8 years 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 Atkins diet is the magic bullet for everyone; or that anyone who follows one will suddenly become skinny and healthy. Many people have been successful by following a low-fat diet as well. For example, the National Weight Control Registry has documented over 5,000 people around the country who have successfully lost weight following a low-fat diet . 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.


Bibliography:

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Low carbohydrate diets most effective in clinical trials for weight loss and blood lipids
Low carbohydrate diets are equally effective for weight loss
Satiety of various macronutrients

Low carbohydrate group lost more weight and usually improved blood cholesterol:

Shai et al. (2008). Weight loss with a low-carbohydrate, mediterranean, or low-fat diet

  • 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.
  • Outcome: Mean weight loss was 2.9 kg for the low-fat group, 4.4 kg for the Mediterranean-diet group, and 4.7 kg for the low-carbohydrate group (P<0.001); relative reduction in the ratio of total cholesterol to HDL was 20% in the low-carbohydrate group and 12% in the low-fat group (P=0.01)
  • 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.

Gardner C.D. et al. (2007) Comparison of the Atkins, Zone, Ornish, and LEARN Diets for Change in Weight and Related Risk Factors Among Overweight Premenopausal Women: The A TO Z Weight Loss Study: A Randomized Trial.

  • 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
  • Outcome: Atkins group: weight loss (4.7kg, CI: -6.3-3.1) significantly greater and Systolic BP significantly lower at 12 months.
  • 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.

Brehm, B. J. et al. (2005). The role of energy expenditure in the differential weight loss in obese women on low-fat and low-carbohydrate diets.

  • 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
  • Outcome: Very low carb group lost more weight (9.79kg +/-0.71 vs. 6.14kg +/-0.91) and more body fat (6.2kg +/- 0.67 vs. 3.23kg +/- 0.67). No difference in energy intake between groups.Low-fat diet caused greater 5-h increase in TEF. Physical activity and REE were similar in both groups.
  • CommentsCalorie unrestricted diet lost twice as much weight as calorie-restricted, low-fat diet. REE, physical activity, or TEF did not account for this difference. Sample size small.

Mcauley K.A. et al. (2005). Comparison of high-fat and high-protein diets with a high-carbohydrate diet in insulin-resistant obese women.

  • 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.
  • Outcome: HF and HP groups had significantly greater reductions in weight (HF -2.8kg, HP -2.7kg), waist circumference (HF -3.5 cm, HP -2.7) and triglycerides (HF -0.3 mmol/l, HP -0.22mmol/l). Insulin decreased in all 3 diets. Energy intake did not differ between groups. 88% completed the study.
  • 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

Yancy, W. S. et al. (2004). A low-carbohydrate, ketogenic diet versus a low-fat diet to treat obesity and hyperlipidemia.

  • 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
  • Outcome: Calorie unrestricted, low carbohydrate diet group had greater weight loss (12.9% of weight vs. 6.7%), greater loss of fat mass (-9.4kg vs -4.8kg), 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.
  • 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

Volek et al. (2004). Comparison of energy-restricted very low-carbohydrate and low-fat diets on weight loss and body composition in overweight men and women.

  • 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)
  • Outcome: Men on VLCK diet performed significantly better in weight loss (-8kg vs. -4.5 kg), 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.
  • 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.

Brehm, B. J. et al. (2003). A randomized trial comparing a very low carbohydrate diet and a calorie-restricted low fat diet on body weight and cardiovascular risk factors in healthy women.

  • 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
  • Outcome: Very low carb group lost more weight (8.5kg +/-1.0 vs. 3.9kg +/-1.0) and more body fat (4.8kg +/- 0.67 vs. 2.0 +/- 0.75kg). No difference between diets in blood pressure, lipids, fasting glucose and insulin.
  • Comments: Total reported energy intake not significantly different, despite low carbohydrate diet being calorie unrestricted. Sample size small.

Foster, G. D. et al. (2003). A randomized trial of a low-carbohydrate diet for obesity.

  • 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.
  • Outcome: Subjects on calorie unrestricted, low-carb diet lost more weight at 3 months (-6.8 +/-5 vs. -2.7 +/-3.7 % body fat), and at 6 months (-7 +/- 6.5 vs. -3.2 +/-5.6 % body fat). 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.
  • 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.

Samaha FF., et al. (2003). A low-carbohydrate as compared with a low-fat diet in severe obesity.

  • Overview: 6-month randomized trial in 132 obese obese subjects (mean BMI = 43).
  • Intervention: Randomly assigned to either calorie unrestricted low-carbohydrate diet (<30 g/day) or low-fat diet, calorie restricted diet (deficit of 500 kcal/day).
  • Outcome: Subjects on calorie unrestricted, low-carb diet lost more weight (-5.8 +/-8.6 kg vs. -1.9 +/-4.2 kg), and greater decreases in triglyceride levels (-20% +/- 43% vs. -4 +/- 31%). Insulin sensitivity (measured in non-diabetics only) also improved more on low carbohydrate diet (6 +/- 9% vs. -3 +/- 8%)
  • Comments: No significant difference in energy intake. 14% of subjects on low-carb lost at least 10% of baseline weight, as opposed to 3% of low-fat group.

Low carbohydrate and low-fat groups lost similar amounts of weight:

Yancy Jr. WS, et al. (2010). A randomized trial of a low-carbohydrate diet vs Orlistat plus a low-fat diet for weight loss.

  • 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
  • Outcome: Weight loss was similar for the LCKD (expected mean change, −9.5%) and the O + LFD (P = .60 for comparison) groups. The LCKD had a more beneficial impact than O + LFD on systolic (−5.9 vs 1.5mm Hg) and diastolic (−4.5 vs 0.4 mm Hg) blood pressures (P = .001 for both comparisons). 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 the LCKD group only; comparisons between groups, however, were not statistically significant.
  • 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.

Foster et al. 2010.Weight and Metabolic Outcomes After 2 Years on a Low-Carbohydrate Versus Low-Fat Diet.

  • 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.
  • 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.
  • Comments: Dietary intake not assessed. No mention of food records or recalls. No significant difference in urinary ketone analysis after 6 months, potentially suggesting a larger carbohydrate increase in the low-carbohydrate group.

Brinkworth, G.D. et al. (2009). Long-term effects of a very-low-carbohydrate weight loss diet compared with an isocaloric low-fat diet after 12 mo.

  • 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.
  • Outcome: LC group lost slightly more weight, but not significant (LC: -14.5 +/- 1.5 kg vs. LF: 11.5 +/- 1.2 kg). LC group had greater decreases in triglycerides, and increases in HDL and LDL cholesterol.
  • Comments: Very high attrition rate (only 59% completed study). Subjects who completed the study (n=69) were very compliant to their respective diets.

Frank Sacks et al. 2009. Comparison of weight-loss diets with different compositions of fat, protein, and carbohydrates

  • 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%.
  • 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
  • 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 typical 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.

Clifton et al. 2008. Long term effects of a high-protein weight-loss diet

  • 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.
  • Outcome: Mean weight loss was not significantly different between groups: (HP: 4.6 +/- 5.5 kg; HC: 4.4 +/- 6.1 kg). 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.
  • 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.

Tay et al. 2008. Metabolic effects of weight loss on a very-low-carbohydrate diet compared with an isocaloric high-carbohydrate diet in abdominally obese subjects

  • 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.
  • Outcome: Weight loss was similar in both groups (VLCHF -11.9 +/- 6.3 kg, HCLF -10.1 +/- 5.7 kg; p < 0.17). Blood pressure, CRP, fasting glucose, and insulin reduced similarly in both diets. VLCHF diet produced greater decreases in triglycerides (VLCHF -56.96 +/- 55.18 mg/dl, HCLF -31.15 +/- 43.61 mg/dl; p <0.01) and increases in HDL (VLCHF 9.75 +/- 10.92 mg/dl, HCLF 3.12 +/- 6.63 mg/dl; p < 0.002). LDL decreased in the HCLF diet but remained unchanged in the VLCHF diet (VLCHF 2.34 +/- 22.62 mg/dl, HCLF -17.94 +/- 27.69 mg/dl; p < 0.001). The apoB levels were the same in both groups.
  • 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.

Jenkins DJ, et al. (2009). The effect of a plant-based low-carbohydrate (“Eco-Atkins”) diet on body weight and blood lipid concentrations in hyperlipidemic subjects.

  • 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)
  • 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
  • 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.

Dasinger ML, et al. (2005). Comparison of the Atkins, Ornish, Weight Watchers, and Zone Diets for weight loss and heart disease risk reduction: A randomized trial.

  • 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)
  • 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%
  • 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.

Satiety and different macronutrients:

Halton, T.L., Hu F.B. (2004). The Effects of High Protein Diets on Thermogenesis, Satiety and Weight Loss: A Critical Review.

  • Overview: Systematic critical review of the evidence that high protein diets are beneficial for satiety, thermogenesis and weight loss.
  • Outcome: 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.”

Weigle DS., et al. (2005). 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.

  • 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)
  • Outcome: Satiety increased with the isocaloric high-protein diet. With the calorie unlimited high protein diet, energy intake decreased by 441 +/- 63 calories per day; body weight decreased by 4.9 +/- 0.5 kg, fat mass decreased by 3.7 +/- 0.4 kg
  • 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.”

Ludwig DS, et al. (1999). High glycemic index foods, overeating and obesity.

  • 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.
  • Outcome: Voluntary energy intake after the high 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
  • Comments: All the diets were essentially equal in calories. Researchers conclude, saying “consumption of high-GI foods incudes hormonal and etabolic changes that limit availability of metabolic fuels and lead to overeating in obese subjects
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