INTRODUCTION: To date, there have been 3 long-term clinical trials (2 years) published over the past 10 years involving “low carb diets”.
The first long-term study that was presented in the previous article (which can be read here) clearly demonstrated that a low carb non–calorie-restricted diet was both safe and effective and produced the greatest weight loss, lower FBS and HbA1C, the most significantly lower TG and higher HDL and lower C-reactive protein (when compared with a low-fat calorie-restricted diet and a Mediterranean calorie-restricted diet).
In this, the second of the three long term studies, researchers looked at the effectiveness of four dietary interventions with different composition of fat, protein and carbohydrate – including one “low carb” diet..
Did this study demonstrate that a “low carb” diet was safe and effective to result in weight loss?
Comparison of Weight-Loss Diets with Different Compositions of Fat, Protein, and Carbohydrates
This study involved over 800 overweight and obese subjects, of which 40% were men. Subjects were between the ages of 30 and 70 years and had a Body Mass Index (BMI) of 25-40, where BMI is the weight in kilograms divided by the square of the height in meters.
BMI =25.0-29.9 is considered overweight
BMI = 30.0-34.9 is Class I obesity
BMI = 35.0-39.9 is Class II obesity
BMI ≥ 40.0 is Class III obesity
Major criteria for exclusion from this study were the presence of Diabetes or unstable cardiovascular disease, the use of medications that affect body weight and insufficient motivation as assessed by interview and questionnaire.
Of the 811 subjects that began the study, at the end of two years, 645 subjects remained enrolled. Approximately 80% of the participants were white, 15% black, 4% Hispanic and the remaining 1% Asian.
The Four Diets – high/low fat, high/low protein
The 811 overweight adults were randomly assigned to one of four diets:
- Low Fat, Average Protein: fat: 20%, protein: 15%, carbohydrate: 65% (202 subjects)
- Low Fat, High Protein: fat: 20%, protein: 25%, carbohydrate: 55% (202 subjects)
- High Fat, Average Protein: fat: 40%, protein: 15%, carbohydrate: 45% (204 subjects)
- High Fat, High Protein: fat: 40%, protein: 25%, carbohydrate: 35% (201 subjects)
Two Diets were Low Fat but Two were not High-Fat Diets
The researchers stated that “two diets were low-fat and two were high-fat”, but it is important to note that none of the diets were “low carb high fat”/ ketogenic diets, which are ≥ 65% fat (not 40% fat). Two of the diets were higher in fat than the recommended dietary intake (in both the US and Canada).
Two Diets were Average Protein but not High Protein
The researchers said that “two diets were average protein and two were high protein” and while the ‘average protein intake’ in the US in 2008 was ~15% (16.1% for men and 15.6% for women), diets such as two of the ones in this study that have only 25% protein are really at the very lowest range of what are considered high-protein diets – which normally contain between 27 – 68 % protein. Also important to note, a “low carb high fat”/ ketogenic diet usually has ~20% protein (considered ‘moderate protein’) and are not high protein diets.
Two Diets were High Carb and One Diet was Moderate Carb
The first and second dietary interventions would both be considered high carb, as they fall within the range of the dietary recommendations in both Canada and the USA, 45-65% carbohydrate, with one being higher protein and one being average protein.
The third diet would be consider “moderate carb” according to Diabetes Canada’s standards, at 45 % carbohydrate, and higher fat and higher protein.
One Diet was Low Carb but not Ketogenic – and not Low Carb High Fat
The fourth diet could be considered ‘low-carb’ at 35% carbohydrate, but it is not a ketogenic diet, as the percent of carbohydrate is too high. A ketogenic diet has between 5-10% carbohydrate. It was not a “high fat diet”, as the fat is only 40%, not ≥ 65% fat.
None of the dietary interventions in this study was 'low-carb high fat' or ketogenic, however one diet was "low carb".
Other Study Goals and Information
Other goals for all the dietary interventions were that the diets had;
– 8% or less of saturated fat
– 20 g or more of dietary fiber
– 150 mg or less of cholesterol per 1000 kcal
Each participant’s calories represented a deficit of 750 kcal per day
from baseline, as calculated from the person’s resting energy expenditure and activity level (which should have promoted a weight loss of ~ 1.5 pounds per week).
Blinding between the groups was maintained by the use of similar foods in each of the dietary interventions.
Staff as well as participants were taught that each diet adhered to principles of a “healthful diet” and that each had been recommended for “long-term weight loss”.
Group dietary counselling sessions were held once a week, 3 of every 4 weeks during the first 6 months and 2 of every 4 weeks from 6 months to 2 years; individual sessions were held every 8 weeks for the entire 2 years. Behavioral counseling was integrated into the group and individual sessions to promote adherence to the assigned dietary intervention.
Participants were instructed to record their food and beverage intake in a daily food diary and in a web-based self-monitoring tool that provided information on how closely their daily food intake met their dietary intervention’s goals for macronutrients and calories.
The goal for physical activity was 90 minutes of moderate exercise per week. Participation in exercise was monitored by questionnaire and by
the online self-monitoring tool.
Body weight and waist circumference were measured in the morning before breakfast on 2 days at baseline, 6 months, and 2 years, and on a single
day at 12 and 18 months.
Levels of serum lipids, glucose, insulin, and glycated hemoglobin (HbA1C) were measured via fasting blood samples, and 24-hour urine samples, and measurement of resting metabolic rate were obtained on 1 day, and blood-pressure measurement on 2 days, at baseline, 6 months and 2 years.
Weight loss and Waist Circumference
The amount of weight loss after 2 years was similar in participants assigned to a diet with 25% protein and those assigned to a diet with 15% protein.
Weight loss was the same in those assigned to a diet with 40% fat and those assigned to a diet with 20% fat.
There was no effect on weight loss of carbohydrate level through the target range of 35 to 65%.
Most of the weight loss occurred in the first 6 months, however 23% of the participants continued to lose weight from 6 months to 2 years.
The change in waist circumference did not differ significantly among the diet groups.
At 2 years, 31 to 37% of the participants had lost at least 5% of their initial body weight, 14 to 15% of the participants in each diet group had lost at least 10% of their initial weight, and 2 to 4% had lost 20 kg or more.
Risk Factors for Cardiovascular Disease and Diabetes
All the diets reduced risk factors for cardiovascular disease and Diabetes at 6 months and 2 years.
At 2 years, the two low-fat diets and the highest-carbohydrate diet decreased low-density lipoprotein (LDL) cholesterol levels more than did the high-fat diets or the lowest-carbohydrate diet, 5% vs 1%. And at 2 years, the highest carbohydrate decreased LDL more (6%) versus the lowest carbohydrate diet (1%).
The lowest-carbohydrate diet increased HDL cholesterol levels more (9%) compared with the highest-carbohydrate diet (6%).
All the diets decreased triglyceride (TG) levels similarly, by 12 to 17%.
All the diets except the one with the highest carbohydrate content decreased fasting serum insulin levels by 6 to 12% – and the decrease was larger with
the high-protein diet than with the average-protein diet (10% vs. 4%).
Blood pressure decreased from baseline by 1 to 2 mm Hg, with no significant differences among the groups.
The metabolic syndrome (defined as elevated fasting blood glucose, elevated blood pressure and abnormal triglycerides or cholesterol levels) was present in 32% of the participants at baseline, and the percentage at 2 years ranged from 19 to 22% in the four diet groups.
Mean reported intakes at 6 months and at 2 years were not at the target levels for macronutrients (fat, protein and carbohydrate). This limits the applicability of the data.
In the Low Fat, Average Protein group (fat: 20%, protein: 15%, carbohydrate: 65%), carbohydrate intake decreased from baseline by 12.8% and by 9.3% from baseline at 2 years and fat intake decreased from baseline by 11.8% at 6 months and 12.0% at two years. As it should have, protein intake hardly changed at 6 months (0.2%) but by 2 years it had increased by 2.1% to 19.6%.
In the Low Fat, High Protein group (fat: 20%, protein: 25%, carbohydrate: 55%) at 6 months carbohydrate intake decreased from baseline by 7.4% and at 2 years, it decreased from baseline by 6.8%. Protein intake increased from baseline by 3.9% at 2 years it had increased by 2.5% – but it is important to note that such a modest increase meant that this group did not consume a diet of 25% protein (but slightly less than 19% at 6 months and 17.5% at 2 years). Fat intake decreased from baseline by 11.8% at 6 months and 12.0% at two years.
In the High Fat, Average Protein group (fat: 40%, protein: 15% carbohydrate: 45%), at 6 months carbohydrate intake decreased from baseline by 5.0% and at 2 years, it decreased from baseline by 2.4%. Protein intake hardly increased from baseline at 6 months (0.5%), but at 2 years it had increased from baseline by 2.1%. Fat intake in this group was supposed to have increased, but actually decreased from baseline by 3.8% at 6 months and decreased from baseline by 2.1% at two years.
In the High Fat, High Protein group (fat: 40%, protein: 25%, carbohydrate: 35%) – which was the only intervention that was “low carb”, at 6 months carbohydrate intake only decreased from baseline by 0.2% and at 2 years, it decreased from baseline by 0.4%. In fact, carbohydrate remained at ~ 43% the entire time. Protein intake was supposed to increase substantially, but only increased from baseline by 4.3%, and at 2 years it had had only increased from baseline by 3.4%. It is important to note that such a modest increase in protein meant that this group did not consume a diet of 25% protein but ~19.3 % at 6 months and ~18.4% at 2 years. Fat intake in this group was supposed to have increased, but actually decreased from baseline by 3.7% at 6 months and decreased from baseline by 3.4% at two years.
Neither of the "high protein" groups achieved anywhere near 25% of daily calories as protein.
Despite the intensive behavioral counseling in this study, participants did not achieve the goals for macronutrient intake of their assigned group and while some data in this study is helpful, the one group that was supposed to be “low carb” (high fat, high protein) was none of those!
The researchers concluded;
“we did not confirm previous findings that low-carbohydrate or high protein diets caused increased weight loss at 6 months”
High Protein Diet “Fail”
The reason that this study failed to confirm whether a high protein diet causes increased weight loss at 6 months is because neither of the two “high protein” diet groups in this study ate anywhere near the target protein level of 25%, but rather ate between 17.5%-19% protein, which is remarkably close to the average protein intake of 15% (16.1% for men and 15.6% for women). Subjects also ate no where near the lower limits of a “high protein” diet, which is 27-68% of daily calories as protein.
Low Carbohydrate Diet “Fail”
The reason that this study failed to confirm that a low carbohydrate diet causes increased weight loss is because the one group of the four diet interventions that was supposed to eat what the researchers defined as “low carb” (35% of calories as carbohydrate) ate ~43% of calories as carbohydrate the entire duration of the study. This as a moderate carb diet, not a low carb diet.
In this long term study, researchers set out to look at the effectiveness of four dietary interventions including a “low carb” diet group, however poor study design failed to produce even one of the four groups that ate low carb.
Sacks FM, Bray GA, Carey VJ et al, Comparison of weight-loss diets with different compositions of fat, protein, and carbohydrates, N Engl J Med. 2009 Feb 26;360(9):859-73