Sustained, long-term weight loss is difficult for many people. One reason is because the body adapts to weight loss, which is known as starvation response.
During starvation response, metabolic rate (or calories burned) is reduced. Dietary factors may also play a part, but studies debate this.
Recently, a team of scientists looked into how carbs affect starvation response and body weight. Here is a detailed summary of their results, published in Obesity.
A group of scientists compared the effects of a moderate-carb diet and a high-carb diet on weight loss, body composition and starvation response.
Basic Study Design
This double-blind, randomized controlled trial investigated how carbs in calorie-restricted diets affect body weight, body composition, resting metabolic rate and starvation response. The participants were obese adults, aged 45 to 65 years.
The study was divided into 4 phases. The first three phases had a controlled diet, provided by the study’s kitchen. During the last phase, the participants were free to eat as much as they wanted to.
- Phase 1. Weight maintenance: This 5-week phase determined how many calories participants needed to maintain a stable weight.
- Phase 2. Experiment: In this 12-week phase, the participants were randomly assigned to one of four diets (see below). These 4 diets were calorie-restricted, and had about 67% of the calories required to maintain a stable weight.
- Phase 3. Weight maintenance: During this 5-week phase, the participants had their calorie intakes adjusted so they would maintain their weight.
- Phase 4. Follow-up: For 12 months, the participants selected and prepared their own meals, but were provided with instructions to follow their assigned diets.
The 4 diets assigned in phase 2 varied the amounts of carbs, as well as the glycemic index (GI). Those who ate fewer carbs ate more fat instead.
- Moderate-carb, high-GI diet: This diet provided 54%, 29% and 16% of calories from carbs, fat and protein, respectively, and had a glycemic index of 80.
- Moderate-carb, low-GI diet: This diet provided 54%, 31% and 16% of calories from carbs, fat and protein, respectively, and had a glycemic index of 51.
- High-carb, high-GI diet: This diet provided 70%, 14% and 16% of calories from carbs, fat and protein, respectively, and had a glycemic index of 86.
- High-carb, low-GI diet: This diet provided 68%, 16% and 15% of calories from carbs, fat and protein, respectively, and had a glycemic index of 59.
Throughout the study, the researchers measured body weight, body composition and resting metabolic rate. Starvation response was calculated as the difference between predicted and measured resting metabolic rate.
Of the 107 participants that initially started the study, 79 completed phases 1–3, but only 60 made it to the end.
Bottom Line: This was a double-blind, randomized controlled trial comparing the effects of moderate-carb and high-carb diets, and high- and low-GI diets, on starvation response, body composition and body weight.
Finding 1: Carbs Did Not Affect Starvation Response
At the end of phase 2, resting metabolic rate (RMR) was 6.5% lower, on average, compared to at the beginning of the study.
Some of the decline in RMR was predicted to happen with weight loss, but the actual measured reduction was slightly greater than predicted.
This difference, which amounted to 54 kcal/day, on average, was assumed to be accounted for by the starvation response. The chart below shows the differences in starvation response during phase 2 across groups.
Bottom Line: Neither the amount of carbs nor the glycemic index had any significant effects on starvation response or the total reduction in RMR during the study.
Finding 2: The Diets Caused Similar Weight Loss
On average, the study participants lost 7.5% of their original body weight during the study, mostly in phase 2. However, weight loss did not significantly differ across groups.
These results indicate that when a calorie-controlled weight loss diet contains moderate to high amounts of carbs, variations in the ratio of carbs to fat do not change the effectiveness of the diet.
This study, however, did not compare the effectiveness of high- or moderate carb diets with a low-carb diet. A low-carb diet is much lower in carbs than the diets tested in the current study.
Bottom Line: Moderate-carb and high-carb diets had similar effects on weight loss. These results cannot be generalized to low-carb diets, which were not examined in the present study.
Finding 3: Glycemic Index Did Not Affect Weight Loss
The glycemic index measures how foods affect the rise in blood sugar after a meal. In the present study, weight loss did not significantly differ by the glycemic index of the diet.
This is supported by previous studies showing that the glycemic index of a particular diet has no effects on weight loss when calories are controlled.
Bottom Line: The glycemic index of the diets had no effects on weight loss in the current study.
Finding 4: Carb Amount Did Not Affect Weight Loss Quality
The quality of weight loss refers to how much fat mass is lost relative to muscle mass.
When you go on a weight loss diet, you usually lose both fat mass and muscle mass (lean mass), but the ratio may vary. If relatively little muscle mass is lost, the quality of the weight loss is high.
In the present study, neither the carb content nor the glycemic index (GI) of the diet had any effects on the ratio of the loss of fat mass and lean mass, as shown in the chart below.
Various factors may affect the quality of weight loss. These include strength exercises, protein intake and eating pattern.
However, the present study makes it clear that carbs are not one of those factors, at least not when the carbs are at moderate or high levels.
Bottom Line: Neither the carb amount nor the glycemic index of the diets had any effects on weight loss quality, or the ratio of the reduction in fat mass and lean mass.
Finding 5: Carb Amount Did Not Affect Weight Regain
During phase 4 of the study, the participants were allowed to eat as much as they wanted, but were given instructions to follow their diets from phase 2.
On average, the participants regained 4.3 kg (9.5 lbs) during phase 4, or about 58% of the weight they lost.
Weight regain did not differ across groups, and was not associated with variations in resting metabolic rate or the starvation response. This means that the amount of carbs eaten did not predict weight regain after dieting had stopped.
Bottom Line: The carb amount of the weight loss diets did not affect weight regain after dieting had stopped.
This study does not have any serious weaknesses in its design or execution.
However, only 56% of participants made it to the end, which means the dropout rate was very high. There are also a few issues that limit how the results can be generalized.
First, the study compared moderate- to high-carb diets. The difference in the amounts of carbs on the high-carb (70%) and moderate-carb diets (54%) may not have been sufficient enough to cause significant differences in weight loss.
Second, the study wasn’t designed to cause a huge amount of weight loss. With greater energy restriction, the results might have been different.
Third, adherence to the diets may not have been perfect. In fact, there were some indications that calorie intake may have been slightly higher than prescribed.
Finally, calories were controlled in the current study. The results may have been different on an ad libitum (eat until fullness) diet.
Bottom Line: This study appears to have been well designed and implemented. However, the results cannot be generalized to low-carb diets, and the dropout rate was very high.
Summary and Real-Life Application
Starvation response was not affected by the amount of dietary carbs or the glycemic index, at least not when carb intake was moderate to high. Likewise, weight loss did not differ across groups.
This is not to say that reducing carbs can’t be beneficial for weight loss. Plenty of studies have shown that reducing carbs can be a very effective weight-loss strategy.
This study, however, replaced carbs with fat, while keeping protein intake unchanged. Additionally, it did not examine the effects of a low-carb diet.
The present findings are of limited use for people trying to lose weight. They are, however, of greater value for science, helping us understand how carbs affect body weight on a calorie-restricted diet.