Friday, June 29, 2012

Is a low carb diet best for weight maintenance?

An interesting study published in JAMA a couple days ago looks at the effect of 3 different diets (low-fat, low-glycemic index, and very low carb) on energy expenditure during weight maintenance. Why is this important?

The basic tenet of weight loss is that if you burn more calories than you eat you will lose weight (1lb for every 3,500 calories), and if you eat more calories than you burn you will gain weight. Burning the same number of calories that you eat is the magic formula for weight maintenance. As far as weight loss is concerned it doesn't matter where these calories come from. Fat, carbohydrate, or protein- a calorie is a calorie. However, the results of this study suggest that it may not be quite so simple.

One of the things that makes weight loss so difficult is that as you lose weight you need fewer calories to power your new smaller body. This means that when you are trying to maintain weight loss you must eat less than before you began your diet, to account for your reduced energy expenditure. Wouldn't it be wonderful if you could diet, but limit this reduction in energy expenditure? That's what this paper addresses.

The Research Question
  • In obese people trying to maintain a 10-15% weight loss: Does varying the amount of fat, carbohydrate, and protein in a calorie controlled diet affect their total energy expenditure?

What they did
  • Weight monitoring phase: For 4 weeks obese study participants were monitored while they ate their normal diet.
  • Weight loss phase: Study participants were put on a Run-in Diet designed for them to lose 12.5% of their body weight in 12 weeks.
  • Weight stabilization phase: Participants were fed pre-prepared diets designed for weight maintenance for 4 weeks.
  • Weight loss maintenance testing phase: Participants were fed 3 different calorie-controlled test diets for 4 weeks at a time in a random order. The percent of calories coming from carbs, far, and protein are shown in the table below.

Diet Carbohydrate Fat Protein
Low Fat
60% 20% 20%
Low Glycemic Index 40% 40% 20%
Very Low Carbohydrate 10% 60% 30%
          All 3 diets had the same number of calories (details of the diets are shown in the table below).
          In this testing phase they measured total energy expenditure (TEE) and resting energy expenditure                 
          (REE). TEE is the total number of calories burned per day. REE is the TEE minus the calories burned           
          during activity. They also measured hormone levels, insulin sensitivity, metabolic syndrome  
          components, and ratings of hunger and well-being.         

Image not available.

The Results
  • As expected, since the participants lost weight, the energy expenditure (TEE and REE) decreased in all groups. The values are shown in the table below.

Diet Decrease in Total Energy Expenditure (TEE) Decrease in Resting Energy Expenditure (REE)
Low Fat ~400 211
Low Glycemic Index ~300 167
Very Low Carbohydrate ~150 138
  • The very low carb diet had the smallest reduction in energy expenditure (both TEE and REE), the low glycemic index diet was intermediate, and the low fat diet had the largest decrease in energy expenditure.
  • These findings suggest that it would be easier to maintain weight loss on a very low carb diet than on a low fat diet (assuming you are eating the same number of calories). However, there are some caveats (see "What does this mean?" below.
  • There was no difference in self-rated hunger or well-being between the diets.
  • Cortisol levels were higher on the very low carb diet. This is a hormonal measure of increased stress. This is bad. It is associated with insulin resistance and cardiovascular disease
  • The low fat diet produced changes in serum leptin (a hunger-related hormone) that would predict weight re-gain. It is worth noting that this diet represents typical US government recommendation for weight loss. But, it had the largest decrease in energy expenditure.

What does this mean?
  • I will start by saying that I think this is a very well done study, with good controls, and statistical analysis of the data. The authors do not over-state their findings, or make medical recommendations that their data can't support.
  • The author's state that the results "challenge the notion that a calorie is a calorie from a metabolic perspective". I agree. But, they don't provide all the answers yet. I think this study has a few limitations, and leaves  unanswered questions that need to be addressed before these results can be translated to recommendations for a weight maintenance diet.
  • While the very low calorie diet has the smallest decrease in energy expenditure (a good thing), it also is associated with increased levels of cortisol (a bad thing). Based on these findings, the low glycemic index diet may be the best option. The authors note that the important factor may be lowering the glycemic index (the very low carb diet has the lowest glycemic index), and that this diet is likely easier to stick to long term.
  • Limitations:
    • Short time frame of test diets. We can not assume that the effects on energy expenditure seen in the first few weeks of a new diet will continue long term. Would the very low carb diet have negative health effects in the long term?
    • Calorie controlled setting. It is hard to say how these results would translate to weight maintenance outside of the controlled setting. Would you really eat the same number of calories on any of these 3 diets? Perhaps those on the low fat diet would eat less to compensate for the reduction in energy expenditure.
    • The author's note that although the very low carb diet had the smallest effect on energy expenditure, the severe carb restriction may not be feasible in the long term. Therefore, outside of the calorie controlled setting, this diet may not perform as well.
    • It is still unknown why the very low carbohydrate diet performed better in maintaining energy expenditure. As this becomes better understood, it may be possible to modify the diet so that it becomes more appealing. Perhaps some carbs have a different effect than others. This seems to be partially true since the low-glycemic index diet did much better than the low fat diet. However, the low glycemic index diet does have fewer carbs than the low fat diet. It would be interesting to compare two diets with identical macronutrient breakdowns (fat, protein, and carb), but a different glycemic index.

The take home message
  • Metabolism is complicated.  The notion that "a calorie is a calorie" may not be entirely correct.
  • There are no easy answers, and this study doesn't tell us which diet is the best for weight maintenance in the long term.
  • My best advice for weight maintenance: continue to adjust your calorie intake until you find what works for you (I would have had the same advice before reading this paper).
*Update: I later wrote about a New York Times interview with obesity expert Dr. Jules Hirsch in which he discussed this article. Check it out if you want another perspective.

No comments:

Post a Comment