by Marion Nestle
Jun 27 2012

Does where calories come from matter to weight maintenance? A new study says yes, but I’m skeptical.

As the co-author of a recent book called Why Calories Count: From Science to Politics, I am well aware of how difficult it is to lose weight.

The problem

  • When you are dieting and losing weight, you require fewer calories to maintain and move your smaller body, and your metabolism and muscle activity—and, therefore, your total energy expenditure–slow down.
  • To maintain the weight loss, you need to eat less than you did before you began dieting.

But what would happen if you could adjust your diet to keep your energy expenditure from slowing down?

Enter Ebbeling et al in JAMA, with a comprehensive study to address precisely that question.  The results of the study and editorial comments on the findings demonstrate how complicated and difficult it is to obtain definitive answers to questions about diet composition and calorie balance.

  • The investigators asked whether calorie-controlled diets containing varying amounts of carbohydrate, fat, and protein, and varying in glycemic load (a measure of rapidly absorbable carbohydrates in foods) affected total energy expenditure in obese people who had just lost 10% to 15% of their weight, but were still obese.
  • They found that the diet lowest in carbohydrate did not slow down energy expenditure as much as did the low-glycemic index diet, or the one lowest in fat.
  • They concluded: “The results of our study challenge the notion that a calorie is a calorie from a metabolic perspective.”

This study took years and involved a very large number of state-of-the-art physiological measurements.

But I want to focus on the question of whether calories from all sources are metabolically equivalent.   Here’s my understanding of the study.

The methods

Ebbeling et al started by offering $2500 to obese volunteers to participate in a 7-month weight-loss trial.   In my view, the 21 subjects who finished the study worked hard for that money.

They had to participate sequentially in a:

  • Weight-monitoring phase for 4 weeks, during which they ate their typical diets while the investigators monitored their weight.
  • Weight-loss phase for 12 weeks, during which they were fed pre-prepared diets calculated to contain about 60% of their usual calorie intake so they would lose about 2 pounds a week.  The average weight loss over 12 weeks was an impressive 14.3 kg (31.5 pounds).
  • Weight-stabilization phase for 4 weeks, during which they were fed pre-prepared diets that provided the reduced number of calories needed to maintain their newly reduced weights.
  • Testing phase of 4 weeks on each of three pre-prepared test diets (total: 12 weeks).  All three test diets provided the number of calories needed to maintain the reduced weight.  During each of the 3 testing periods, investigators measured—not estimated—the subjects’ total daily energy expenditure (resting metabolism plus activity).

The composition of the diets

Weight-loss 45 30 25  Not reported
Low-fat (high-carb) 60 20 20 ~400
Low-Glycemic Index 40 40 20 ~300
Very low carb (high-fat) 10 60 30 ~100

Note that whenever one component of a diet gets changed, the other two components change too.   Because protein usually occurs in foods in relatively low amounts, a low-fat diet is necessarily a high-carbohydrate diet, and vice versa.

The results

  • The weight-loss part of this study showed that when overweight people were allowed to eat only calorie-controlled pre-prepared diets, they lost weight quickly and maintained the weight loss.
  • The test-diet part of the study showed that the diet lowest in carbohydrate (and, therefore, highest in fat) had the least effect in slowing down total energy expenditure.   The diet that slowed down overall energy expenditure the most was the one lowest in fat.

If these results are correct, people eating high-fat, low-carbohydrate diets are likely to have the easiest time maintaining weight loss.  In contrast, people on low-fat, high-carbohydrate diets are likely to have a harder time maintaining weight loss.

But does this mean that calories from different sources have different effects on metabolism?  Proponents of the Atkins (high-fat, low carb) diet say yes, according to an account in USA Today (in which I am also quoted).

I’m still skeptical.  The subjects in this study lost and maintained weight under highly controlled, calorie-restricted conditions, in which the calories came from a relatively low-fat, moderate-carbohydrate, high-protein diet (average diets contain 10% to 15% protein).

The accompanying editorial notes that heat losses are greater for protein than for carbohydrate or fat, and also raises questions about whether physical activity declined more with the low-fat (high-carb) diet than the others.  It also notes:

Each diet was consumed for only 4 weeks. A weight stabilization protocol…may not have adequately accounted for changing energy needs associated with readjustment to new diets.

These provocative results…emphasize the current incomplete knowledge base regarding the importance of dietary macronutrients and energy expenditure, especially after weight loss.

Under the relatively short, highly controlled feeding conditions of this study, the composition of the diet may indeed matter to metabolism.  But does diet composition matter for weight maintenance in the real world?

Other longer term studies of “free-living” people out and about in their communities show little difference in weight loss or maintenance between one kind of diet and another.

More research needed!

The bottom line

  • If you want to lose weight, eat less (it worked well for the subjects in this study).
  • It may help to avoid excessive consumption of sugars and easily absorbed carbohydrates.
  • Once you’ve lost weight, adjust your calorie intake to maintain the weight loss.
  • And understand that science has no easy answers to the weight-loss problem.
  • Brad,

    Your comment seems right, but misses my point. I was questioning the accuracy of Dr. Nestle’s summation of the outcomes of this study.

  • Ouch … egg on my face … after reading ten JAMA study and looking again at Dr. Nestle’s chart adn summation … she is right … I misread her table and the ratter confusing descriptions of the outcomes in initial press reports.
    Mea culpa!

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  • Meg

    As someone who has maintained a 70 lb weight loss for 4 years on what would likely be classified as a high fat/lower carb diet, I have to agree with the findings. And losing the weight by changing to that way of eating was not deliberate. I was diagnosed celiac and just didn’t bother to start eating substitutes for the foods I gave up. I ate more meat and vegetables instead. The weight just fell off and hasn’t come back. And I haven’t been in a test environment. The only controlled thing about my diet is what I can’t eat because it will make me sick.

  • @Calorie
    What it means is that a calorie of protein will generate the same energy when metabolized in a living organism as a calorie of fat or carbohydrate. When talking about obesity or why we get fat, evoking the phrase “a calorie is a calorie” is almost invariably used to imply that what we eat is relatively unimportant. We get fat because we take in more calories than we expend; we get lean if we do the opposite. Anyone who tells you otherwise, by this logic, is trying to sell you something.

  • El Cid

    Even if it’s the case (or could be the case) that the sources of calories changes metabolism or such, for the vast majority of people wouldn’t it be the base-level starting point that *whatever* the relative advantages of this or that calorie source the overall caloric intake must be decreased in order to lose undesired or (measured or assumed) unhealthy weight?

    The actual scientific research about diets and metabolisms etc. is obviously of its own importance, whatever the direct applicability might or might not be.

    But there is an aspect of this which has people thinking (and the sales in the diet industry apparently support the notion) that they need to find some perfect or ideal diet or nutrition plan.

    Different things work better and worse for different people, and at any point it may be clearly proven that different food sources have wildly differing metabolic effects on all or some people, but it would seem to be quite urgent to get most people to understand that until that perfect day comes along it will be necessary to know how much you likely may need to eat each day and what are you or should you be eating to more properly stay within that desired range.

    I.e., maybe there is a perfect diet for one or some or most persons, but somehow or other fewer calories have to be taken in or radically higher energy expenditures have to be made, the latter of which I think is extremely improbable for all but a few people.

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  • Vivek Spv

    Awesome article. My wife and I just published an article on PCOS diet. Will link to this this article. I loved the post about low GI (glycemic index) foods