Carbs, Calories, Weight, and Diet Quality: What A New Study Really Means

A fascinating and well-run randomized trial of 164 adults just published in the BMJindicating that eating a low-carbohydrate diet for maintenance of weight loss can increase energy expenditure has predictably generated widespread media attention. Some of the highest-profile entries in that barrage of brief attention, notably coverage in The Chicago Tribune and in The New York Times, massively misrepresent the study findings.

Both of these articles about the study play to, and propagate, the popular fantasy that when the right magical formula comes along, weight loss can be uncoupled from calories. That contention, made in the media coverage of the study, but not in the well-written study paper itself, is completely at odds with how the study was done. 

The authors enrolled overweight adults at the start, and put them all on the same weight loss diet. How did they get people to lose weight? By restricting calories, as they state clearly and explicitly in their own words: “During the run-in phase, energy intake was restricted to promote 12% (within 2%) weight loss over 9-10 weeks.”

Participation in the new study was then limited to those who achieved the intended weight loss, making the study cohort a somewhat rarefied group, as is often the case in randomized trials (an important limitation routinely overlooked by those who clamor that RCTs are the most robust way to answer all nutrition questions). How rarefied? Again, in the authors’ own words: “Of 1685 people screened, we enrolled 234 participants for the run-in phase. Of these, 164 achieved 12% (within 2%) weight loss and were randomly assigned.” So, of those interested enough in the study to turn up for screening, only 10% ultimately “qualified.” 

Since both the authors, and the media, suggested ways the study findings could confer benefit in the “real world,” this is a crucial point. The study participants represent some very small fragment of the general population, and so extrapolating from the small minority of people who could pass through the study’s eligibility filters to the rest of us is fraught with the customary peril and doubt (for those who care to know, this is a general concern called “external validity”).

Back to the issue of calories, the researchers again turned to calories when it was necessary to adjust weight during the intervention. In words copied directly from the research paper: “We randomly assigned participants who achieved the target weight loss to high, moderate, or low carbohydrate test diets for a 20 week test phase. During the test phase, participants’ energy intake was adjusted periodically to maintain weight loss within 2 kg of the level achieved before randomization.”

Perhaps, in light of all the silly noise being made about calories, that last bit bears repeating one more time: “… participants’ energy intake was adjusted periodically…” In other words, even in this study looking explicitly at the metabolic effects of carbohydrate, when it was time to induce weight loss, or make sure it was maintained, the researchers turned to calories both times.

So much for the silly, self-serving, and largely misleading media coverage; what of the actual study findings? Essentially, the researchers randomized this group maintaining weight loss to diets varying in total carbohydrate across three levels: 60%, 40%, and 20%. The paper itself did not include details about the diets; those were published elsewhere. I have not yet tracked down those particulars, so thus far, like other readers of the study, I know little to nothing about the composition and quality of the diets. First the study results, and then we’ll return to consider diet quality.

The study reported that total energy expenditure, using a lab technique called doubly labeled water, was higher when carbohydrate was reduced. The magnitude of the effect- getting up above two hundred calories daily- was greatest when carbohydrate restriction was greatest, and among those with the greatest adherence, and among those with the least insulin sensitivity (i.e., highest insulin levels) at baseline. 

This increase in daily energy expenditure was not attributable to resting energy expenditure (the energy cost of keeping all of our cells alive, essentially) – and so must be due to…something else. The authors could only speculate where this energy was going, since the study did not reveal that. Importantly, while noting the differences were not due to exercise, the paper did not report exercise in detail. Nor did it report on body composition. An increase in muscle mass- from resistance exercise- is known to raise energy expenditure, so body composition measures were an unfortunate omission. No study, no matter how good, can measure and report everything of interest and relevance, of course. The authors do note that “total physical activity, and moderate to vigorous intensity physical activity were marginally higher in the group assigned to the low carbohydrate diet.”

The signature finding of this study is that energy expenditure- by means as yet unknown- increases as carbohydrate intake decreases. That is not a refutation of the importance and relevance of calories and energy balance, but rather a reaffirmation of it. Calories count, and if you want to lose weight and keep it off- you need to burn first more than, and then as many as, you take in daily. If a lower carbohydrate diet helps to increase calories “out,” it would allow for weight loss and maintenance, at higher levels of calories “in.” An increase in energy expenditure is not, therefore, the “prize.” The prize would be an easier way to lose weight, and keep it off.

The study authors speculate at some length about just this implication of their paper, suggesting that reducing carbohydrate might, and apparently should result in better success with long-term maintenance of weight loss. If this were our first foray into the low-carb rodeo that might be the novel, provocative concept the media- ever interested in titillating us with diet magic- are making it out to be. But, alas, it is not.

A vast amount of information about weight, in populations and individuals, in clinical trials and out in the world, shows no such effect. Millions upon millions of Americans have gone on every conceivable kind of diet over recent decades, including many versions of a low-carb diet since the 1970s at least; but weight loss maintenance is the exception not rule. 

Despite the sequential popularity of many branded low-carb diets (e.g., Atkins, South Beach, etc.), obesity prevalence for the overall population only continues to worsen. In RCTs that compared diet types, those assigned to low carb gained back weight just as readilyas those in all other diet assignments. 

So, this interesting new study in the BMJ shows a metabolic effect. The report then speculates about the real-world benefit of that metabolic effect. The sad reality, though, is that we have been looking for that benefit for decades, and it does not tend to turn up, or is negligible over time. There is, therefore, a new set of interesting and valid research questions to pose now: why does the metabolic advantage associated with carbohydrate restriction in this well-conducted study NOT show up as a sustainable, real-world weight-control benefit? Is it because the metabolic effect wears off not long after the 20-week duration of this study? Is it because people don’t maintain a carbohydrate restricted diet? Is it because of compensatory metabolic mechanisms unassessed in this study? For now, all we can say is: we don’t know, and answers would be of value.

What actually does work for maintaining weight loss in the real world? One of the best sources for this answer is The National Weight Control Registry, which has been aggregating data from successful weight-loss maintainers for years. If you are a seeker of dietary magic, you aren’t going to like the answer: the people most successful at maintaining weight loss for the long haul eat sensible, balanced, carefully portion-controlled diets, and exercise routinely. Sorry.

Lastly, let’s return to the matter of diet composition and quality, noting in passing that diet quality is all about foods, and not at all about macronutrient thresholds. A diet rich in diverse leafy greens, avocado, nuts, legumes, and wild fish is one way to get to “low carb;” nothing but pepperoni would be another. So, too, for low fat, which can be achieved by eating an optimal, balanced array of natively low-fat foods- vegetables, fruits, whole grains, beans, legumes, and so on- or by living on Coca Cola and cotton candy. So, no, macronutrient threshold is not a reliable indicator of diet quality; is largely uninformative; and is prone to all manner of mischief, manipulation, and misdirection.

How does that pertain here? A recent study called DIETFITS showed no difference in weight loss among adults assigned to low-carb and low-fat diets, both put together well (a very rare achievement, since with most such comparisons, researchers favor one over the other and thus surrender to the temptation to put one together well, and the other…not so much. These “straw man” diet assessments are highly prevalent in the peer-reviewed literature, alas.) The authors of the new paper mention DIETFITS, and say the following: “In that study, however, which relied on nutrition education and behavioral counseling, participants were instructed to ‘minimize or eliminate refined grains and added sugars and maximize intake of vegetables’ and other minimally processed foods. Probably for this reason, the reported glycemic load of the low-fat diet was very low for a diet that is by nature higher in total carbohydrate, and similar to the value for the lowest glycemic load diets in some previous intervention studies. Thus, the effects of predisposing risk factors might be attenuated on diets that are generally healthy and specifically low in glycemic load.”

Well…wow. Those few lines basically say that good effects might be seen with good diets. They suggest that the same advantages might issue from a high-quality diet- wholesome foods in some sensible combination- high in carbohydrate but low in glycemic load, as from a good diet low in carbohydrate. When I reviewed this matter extensively, that was my conclusion. Studies have examined this specifically and reached that same conclusion, but further suggested greater cardiometabolic benefit from high-quality, high-carbohydrate diets. This is maybe not surprising when one considers that all whole plant foods are carbohydrate sources. Of note, the new BMJ study did not report on biomarkers for cardiovascular risk.

I think this new paper in the BMJ is of very high quality, and very intriguing. Might macronutrient shifts, in the context of wholesome diets, shift energy expenditure? Might there be more than one to achieve this? Do such effects generalize, or only pertain to a select few? Are such effects maintained, or blunted over time by compensatory mechanisms? Important, worthy questions, all.

But it was calories these very researchers relied on when they needed their study participants to lose weight. Yes, calories still count. Restricting carbs may be advantageous for some, but making sure a high-carbohydrate diet is of high quality may work as well, or better. These statements are concordant with the findings in this paper- they just make poor click-bait.

Calories do count, even if counting them is not the best way to control how many you consume; for that, focus on eating high quality foods. Macronutrient thresholds tell us little about diet quality, and diet quality is what matters most to health outcomes over a lifetime. There is a mechanism suggested here by which restricting carbohydrate could help maintain weight loss, but weight loss has not been maintained in most of the many millions who have tried low-carb diets. Weight loss is generally maintained most successfully by those who eat sensibly and with care, and exercise routinely.

Pepperoni isn’t good for you. Donuts aren’t good for you either. But maybe diet-study-headline-whiplash is worse for you still.

Dr. David L. Katz

Author, The Truth about Food. All book proceeds go to support the True Health Initiative, a federally authorized 501c3 non-profit.