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Why Do Obese People not Lose More Weight When Treated with Low-Calorie Diets?

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Heymsfield SB et. al. Why do obese patients not lose more weight when treated with low-calorie diets? A mechanistic perspective. Am J Clin Nutr. 2007 Feb;85(2):346-54.


Maximal weight loss observed in low-calorie diet (LCD) studies tends to be small, and the mechanisms leading to this low treatment efficacy have not been clarified. Less-than-expected weight loss with LCDs can arise from an increase in fractional energy absorption (FEA), adaptations in energy expenditure, or incomplete patient diet adherence. We systematically reviewed studies of FEA and total energy expenditure (TEE) in obese patients undergoing weight loss with LCDs and in patients with reduced obesity (RO), respectively. This information was used to support an energy balance model that was then applied to examine patient adherence to prescribed LCD treatment programs. In the limited available literature, FEA was unchanged from baseline in short-term (<12 wk) treatment studies with LCDs; no long-term (>/=26 wk) studies were found. Review of doubly labeled water and respiratory chamber studies identified 10 reports of TEE in RO patients (n = 150) with long-term weight loss. These patients, who were weight stable, had a TEE almost identical to measured or predicted values in never-obese subjects (weighted mean difference: 1.3%; range: -1.7-8.5%). Modeling of energy balance, as supported by reviewed FEA and TEE studies, suggests that obese subjects participating in LCD programs have a weight loss less than half of that predicted. The small maximal weight loss observed with LCD treatments thus is likely not due to gastrointestinal adaptations but may be attributed, by deduction, to difficulties with patient adherence or, to a lesser degree, to metabolic adaptations induced by negative energy balance that are not captured by the current models.

My Comments

Over decades of their use, the simple fact is that low calorie diets (LCDs) mostly fail. And I’m not simply talking about the fact that most people will regain any weight lost. The simple fact is that even the total weight loss seen with such diets is often fairly small. A total loss of 5-10kg (roughly 10-20 lbs) over a year’s span is a typical result. And while that might be great for someone who who is lean, for someone at 250-300 lbs, it’s not terribly significant.

Tangentially I’d note that from a health standpoint, even small weight losses of 5-10% of body weight can improve various health and metabolic  markers. Even if most people probably won’t be satisfied with that rate of loss.  One of the questions is why the results are so poor, especially given that, based on the caloric deficit created, a much greater loss should occur.

Setting out to examine the (surprisingly) limited amount of data on this topic, this paper first examined representative weight loss studies of LCD’s. Although the study sample was far from comprehensive, they typically showed a weight loss of anywhere from 25-50% of what was predicted based on the deficit created. I should note that this included both high and low-carb diet studies so this shouldn’t be taken as a ‘the calorie theory of weight loss is wrong’ kind of argument.

The study, thankfully mentions that there are three different ways of setting up a deficit, something I’ve discussed variously. The first is by making an absolute reduction in food intake (i.e. 500 calories/day). The second (my preferred method) is to reduce food intake by some percentage (for example 20%) below baseline. The third, and worst in my opinion, is to use an absolute level such as ‘Women get 1200 calories/day and men get 1700 calories/day’.  I discuss this topic in some detail in my first book The Ketogenic Diet.

The paper then set out to examine three major possibilities (based on a host of assumptions that they discuss in some detail but that I’m going to skip over) for the failure of actual weight loss to reach what is predicted.


Possibility 1: An Increase in Food Absorption

The first possibility considered is that there is an adaptation towards increased absorption of food in the gut with dieting. However, research fails to support this contention. Absorption rates of ~95% or so are seen for most ingested nutrients and this is true in both the pre- and post-obese (dieted down individuals). The study also points out that even a 5% increase in food absorption would only amount to about 100 calories per day extra which still wouldn’t be able to explain the observed results.  So possibility 1 is unlikely to be the cause of the observed results.


Possibility 2: A Reduced Metabolic Rate

The second possibility considered was that a reduction in total energy expenditure is the cause. That is, some studies have shown that individuals who are actively dieting have a lower than predicted metabolic rate (as well, calories burned during activity goes down as you lose body mass) than individuals at the same weight who aren’t/haven’t had to diet.

However, and I’ll come back to this below, the study chose to examine studies looking at post-obese individuals at weight maintenance in their analysis. And such studies invariably show a fairly small difference (maybe 1-5% tops, I should note that these are studies in obese individuals, much greater drops have been seen in leaner individuals) between predicted energy expenditure and actual energy expenditure in the post-obese.

However, I think this is misleading because it’s the drop during active dieting that would be contributing to the difference in actual versus predicted weight loss. In their defense, the researchers did address this in the discussion but why they chose this particular data set for their analysis I’m not sure.

The authors do thankfully note that the work they examined on total energy expenditure in the weight stable state isn’t exactly the same as what happens during active dieting. Numerous (but not all) studies show a drop in energy expenditure (both in terms of basal metabolic rate as well as overall activity, people tend to move less when they diet) during active dieting.

Additionally, there is work showing a fairly large variability in this (a topic I’ve discussed at length before). In one study I recall, subject’s metabolic rates dropped anywhere from 50-200 calories/day by week 2. Clearly the subjects with the larger metabolic rate drop are going to get less weight loss over time because their net deficit is smaller.

Put differently say you put two people on a 500 calorie per day deficit and one person shows a drop in metabolic rate of 50 calories/day while another shows a drop of 200 calories/day.  The first is still maintaining a 450 cal/day deficit, the seconds deficit has been cut to 300 calories.  Over a month’s time that will add up to 1.5 pounds difference in fat loss between the two despite being on ‘the same diet’.

At the same time, the researchers point out that “..changes occur in EE (energy expenditure) that could reduce the prescribed energy deficit, but these decrements in EE appear to be tied to the energy deficit, the rate of weight loss, or both, and thus they would slow weight loss but not result in a premature plateau because, by definition, that plateau occurs at the point of energy balance.”

Which is a convoluted way of saying that, even if you’re ona 50% daily deficit, and energy expenditure drops by 25%, that’s still insufficient to STOP weight loss. Because you still have a 25% deficit. The drop will simply slow things down.  That is, the decrease in metabolic rate with dieting can only ever slow the rate of fat loss, it can’t ever stop it completely.


Possibility 3: Adherence

Thus, by default, they basically conclude that the failure of LCD’s comes down to problems with adherence. That is, people lose less weight on the diets because they are not really following them. This would also explain the tendency to start regaining weight at some point, people start reverting to old eating habits.

Related to this, in the discussion, they discuss a paper which used self-reporting of dietary compliance (relative to Zone, Atkins and carb-based diets) in terms of results. The first observation in that study is that the subjects who started with low levels dietary compliance ended up with even worse levels. Put bluntly, the people who started the diet half-assed, ended up doing the diet all-assed.

Additionally, the subjects reporting the greatest dietary compliance lost 20kg (45 lbs) whereas the low compliance folks lost negligible weight.  It’s a conclusion that is unbelievably obvious but really bears making explicit: people who actually follow/stick to a diet lose weight and those who don’t don’t.



So overall, it appears that the major determinant of overall diet success is simply one of adherence.  Changes in food absorption can’t explain the difference between predicted and actual weight loss and, with the caveats mentioned above, it doesn’t appear that metabolic rate can either (although individual variance in metabolic rate change might explain differences between individuals on the same diet).

The researchers did acknowledge the limitations of their study, the limited amount of data, the short-time frames over which most studies are done (a year or less) and the fact that they didn’t examine exercise/physical activity in terms of how it impacts or changes during dieting.

This is important for a number of reasons not the least of which is that some people will start to decrease their daily activity (subconsciously) on a diet, reducing their daily activity energy expenditure.  While this wouldn’t show up as an actual measurable drop in resting metabolic rate, it would still serve to reduce the net deficit.

The paper concludes:

A critical need therefore exists for elucidating the basis of poor patient adherence to prescribed energy deficits. This research effort could lead to even greater clinical benefits for the many obese patients with weight related comorbidities.

Basically, the question is no longer one of what type of diet to put someone on, it’s a question of how to actually get people to stick to a diet in the long-term.  I couldn’t agree more and expressed that very opinion in A Guide to Flexible Dieting.

After decades of research, we know about pretty much all there is to now about dieting and fat loss in my opinion at this point.  We know how, from a biological standpoint, to get people to lose weight and/or fat.

I think the bigger question is figuring out why people are so bad at changing habits in the long term or the short-term for that matter. Until that question is answered, it seems that only the small percentage of people willing to change their habits and, most importantly, keep them changed FOREVER are going to succeed.

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