It’s been known for years the actual weight losses from low calorie diets are always less than predicted in both lean and overweight individuals. This is often used by the clueless to argue against calorie based models of weight loss or energy balance. They’re wrong, mind you. But it still raises the question of why this happens: why don’t the obese (people with obesity) lose more weight on low-calorie diets.
Weight Loss from Low-Calorie Diets
As I mentioned above, it’s been long observed that the predicted weight loss from low-calorie diets and the actual weight loss are often significantly different. Quite in fact, they are often one half as much as would be predicted. The question is why.
To address the issue, I will be examining a 2007 paper titled Why do obese patients not lose more weight when treated with low-calorie diets? A mechanistic perspective.
As of the writing of the paper, there was a surprisingly limited amount of research on the topic in the sense of looking at the reasons why the weight loss is so much lower than predicted.
On average, the studies examined showed an actual weight loss of 25-50% of predicted based on the dietary deficit created. I’d 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”t kind of argument that stupid people make.
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. Let me note that there are assuredly other possibilities. These are the three the paper examines.
Reason 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. But, overall the research failed to support this as a reason. Most foods are absorbed extremely well by the gut. Animal proteins are in the realm of 90-95% absorption, dietary fat is 97% and carbs can vary.
Even if nutrient absorption in the gut went up, it wouldn’t change much in the big picture since efficiency is already so damn high. The paper 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.
The conclusion is that this is unlikely to explain why weight loss is so much lower than predicted on low-calorie diets.
Reason 2: A Reduced Metabolic Rate
The second possibility considered was that a reduction in total energy expenditure/metabolic rate is the cause. In addressing this, the paper did it in what I consider a problematic way. Primarily they looked at the difference in resting metabolic rate in post-dieted individuals who were at weight maintenance. And in this situation, the reduction in metabolic rate is usually quite small, maybe 1-5% tops at least in obese individuals. In leaner individuals it can be much much greater.
In the big picture this may only amount to 75-100 calories per day. But it’s also only looking at resting metabolic rate. There are adaptations in other components of energy expenditure as well. So this may be underestimating the overall picture.
But that’s not the big issue to me. Because the drop in energy expenditure after the diet is over is not what is important. Rather it’s the adaptive component of metabolic rate that occurs during the that is reducing weight loss during active dieting. And this is often much larger than what is seen at weight maintenance.
So if during dieting you see not only a reduction in resting metabolic rate but a decrease in calorie burn during exercise and/or a reduction in NEAT, then you will get a much lower rate of weight/fat loss than predicted. But it won’t show up in your calculations if you only look at people after the diet is over.
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. That alone lowers the effective daily deficit which causes actual weight/fat loss to be different than what is predicted based on simple math.
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 on a 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. At some point of course, the body does come back into balance and a plateau will be reached.
Reason 3: Poor Adherence
By the paper’s logic, if food absorption and metabolic rate slowdown can’t explain the low weight loss from low-calorie diets, then adherence is the primary issue. Let me note again that their conclusion about metabolic rate is based on a slightly flawed data set. That doesn’t in any way eliminate adherence as an issue but this is clearly a complicated issue.
This same issue with adherence would also explain the tendency for most to start regaining weight at some point, they simply start reverting to old habits. That said, it’s clear that there are multiple physiological adaptations that occur to promote weight/fat regain after a diet.
Related to this, the review describes a paper in the discussion which used self-reporting of dietary compliance to various diets in terms of results. Noting the limitations of self-reported anything, it found that people who started with low levels of dietary compliance ended up with even worse levels. That alone predicts that many will fail in their efforts.
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. Those people who adhere to their diet lose weight and those that don’t keep it off. By extension, those who stick to their diet in the long-term maintain that weight loss and those that 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 (a topic not discussed is individual variance in how metabolic rate change which could explain the often large differential between two people’s results 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. As I noted above, changes in activity, an especially NEAT are a key component of daily energy expenditure. Leaving that out of the model may have led to a slightly incorrect conclusion.
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 daily deficit reducing the real world weight loss from the predicted level.
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. This is an idea I’ve thumped on about for years and discussed in some detail in A Guide to Flexible Dieting.
Simply, 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. We change the way they eat, we incorporate exercise (ideally), to create a long-term deficit. It may not be simple in practice but it’s simply in premise.
The bigger issue is why we can’t get people to keep the weight off in the long-term. Because while there are clearly biological drivers on weight regain, a major proportion seems to be people’s difficulty at changing their habits in the long-term. Once we figure out why that is the case, we’ll be a step closer to generating permanent weight and fat loss results.