Limited-calorie weight loss diet:
NOTE: I am presenting this by way of explanation of the common model. I am not trying to justify this approach, which I think has been very unsuccessful and beyond that, has led to a lot of harm. Still, some people have found this approach useful and prefer this approach over any other.
This is how it is (was) supposed to work in theory.
At the beginning, an educated assessment is made of approximately what number of calories a person may need in a day in order to keep their weight from going either up or down (stable). In order to achieve weight loss, the body must burn some of its fat stores for fuel and, at the same time, not store incoming food energy as new fat stores. The simplistic view (which turns out not to work nearly as well as the theory, and common sense, predicts) is that by having an intake of calories below the amount needed for the day, this will result in burning of body fat stores for energy, thus leading to a reduction in body fat stores over time. This is called a “calorie gap”.
A diet is provisionally-prescribed that sets a limit of daily calorie intake somewhat below the amount thought to be needed for the day. This is just step one and does not, by itself, make for a diet prescription. It is the “beta” trial. At follow-up, the response to the diet is determined. Ideally, it would be easy to do an assessment that would accurately show any changes in body composition, such as loss of body fat (and where) or loss of muscle mass. Don’t we wish. No such luck. Such an assessment technique would make a world of difference to figuring out the best interventions. In reality, an imprecise tool is used, such as repeated measurement of body weight and possibly body measurements, such as waist circumference. As best as possible, a determination is made as to whether any weight has been gained or lost, and how much. Measurements of body composition by scales that calculate body fat percent, body water percent, etc. are much less accurate than we would hope them to be and aren’t useful in figuring out any changes in your body composition from, say, a week or two ago. (Sorry.)
If there has not been weight loss, and at the desired speed, the prescribed number of calories is adjusted down and the person is again seen for follow up, usually after another 2-3 weeks or so. The process of assessment at follow-up visits and calorie prescription adjustments is continued until weight loss (hopefully mostly as fat loss) is proceeding at a satisfactory rate. Of course, this is an idealised version of events – this describes the way things are meant to go. In reality, the situation can be complicated by many factors, two of the chief ones being (a) the person eating more than the set calorie prescription and (b) slowing metabolic rate and/or reduced activity leading to dropping calorie needs.
The “calorie gap” is a theoretical number that we know we can’t calculate or measure accurately, but is never-the-less a useful concept. The key is for all players to know that it is a rough approximation, not presented as an accurate number. It is part of the “try-adjust according to results – try again” model, and therefore accuracy would be convenient but lack of accuracy is what we have to cope with and doesn’t make it useless. There is lack of accuracy in knowing how many calories have been absorbed from the food eaten in a day (input). There is equal or greater lack of accuracy in estimating anyone’s energy output in a day (and how that might change with calorie restriction), so we know that any recommendation for a calorie amount is only a “trial of therapy” and subject to the try-assess-try again approach.
The (estimated, intended) calorie gap can be set at various levels. Often it is set at a level that, as near enough as can be guessed without good tools to use, is about 500 calories. That is, there are 500 calories missing from what they need for the day. Theoretically, if things went according to what we can readily see of the situation (and they often don’t, our tools to monitor metabolism are not up to the task), a 500 calorie gap would produce a weight loss of about one pound per week. Again, not presented as definitive, just an approximation. Most people who have medical reasons to lose weight want to lose weight at least as quickly as this.
The only point in using calorie numbers is to arrive at an eating plan to try out. It is a big mistake to think that these numbers are meant as scientific facts as if we could subject every thing about the person, their environment, their food and their activity to the highest laboratory investigation and scrutiny. It only has to be an estimate. If we had to spend $20,000 investigating every day of every person’s life to come up with accurate numbers, nothing would get done.
So, it is known and understood that the numbers used in the calculations are ever only approximations, and not necessarily very good approximations at that.
The first number in the calculation - ”Calories needed in a day to stay at a stable body weight” is almost always little better than a calculated guess. Yes, in a research setting, or in some clinics with very expensive equipment, measurements can be made that can get quite close to the truth, but even then there is room for error. Calculations exist to come up with a number of calories based on such factors as height, weight and reported daily or weekly activities. These calculations give a number that can actually be in error by hundreds of calories.The second number in the calculation – “Caloric intake” is also a measurement that is problematic in many ways. The real number one is interested in knowing is how many calories are absorbed into the body from the digestive tract, which is not the same as the number of calories calculated to be in the food eaten. This then would have to be adjusted for the energy cost of digestion and the varying energy costs of the metabolic handing of various molecules. (To further complicate things, some people have a talent for “wasting” excess calories, such as by turning up brown fat production of heat. However, we have a name for people like that. We call them slim and they are not likely to be on a medically-prescribed weight loss diet.) The third number in the calculation – “The amount of calorie gap per day that will give an estimated amount of weight loss” – for example, “a 500 calorie gap daily for a week will be enough missing calories to correlate with a one pound weight loss”. This type of calculation is now recognized as misleading and not of much practical use day by day in predictions regarding a specific individual. Doesn’t this, taken together, mean that the concept of calories is too flawed to be of use when it comes to decisions at the level of the individual. Some people have argued that this is true. I am not one of them. That fact that these three numbers all can only be approximated is not, in practice, fundamentally an insurmountable problem. Why? The answer is simple. Just go back to the initial model of making adjustments to the calorie prescription based on feed-back from the person’s body over time. The feedback information is the change (or not) in body weight in response to the changes in calorie intake. The use of this strategy fully compensates for the inaccuracies in the numbers used to arrive at the initial provisional calorie prescription. It is a little like learning to throw a ball at a target on a wall. The outcome of your efforts will teach you. You don’t necessarily need to know the exact distance to the wall or the calculated energy output of your arm muscles. Not being able to accurately state these numbers does not mean that it doesn’t matter how much you increase or decrease the distance to be thrown. No, it is not the difficulties with calorie calculations that is the problem.
The trouble is that, at a 500 calorie gap (or even with less missing calories than that) all people, or most all people, or very nearly totally everybody — gets hungry.
No, that is not quite true. When eating the mix of foods that is generally eaten in western societies (the so-called Standard American Diet), very nearly everybody who eats with a 500 calorie per day calorie gap will get hungry.
Hunger? What of that? Aren’t we supposed to ignore the hunger and “stick to the diet plan”. Isn’t that the advice that pervades society, from agencies and health practitioners alike, repeated over decades? Isn’t the hunger little more than a nuisance, to be ignored or managed as best as possible?
There are rumblings (?angry screaming?) that the persistent, excess hunger (and the accompanying attitude that the hunger can be safely ignored) is an unsuspected, slow-onset, toxic bomb that goes off in the individual and in society over the course of months and years.