The Calories In, Calories Out Theory
The calories in/calories out theory of weight management is currently the most widely accepted approach to weight management. Many powerful and influential organizations support this theory. For example:
American Dietetics Association: "Maintaining a healthy weight is all about balancing food intake with physical activity."
US Department of Agriculture: "Calorie balance over time is the key to weight management."
World Health Organization: "The fundamental cause of obesity and overweight is an energy imbalance between calories consumed and calories expended."
Sounds logical enough. Eat a bit less everyday. Exercise a bit more. Even simple lifestyle changes like parking further away from the mall entrance or taking the stairs instead of the elevator should add up to dramatic results.
For example, change your morning latte from whole milk to skim milk and you should be 12 pounds lighter this time next year. Go up the stairs four or five extra times per day and weight loss should increase to 20 pounds. Sounds like an easy and effortless way to drop those last 20 pounds, doesn't it? But this doesn't work.
I am not implying that what we eat and how much we exercise has nothing to do with how fat or how lean we are. If you stuff yourself full of beer and pizza while doing little more than changing positions on the coach from time to time, you will assuredly gain weight.
The opposite is also true. Meal replacement diets and gastric bypass surgeries will both cause dramatic reductions in caloric intake and a related loss of body weight. But these situations are extreme scenarios. On a day in and day out basis, a difference of two, three, or even five hundred calories in either direction will not have a measurable effect on weight.
What Really Happens
So what is the problem with a calories in, calories out approach? Why doesn't it work? Here are three reasons we need to look beyond the eat less and exercise more paradigm.
3) Some calories are wasted.
The calories in, calories out myth relies on many assumptions that are simply not true. One of these assumptions is that the calories we eat are not wasted. But many times they are.
One example is the physiological state of ketosis, which the body enters when you have severely restricted carbohydrates or calories. When blood glucose drops low enough, the body will begin to turn fat and protein into ketone bodies as an alternative source for cellular energy. Excess ketone bodies are excreted in urine and lost through respiration. This represents wasted calories that are not used as fuel or stored as body fat. And if you are overweight, wasted energy is exactly what you want.
Although the body readily excretes ketone bodies, it very efficiently re-absorbs glucose. Even small amounts in the urine are cause for concern.
2) Calories are used for purposes other that fuel or storage.
The calories we eat are often used for other purposes. Fat and protein are used by the body for many other purposes. Protein can be used to make cell membranes, digestive enzymes, and hormones. Fat insulates neurons and is used for cellular growth and repair, among other things. Carbohydrates, however, are primarily burned or stored.
1) Basal metabolic rate does not remain constant.
Our BMR increases when we eat more and it decreases when we eat less. This adaptation happens very quickly in response to fluctuations in energy intake. There are many examples of this in the scientific literature. Here is one example:
In 1964 researchers at the Vermont College of Medicine overfed prisoners. Inmates involved in the study were fed 8000-10,000 calories per day for ten weeks. This represented a calorie excess of 6500 or more calorie per day over the standard 2500 calories prison diet. Using the calories in, calories out model, the prisoners should have gained 130 or more pounds! Yet the average weight gain was 36 pounds. Similar starvation studies also support this idea. But when they were done in the 1940's, a 1560 calorie per day diet was considered starvation. Now, a 40 year old 200 pound male on the weight watchers points program would be expected to eat only 1460 calories.
Speaking of Weight Watchers, the typical person who follows this program for two years loses six pounds. Dedicated participants who attend at least three meetings per month lose eleven pounds. Eleven pounds in two years on a diet low enough in calories that 70 years ago it would have been considered a starvation diet.