Body Weight and Body Fat Control


  1. Introduction and General Issues
  2. Why Body Mass Index (BMI) is irrelevant
  3. Energy Use and Energy Intake (Calories)
  4. The Basal Metabolic Rate and the Yoyo effect

1. Introduction and General Issues

Weight control in the society can have several motivations like social norms, aesthetics, and also, as we can see from the resources cited in the health and diet page: health. Being overweight, or obese is indeed a risk factor for many diseases, including cancer, cardiovascular problems and diabetes.

Being overweight, in relation to social norms, carries some social stigma, and therefore often involves, at the psychological level, feelings of guilt and shame. It is known in psychology that these feelings of guilt can have counterproductive effects for an individual to work towards a solution, be it on the self-acceptance level, on on the physical level. These questions can also lead to other weight related problems or pathology with underweight people.

Somme argue that those social norms about weight, which often impose harshly especially on women and girls, are "natural" beauty criteria. It should be noted that those criteria are dependant on the society. From the point of view of evolutionary approaches to psychology, it can be argued that an "attractive" human (with whom you are most likely to desire breeding) is a healthy human. From that perspective, a "naturally attractive" body must be a healthy body, and therefore it cannot be underweight. From that perspective too, avoiding having too much fat should be achieved through a combination of physical activity and a balanced diet, rather than by food intake restrictions only.

At last, if you return to your original lifestyle after losing weight, you will revert to your original state, sometimes even worse, as can be understood by considering The Basal Metabolic Rate and the Yoyo effect. So, weight control should be considered a permanent change of lifestyle, rather than going through a diet stage in the spring and revert later...

2. Why Body Mass Index (BMI) is irrelevant

The Body Mass Index (short:: BMI) is a commonly used measure to determine if that person is, medically speaking, "normal weight", "underweight", "overweight", "obese", etc. The BMI is calculated from the height of a person and the weight of that same person.

BMI = weight(mass in kg) divided by (height in m)2
(that is: weight in kg divided by squared height in meters)

The BMI, so computed, is often used by insurance companies to assess risk for disease, since being overweight/underweight is a risk factor for many health problems.

It is fairly common, among people who can afford the time to make money as a coach, to post before/after photographs to show how they lost weight. I'll post this same before/after photographs, but only to point out that my weitght is more or less the same on the two pictures.

The same person with roughly the same BMI
Left: 2006; Right: 2018

In particular, since my size hasn't changed much either, I have the same BMI on the two pictures and, by using the BMI based definition of obesity, I'm almost obese on those photographs!

The problem comes from the fact that your weight is roughly the sum of the weights of your bones, the fat, the muscles (including the water within them). So, if, through exercise, you change some fat into muscles with the same weight, your overall weight (and therefore your BMI) doesn't change.

This shows that a far better measure do decide whether a person is "normal weight", "underweight", "overweight", "obese" would be to consider the silhouette of the person, rather than doing the maths with the BMI. A more precise and numeric measure is provided by the body fat percentage in the body, which can be evaluated through the electrical impedance of the body.

3. Energy Use and Energy Intake (Calories)

Before continuing with this section, let me make clear that:
your body needs fat!!!

Energy is what you need to create motion with your body and heat in your body. For example, when you do some weight lifting, you need some energy to lift your limbs, to lift the weights, and there is some "wasted" energy in the form of heat, which will eventually be evacuated through vaporisation of your sweat (see enthalpy of vaporisation)

So, as your body uses energy, be it going about regular business or while exercising, you need some energy input, and it comes in a chemical form when you eat. The amount of energy contained in the different foods you eat is measured as kcal (beware that there are variants for Calories units)

If you eat significantly more calories than you spend daily, you will gradually put on weight. If you eat significantly less calories than you spend daily, you will gradually lose weight. So, we should mind the calories that we eat when trying to control one's weight. Now, yet another hurdle: different kinds of food don't contain at all the same amount of calories.

The following table shows examples of the amount of Calories contained in 100g of some fat, some starchy food, and some green vegetable. As we can see, the fat contains more calories than the starchy foods, which contains more calories than the vegetable.

Kind of food Energy for 100g
Butter 715 kcal for 100g
Wheat bread 270 kcal for 100g
Spinach 26 kcal for 100g

To conclude on this, reducing calories is not about reducing the amount of food in your plate, but attention should be paid to the kinds of foods that you put into your plate.

In fact, you could have as much vegetables (such as spinach) as you want (or as you can afford) without putting on any weight or fat. It will bring you a lot of good nutrients and will also make you feel sat (prevent you from feeling hungry because your stomach if full) and prevent you from any snack urge... Here is another section about what you need and what you don't want in your plate generally.

4. The Basal Metabolic Rate and the Yoyo effect

The Basal Metabolic Rate (short: BMR) if the amount of energy per unit of time (for instance in kcal per minute) that your body uses to maintain its functions while you are at rest (e.g. lying on your bed at some given temperature). The energy that you spend everyday can be obtained as the sum of your BMR, plus the energy you need to digest food, plus the extra energy you need for efforts (motion, intensive though or emotions, exercise).

Hence, when thinking about eating less/more energy than we spend to control our weight, attention must be paid to our BMR. Now, our BMR depends more or less directly on our muscle mass, as can be seen for example in the Katch-McArdle formula for the Resting Daily Energy Expenditure.

If we try to lose weight through food intake restrictions only (whatever the diet, but without exercising), we lose not only fat, but also muscle mass. Therefore, our BMR decreases. As a consequence, we should eat even less in order to only stabilize our weight (keep it as it is). That is the main reason for the so called "yoyo effect", by which when trying to lose weight by food restriction we end-up putting on even ore weight when returning to a regular diet.

As a consequence, we see that we can durably control our weight only by a combination of Calories control and exercise to maintain (or increase) our muscle mass, so that our BMR does not decrease.