Fat Loss Versus Weight Loss – Bodyfat Vs. Bodyweight

Most individuals starting a diet as well as most diet books tend to focus solely on one measure of progress: changes in bodyweight. The scale has been used for years as the only indicator of whether or not a diet is working. While this is a problem on any diet (for reasons discussed below), it can be of even greater importance when discussing low-carbohydrate diets and low-carbohydrate diet studies, due to shifts in water weight.

To accurately know whether a diet is working or not, we have to be more specific in our measurements than simply bodyweight.

Bodyfat vs. Bodyweight

The primary distinction that dieters should consider is between weight loss and fat loss.

Weight loss is easy

Don’t drink any water for three days and you will lose three to five pounds by the scale. This obviously isn’t ‘real’ weight loss since it returns when you drink water again. Whether they know it or not, most dieters want fat loss to occur.

Fat loss is a more specific type of weight loss

While this seems a trivial distinction, it is not. Without knowing where the lost weight is coming from (fat, muscle, or water), an individual cannot know whether their diet and exercise program is working optimally. Ideally, lean body mass (which includes muscle mass) will increase or stay the same while fat is reduced. In practice this rarely occurs. Any calorie restricted diet will cause the loss of some muscle through a variety of mechanisms, more so if exercise is not included.

Body Composition

More than changes in scale weight, we need to focus on overall change in body composition. Body composition (or bodyfat percentage) represents the ratio of bodyfat to total body weight.

The body is generally divided into two components:

1. Fat mass (FM): the sum of the body’s fat stores
2. Lean body mass (LBM): everything else including bone, muscle, body water, minerals, the brain, internal organs, muscle glycogen, etc.

Total bodyweight (TB) = FM + LBM
Therefore, Bodyfat percentage = FM/TB

A sample body composition estimation

An individual is measured and found to have 15% bodyfat at a bodyweight of 180 lbs.

• He has: 180 lbs * 0.15 = 27 lbs of fat
• 180 lbs – 27 lbs = 153 lbs of lean body mass

For an individual desiring a specific bodyfat percentage, there is an equation that will determine how much fat must be lost assuming 100% of the weight lost is fat and there is no change in lean body mass.

• Desired bodyweight = lean body mass / (1-desired bodyfat)
• Fat loss needed = current body weight – desired body weight

So if our 180 lb., 15% bodyfat individual (with 153 lbs of lean body mass) wished to reach 10% bodyfat, he would have to lose the following:

• Body weight needed = 153 / (1 – .10) = 153 / 0.90 = 170 lbs
• Fat loss needed = 180 lbs -170 lbs = 10 lbs fat loss

Problems with the scale

The primary problem with the scale is that it does not differentiate between what is being lost (or gained) on a diet. With regular exercise, especially weight training, there may be an 87 increase in lean body mass as fat is being lost. Although body weight may not change, body composition is changing.

Let’s say our 180 lb individual at 15% bodyfat begins a basic exercise program of weight training and aerobic exercise. Over the first eight weeks, he gains 4 lbs of lean body mass and loses 4 lbs of fat at the same time. Looking only at weight, nothing appears to have happened. But looking at changes in body composition (in table 1), it is obvious that the program is working.

Similarly, consuming carbohydrates after a period of low carbohydrate dieting will cause a large, but transient, increase in bodyweight from increased body water. This weight gain also should not be confused with true fat gain.

Although the measurement of body composition may not be convenient for many individuals, as it requires special equipment, a similarly objective method exists: how clothes are fitting. Many individuals beginning an exercise program, especially if it includes weight training, will lose inches with no change on the scale. Since one pound of muscle takes up less space than one pound of fat, this reflects a gain in muscle that equals or exceeds the loss of fat. Keep in mind that a large gain in muscle may cause clothes to fit tighter and should not be misinterpreted as fat gain.

*info and tables adapted from “The Ketogenic Diett a complete guide for the Dieter and Practitioner “by Lyle McDonald

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