Section 1 Fat loss versus Weight loss

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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 (that are discussed in later chapters), more so if exercise is not included. In fact, if a person loses weight without exercising, over half of the total weight loss is muscle and water, not fat.

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):

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.


Bodyfat percentage = FM/TB

The ultimate goal in dieting is to see a drop in bodyfat percentage primarily through a decrease in fat mass. Increases in lean body mass will also cause bodyfat percentage to decrease.

A sample body composition estimation

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

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 (2).

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 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.

Table 1: Changes in body composition




Total weight (lbs)



No change

LBM (lbs)




Fat (lbs)




% bodyfat




Individuals beginning a weight training program often gain one or two pounds by the scale from increased water storage in the muscles. This weight gain is temporary and should not be confused with true fat gain. 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.

Subcutaneous vs. Essential fat

Before discussing how body composition is measured, it is necessary to know where the bodyfat is located. Total bodyfat is typically divided into subcutaneous fat (under the skin) and essential fat (in the spine, brain and around the internal organs). Average levels for these two types of fat appear in table 2 below along with the bodyfat levels recommended by health organizations (1). Bodybuilders are included as a reference point only for those individuals seeking extreme levels of leanness. In all likelihood, maintaining this level of leanness is unhealthy, especially for women.

Table 2: Comparison of bodyfat levels


Essential fat (%)

Average bodyfat (%)

Recommended (%)









Male bodybuilder




Female bodybuilder




Measuring bodyfat (3)

Over the years, many methods have been used to measure bodyfat. The method currently considered the 'Gold Standard' is underwater weighing. Based on the fact that fat floats better than muscle (technically, fat is less dense than muscle), bodyfat percentage can be estimated by weighing an individual on ground and again when submerged underwater.

Many assumptions and estimations are made during underwater weighing. Although the subject is instructed to exhale all air from their lungs, there will always be a small amount left. A correction for this residual air must be made. Additional estimations are made on the density of bone, fat and muscle but these vary greatly depending on race, age, activity level, etc. Many of the assumptions which are being made for underwater weighing have been found to be incorrect (3).

The other methods of bodyfat measurement are generally based on underwater weighing. Descriptions of the most common methods of measuring body composition appear below.

Girth measures

Girth methods of bodyfat estimation typically use weight and several girth measures (i.e. circumference of the waist, wrist, or hips) to estimate bodyfat. While these measurements are fast and easy to perform, they are notoriously inaccurate as they cannot distinguish between an individual with excess fat, excess muscle, or simply a large bone structure.

For example, an equation which used hip circumference would vastly overestimate the bodyfat percentage of an individual with genetically wide hips, even if that person were very lean. Although girth measures can provide another measure of progress on a diet, they should not be used to estimate bodyfat percentage.

Bioelectrical impedance (BIA)

BIA estimates bodyfat by running a small electrical current through the body based on the fact that muscle, fat and water conduct electricity at different rates. With several other variables (height, age, weight), the BIA machine will estimate body fat percentage. BIA can be severely affected by hydration state. Due to changes in hydration during ketogenic diets (especially cyclical ketogenic diets), BIA is not recommended.

Infrared Reactance (IR)

Infrared reactance attempts to measure bodyfat thickness (and by extension estimate overall bodyfat levels) by measuring the scatter of light through the tissue. Little research supports its use or accuracy. IR is not recommended.

Skinfold calipers

Because the majority of bodyfat is under the skin, an individual's total fat can be estimated with skinfold measurements taken with calipers (spring loaded pinchers). Three or more measurements are made at specific sites on the body which are entered into an equation with weight, age and gender to estimate bodyfat. The estimation equations are accurate to within plus or minus 5% (3) and tend to become less accurate at extremes of both leanness and fatness. Although current research is questioning the overall accuracy of skinfold equations (3,4), they are still considered the most accurate of the methods described above.

Due to the inherent inaccuracies involved in caliper estimations, it is suggested that regular skinfold measurements be used to track relative changes rather then to provide absolute measurements of bodyfat percentage (5). Table 3 compares the two skinfold measurements below.

Table 3: Comparison of skinfold measurements

Skinfolds in millimeters
















We can see that this person has lost bodyfat, irrespective of the estimated bodyfat percentages. Individuals using regular skinfold measurements should develop the habit of just comparing measurements rather than relying on estimations of bodyfat.

With regards to bodybuilders (arguably the athletes most concerned with bodyfat levels), a caliper measurement of 3-4 millimeters represents essentially zero subcutaneous fat. However experienced bodybuilders know not to rely only on the numbers, but rather on appearance. Striving for excessively low skinfold measurements may decrease muscle size, fullness and symmetry. Ultimately, bodybuilders aren't being judged on who has the lowest bodyfat level. Rather they are judged by who looks the best on stage.


Although most dieters rely solely on the scale to gauge the progress of their diet, this method leaves much to be desired unless coupled with another method of measurement. Ideally dieters should keep track of their body composition changes which give a better picture of what is being lost and gained during the diet (i.e. muscle or fat). Although no current method of measuring bodyfat is perfect, skinfold calipers seem to be the best choice, especially considering the fluid shifts which can occur on a ketogenic diet. With practice, most individuals should be able to keep track of their own skinfold measurements at home with a pair of inexpensive calipers.

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