Obesity is a well-known health risk, contributing to cardiovascular disease, cancer, and diabetes, among others, but some people who are not overweight have the same risks. Those are people with the unhealthy skinny-fat phenotype.
Obesity is associated with insulin resistance, and whether as cause or effect, insulin resistance is probably behind many of the adverse effects of health of obesity.
The causes of both insulin resistance and obesity are hotly debated, but it’s also apparent that insulin resistance can appear in non-obese people, in whom it will increase the risks for disease just like obesity will.
People who have the metabolic abnormalities such as insulin resistance but who are not themselves obese have been termed “metabolically obese, normal weight” (MONW).
In more common language, these people are skinny-fat.
“Phenotype” means simply the observable characteristics of an organism, resulting from the interaction of the organism’s genes with its environment.
“Skinny-fat” refers to a person with a normal body mass index (BMI), who has a low muscle mass and a high amount of body fat.
In the case of a skinny-fat person, BMI does not capture the true health risks associated with that person’s body composition, i.e. the fractions of lean mass and fat mass that make up his or her body.
BMI is a rough-and-ready measure of where someone fits into weight categories, with their attendant health risks, but is actually a surrogate measure for the two most important body components.
The real factors in the relation between body weight and health are
I showed extensively in my book Muscle Up just how and why having a relatively large fraction of body weight as muscle, and a low fraction as fat, is extremely important for health. That doesn’t mean you need to look like or be a bodybuilder, only that virtually everyone should do resistance training to enhance their muscle mass and keep muscles in good metabolic shape.
A skinny-fat person is simply someone whose relative proportions of fat and muscle are skewed in the wrong direction, i.e. too much fat, not enough muscle, while being of a “normal” weight.
People in the highest tertile (third) of body fat had 4 times the risk of metabolic syndrome, the prelude to diabetes, than did people in the lowest tertile.1
For men, that was at a body fat of >23%, and for women, >33%.
When these people were grouped by waist size, the results were even worse. See chart below.
Those in the highest third of waist size had around 60 times the risk of cardiovascular disease than those in the lowest third.
Keep in mind that all of these people had a normal body weight, with body mass index of <24.9. Also keep in mind that by definition, one third of all people (in that study, which should be representative) were in the top third of body fat percentage.
For good health, you must avoid the skinny-fat phenotype.
As is to be expected in any case of body composition, the causes of the skinny-fat phenotype can be boiled down to two: diet and exercise.
Diet: Lack of sufficient protein and excess carbohydrates will tend toward the loss of muscle and the gain of fat. Excess carbohydrates also adds a double whammy in that they will predispose to insulin resistance, the cardinal feature of the metabolic syndrome. In turn that leads to hypertension, abnormal blood lipids, and eventually atherosclerosis.
Exercise: Insufficient exercise also leads to loss of muscle and fat gain. Not only insufficient exercise, but doing the wrong kind of exercise, namely, so-called aerobic exercise or “cardio”.
How do you know whether you’re skinny-fat?
First, you must have a normal BMI, less than 25. If you are over this, you’re just plain fat — although there are exceptions, such as in very muscular people, mainly amateur or professional bodybuilders or other athletes.
The gold standard for determining normal weight obesity, or the skinny-fat phenotype, is body fat percentage, and to be done accurately requires a DEXA scan, which can be done at clinics or hospitals.
A faster, cheaper method is to measure your waist circumference.
In the study cited above, tertiles of waist circumference, from lowest to highest, and in men and women respectively, were:
(See chart above for dimensions in centimeters.)
In terms of body fat percentage, the lowest (safest) tertile for men was <18%, and for women, <27%.
If you fall into any category other than the lowest in terms of waist size or body fat percent, you have work to do.
If you are not already, increase your protein intake to 1.2 to 1.8 grams per kilogram of body weight. For a 70 kg man, that comes to 84 to 126 grams a day. Do this by eating meat, eggs, and dairy. Eating adequate protein increases muscle growth and decreases fat mass.
Cut out the refined carbohydrates and sugar, which lead to increased fat mass.
Forget the aerobics/cardio. Lift weights.
Aerobic exercise can lead to muscle loss, which you’re trying to avoid, and will not put on muscle.
Lifting weights will add muscle and decrease fat. There’s nothing like it.
By lifting weights, you’ll improve your insulin sensitivity in two ways:
As we’ve seen, the top one-third of people in waist size or body fat percent, with a normal body mass index, are at risk for a slew of diseases, through the mechanism of insulin resistance.
So don’t count on the fact of a normal BMI to keep you in good health.
Even if you’re not overweight, you need to keep your body fat percent down.
And you can do this through eating right and lifting weights.