“Weight loss” is a misnomer: when people want to lose weight, it means they want to lose fat. Unfortunately, when fewer calories are consumed than expended, along with fat, muscle is lost as well. This is why resistance training is critical for weight loss.
Typically, one quarter to one third of the loss of body weight is comprised of muscle. This is to be avoided at all costs, since it lowers the metabolic rate, since muscle is a highly metabolically active tissue, and fat is not.
Furthermore, as I documented extensively in my book, Muscle Up, by virtue of being so metabolically active, muscle plays a large role in glucose uptake, and hence in insulin sensitivity. Good metabolic health requires at least normal muscle mass and healthy muscle tissue. Generally, a greater-than-normal muscle mass is even better for health. Health risk due to high body mass index is due solely to excess fat, not muscle.
A recent study looked at the effect of a normal protein, low calorie diet versus a higher protein, low calorie diet, on overall weight loss and muscle loss. (1)
The subjects were all overweight, and all consumed diets restricted in calories by 25%, which is a considerable amount.
One group consumed normal protein at 0.9 grams per kilogram of body weight, and the other consumed high protein, at 1.7 grams per kilogram. The latter protein consumption is similar to what a bodybuilder might eat.
Results: both groups lost the same amount of weight, about 9 kg, and both groups lost the same amount of muscle, about 2 kg. Leg strength declined equally in both groups.
A 2 kg (4.4 lbs) loss of muscle is a large one, and likely significantly impairs health.
The groups in the above study performed no exercise during their weight loss. Could exercise help preserve muscle?
Yes, but only the right kind of exercise, strength training (resistance training, weightlifting).
Aerobic exercise does not preserve muscle during weight loss.
A study was designed to find any difference in terms of muscle mass between aerobic and resistance exercise during weight loss: Effects of Resistance vs. Aerobic Training Combined With an 800 Calorie Liquid Diet on Lean Body Mass and Resting Metabolic Rate. (2)
The subjects ate a very low calorie diet at 800 calories a day, and it consisted of 40% protein. One group did aerobics – walking, cycling, and stair climbing. The other did resistance training: “3 days/week at 10 stations which included four lower body and six upper body exercises for 12 weeks”.
The aerobics group actually lost quite a bit more body weight than the resistance training group, 18 kg vs 14 kg. Most people might think that this gives the advantage to aerobic exercise.
But there’s a catch.
The aerobics group lost 4 kg of muscle. The resistance training group lost none.
All of the weight lost by the aerobics group in excess of the resistance training group was muscle.
Will the aerobics group ever regain that muscle? Seems doubtful. This is probably a permanent loss for them. Even if they regain weight, it might be almost all fat tissue.
The conclusion has to be that if you lose weight without doing resistance training, you’re damaging your health.
Besides helping retain muscle during weight loss, resistance training and bodybuilding can help one stay lean and avoid fat gain to begin with, and that’s due to higher muscle mass.
Check out the following chart:
The chart shows two things: 1) there’s a straight line correlation (r=0.89) between the amount of muscle mass and the resting metabolic rate; 2) even with the same muscle mass, bodybuilders burn more energy than controls.
So having more muscle mass from lifting weights greatly increases 24-hour energy expenditure.(3) This leads to fat loss, other things (like diet) being equal.
To lose fat, add muscle.
Weight loss without strength training leads to the loss of muscle as well as fat, an unhealthy situation, and higher protein intake doesn’t help.
Weight loss with strength training leads to no muscle loss.
And strength training leads to a higher resting metabolic rate and with it the ability to avoid fat gain.
There’s much more in this vein in my book, Muscle Up: How Strength Training Beats Obesity, Cancer, and Heart Disease, and Why Everyone Should Do It.