When You Lose Weight, How Much is Fat & How Much is Muscle Loss?

image of women body fat belly between before and after weight loss

The goal of losing weight, of course, is to lose body fat, but not to shed metabolically active muscle. When muscle loss happens, you also lose the subtle, metabolic boost that comes from having more metabolically active muscle tissue Yet, we know that when the number on the scale drops, we shed a certain amount of lean body mass too, mostly muscle. In the best of all possible worlds, the body fat would melt away and leave the muscle intact but that’s not the way it typically works! Along with the fat, comes some muscle. You might wonder how much of what you lose when you shed body weight is actually fat and how much is muscle? Also, is there a way to reduce the quantity of muscle loss that happens when you eat a lower calorie diet?

Statistics available online state that approximately 25% of what you lose when you lose weight is muscle and the rest body fat. However, this is an overgeneralization that doesn’t apply to everyone When researchers at the Pennington Biomedical Research Center in Baton Rouge, Louisiana did a literature search, they discovered the ratio of body fat to muscle one loses is variable. In fact, the ratio depends on a variety of factors, including genetics, age, diet, and, maybe most important, whether a person is physically active and the type of exercise they do. You may not have control over your age or genetics, but you can change your diet and degree of physical activity and this, in turn, impacts how much body fat relative to muscle you lose.

We sometimes look at the number on the scale and feel good when it’s within a healthy range. Yet, total body weight, as determined by your bathroom scale, says nothing about body composition and that’s ultimately what’s important. You can be within your ideal body weight range and still be unfit and unhealthy, thanks to a common phenomenon called sarcopenia, a condition strongly linked with aging and inactivity. As you know, your body composition changes as you grow older. After the age of 30, you lose muscle mass and, due to hormonal changes and a lack of physical activity, gain body fat. That’s an unhealthy combination! Sarcopenia simply means loss of muscle due to aging. In fact, people who are sedentary can lose as much as 5% of their muscle mass every ten years after the third decade of life.

When an older person who is inactive diets to lose weight, the extra muscle mass they lose is detrimental from a health standpoint. That’s because they already have less muscle relative to what they had when they were young, and the loss continues throughout life. Loss of additional muscle mass increases frailty and the risk of falling.  It also is harmful to metabolic health, as lean body tissue helps take up more glucose from the bloodstream. And may lower the risk of pre-diabetes and type 2 diabetes.

Factors that Impact Muscle Loss When You Lose Weight

Age and genetics are two well-described factors that impact how much muscle you shed when you lose weight – but how rapidly you lose that weight is a factor too. In one study, participants that ate a very low-calorie diet of only 500 calories per day (don’t try this) and lost weight quickly, shed around 3.5 pounds of lean body mass while another group of dieters who ate a 1,250 calorie diet daily, and shed the pounds more slowly, lost only 1.3 pounds of muscle mass. So, if you’re trying to preserve muscle mass, don’t drastically reduce your calorie intake. If you don’t supply your body with enough calories, it can elevate cortisol, a catabolic hormone that contributes to muscle loss. Such an approach isn’t sustainable or healthy.

The composition of what you eat matters too. In one study, participants that ate a low-calorie diet that was higher in protein (2.4 grams/kilogram body weight) lost more body fat and less muscle than those that ate a diet lower in protein (1.2 grams/kilogram body weight). A diet higher in protein has a protein-sparing effect and helps you hold on to more muscle when you’re cutting back on your food intake.

Physical Activity Matters Too

As you might expect, studies show that strength training reduces the loss of muscle tissue during periods of calorie restriction. In one study of obese individuals, those who strength trained lost as much weight as those who didn’t, but those who didn’t strength train lost twice the amount of muscle tissue. In other words, strength training provides the stimulus your muscles need to maintain their size. That’s important since metabolically active muscle tissue modestly boosts resting metabolic rate and can help with weight loss maintenance.

Another study published in the Journal of the American College of Nutrition showed that a high-intensity resistance training program preserved muscle and resting metabolic rate when participants ate a very low-calorie diet. A small study by Australian researchers found that muscle protein synthesis declined by 27% after only 5 days of following a low-calorie diet. The good news? Resistance training reversed this trend.

Of course, you shouldn’t drastically reduce your calorie intake but it’s nice to know that strength training helps with muscle preservation when you lose weight. The best approach would be to focus on compound movements, like deadlifts, squats, rows, push-ups, and lunges, that work multiple muscle groups at the same time. Doing so will burn the most calories while preserving muscle.

The Bottom Line

The amount of muscle loss that happens when you go on a low-calorie diet varies, but you can swing the odds toward retaining more muscle by consuming more protein, at least double the recommended 0.8 kilograms per body weight for sedentary people) and by doing high-intensity strength training.



WebMD. “Fast Weight Loss May Mean Muscle Loss”
Med Sci Sports Exerc. 2008 Jul; 40(7): 1213–1219. doi:  10.1249/MSS.0b013e31816a85ce.
Obes Rev. 2014 Apr; 15(4): 310–321.
Am J Clin Nutr. 1997 Sep;66(3):557-63.
J Am Coll Nutr. 1999 Apr;18(2):115-21.
The Globe and Mail. “Leslie Beck: How do I lose weight but maintain – and gain – muscle?”


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