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Intermittent Fasting and Potential Loss of Muscle Mass

Lean body mass as well as fat stores are lost, according to a new study.

Key points

  • In one study, those who followed intermittent fasting lost equal amounts of weight from fat stores and from muscle mass.
  • Because muscle mass burns calories, a decrease in lean body mass makes the dieter more vulnerable to subsequent weight gain.
  • More studies are needed to assess the impact of intermittent fasting on mental health and cognitive function.

For those of us who dread enforced fasting, either for medical (before medical tests or surgery) religious, or cultural ("Everyone is doing it!") reasons, news that it may not be an optimal way of losing weight loss is a relief. It also makes it easier to invite someone for need to ask if a specific day of the week is a fast or feast day.

The evidence that fasting to lose weight is no more (and maybe less) effective than a conventional calorie deficit diet comes from a study published this month. A large research team of physiologists from the University of Bath enlisted healthy, normal-weight volunteers to participate in a three-week study.

One group was told to fast on alternate days and on the other days, increase their calorie intake by 150%. A second group followed a calorically deficient diet continuously. The food plan had them eating only 75% of the calories they needed to sustain their weight each day. And the third group fasted on alternate days, but then ate 200% of their needed calorie intake on the non-fast day.

On average, Groups 1 and 2 ate 25% fewer calories than their bodies needed, but the fasting group did so by eating nothing on one day and 50% more than they needed on the fed day. The second group, a control group, also ate 25% fewer calories than their bodies needed and did so every day. And the third group, also a control group, fasted on one day but because they ate twice as much as they needed on the fed day, their average calorie consumption was enough to maintain their weight.

Intermittent fasting has become a popular method for losing weight, and it promises metabolic advantages that translate into more effective weight loss than simply eating less every day. And indeed the test group did lose weight; 1.6 kg in three weeks. But the group that did not fast, but ate fewer calories every day also lost weight. Interestingly, they lost 1.9 kg in three weeks. The difference in weight loss was not as important as what was actually lost. All of the weight lost by the group that did not fast was from fat stores. In contrast, the intermittent fasting test group lost equal amounts of weight from fat stores, and also from lean body mass; ie. muscle mass. Not surprisingly, the third group lost no weight.

If the dieter is simply interested in seeing the numbers go down on the scale and clothes feeling loose rather than snug, then both types of diet programs are effective in achieving these goals. But since ideally (if not realistically), the dieter should plan on keeping off the weight lost as long as possible, the decrease in muscle mass from intermittent fasting is worrisome. Since it is muscle not fat that burns calories, a decrease in lean body mass makes the dieter more vulnerable to gaining weight after the diet is over. Decreased muscle mass also may make it harder to exercise and from a cosmetic standpoint, even affect body shape and posture. And if enough lean body mass is lost, it may enhance bone loss as well.

The study referenced here did not talk about effects of intermittent fasting on physical activity, cognitive function, and emotional state. There may be some negative consequences. Citing the positive effects of exercise on both physical and mental function, the authors then reviewed studies indicating that intermittent fasting may have a negative impact on physical performance, and thus on the ability to cope with stress and decision making.

On the other hand, the scientific (and not so scientific) literature is filled with studies and opinions that promote intermittent fasting‘s benefit on cognition, emotional state, and physical well-being. Mice presented with a feeding schedule involving daily reduced calorie feeding, or intermittent fasting, apparently showed significant improvements in their memory.

One problem with interpreting the various studies on the effects of intermittent fasting is that calorie intake on the fast days may be limited to broth or other non-caloric liquids, or may allow a calorie intake of 500-700 calories. Moreover, if the food intake on non-fast days is not limited to healthy choices but includes high fat, sugar-rich items, due in part to the need to satisfy hunger quickly, the impact of the fast days may be inhibited by the feasting on the non-fast days.

Fasting itself may not be a pleasant experience especially at the beginning. A recent online review on the side effects of intermittent fasting in HealthLine lists many of the side effects. Obviously hunger is a side effect, but also light-headedness, headaches, digestive issues such as bloating, constipation, diarrhea, nausea, irritability, and other negative mood changes. Sleep disturbances are also a common side effect.

These behavioral and physiological changes may eventually become more tolerable or disappear if intermittent fasting is followed for many months. And one rarely mentioned benefit is not having to worry about what to make for dinner every other day. But does it have benefits that are not available by eating a calorically and nutrient sensible diet every day? The Templeman et. al. study demonstrated an adverse effect on the maintenance of lean body mass when compared with a calorically deficient diet followed daily. We need more such studies to be convinced of its impact on our mental health and cognitive function.