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Higher Body Fat Percentage Helps Retain Muscle Mass In Physically Active Humans

You and some friends are about to take off for a multi-week expedition in the mountains. How much and what kind of food do you pack? Traditional ration plans are simplified into categories based on environmental conditions and physical activity level. For example, it is recommended that for expeditions involving long hikes in cooler-to-cold climates you should pack 2 lbs of food (roughly 3,500 kcal) per person per day. But this assumes that all of the people on the expedition are of the same body size and have the same nutritional demands.

What if one of your friends is a body builder while another is a heavyweight powerlifter? These two individuals will likely not only have different body weights, but they will also have very different body compositions, the ratio of body fat to muscle mass. As such, their nutritional needs and physiological reactions to high levels of physical activity will also be different.

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My recent work focused on individuals taking part in a 3-4 month long outdoor leadership course run by the National Outdoor Leadership School (NOLS). I measured how many calories the NOLS students burned while taking part in high levels of physical activity such as hiking and cross-country skiing with a fully loaded pack. I also measured body composition and how it changed throughout the course as well as the number of calories they consumed.

How Initial Body Fat And Diet Impact Muscle Mass Reduction In Cold Environments

There were four fascinating results from this research. First, these courses are incredibly calorically demanding. Individuals were expending an average of over 3,500 calories per day while hiking in the mountains during the spring and over 4,700 calories per day hiking in the mountains during the winter. NOLS students were expending more calories in the winter because of the need to stay warm and because cross-country skiing with a load and snow shoveling are incredibly demanding activities.

Second, NOLS students were eating 1,000 calories or more too little each day. This creates a negative energy balance. When in a negative energy balance the body relies on its own fuel storage to keep moving. These fuel stores come in the form of glycogen (a form of carbohydrate storage in the liver), body fat, and muscle mass. If the negative energy balance lasts long enough especially during a time of high physical activity, a person quickly burns through both the glycogen and fat stores, leaving only muscle to make up the caloric deficit. Unfortunately, muscle catabolism can hamper physical performance, suppress the immune and digestive systems, and alter hormone levels – not ideal for backcountry expeditions. NOLS students lost body weight through a loss of body fat, lean muscle mass, or both.

Third, an individual’s body composition before taking part in the NOLS course affected the body composition changes they experienced during the course. Students who started with a low percentage of body fat lost muscle mass, while those with a higher percentage of body fat did not. Furthermore, women, who on average have more body fat than men, managed to lose body weight in the form of fat, but actually increased their muscle mass. This suggests that a higher percentage of body fat can protect an individual from muscle catabolism during rigorous expeditions, and even allow for the addition of new muscle mass. Fourth, in the face of such a large negative energy balance, macronutrient content, even dietary protein, didn’t protect against muscle mass loss.

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So, what’s the take away from all of this? If you are looking for a good calorie burn, take your exercise outside during the winter. However, if you are planning a big expedition, you might want to take body composition into account when preparing rations, because one ration plan does not fit all. Those of you with lower body fat percentages will want to pack some extra calorie dense foods to ensure that you don’t put your muscle mass, performance, and health at risk.

The study, Body fat attenuates muscle mass catabolism among physically active humans in temperate and cold high altitude environments was recently published in the American Journal of Human Biology.

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