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Why Some Runners Eat Lots -- And Don't Get Fat

The Athlete’s Kitchen
Copyright: Nancy Clark MS RD CSSD

Some of my clients seem jealous of their teammates. “They eat twice as much as I do and they are skinny as a rail. I just smell cookies and I gain weight,” spouted one collegiate runner. She seemed miffed that she couldn't eat as much as her peers—and she couldn't understand why. They all ran the same mileage, did the same workouts, and were similar in body size. Life seemed so unfair!

Yes, life is unfair when it comes to weight management. Some runners gain (or lose) body fat more easily than others. Unfortunately, fat gain (or loss) is not as mathematical as we would like it to be. That is, if you persistently overeat (or undereat) by 100 calories a day, in theory you will gain (or lose) 10 pounds of body fat a year. But this theory does not hold up in reality. People vary greatly in their susceptibility to gain or lose body fat in response to over- or under-eating.

In general, when people overeat, research has suggested about 85% of the excess calories get stored as fat and the rest gets lost as heat. Overfed fat cells grow in size and in number and provide a storehouse of energy. Obese people commonly have enough fat stores to last a year or more; even lean runners have enough fat stores to fuel a month or more. Fat can be advantageous during a time of severe illness or a famine.

Burning Calories

Lets take a closer look at the four primary ways you burn calories:

1. Basal metabolic rate (BMR). BMR refers to calories burned when you are at rest and unfed, such as upon waking in the morning. Your heart, lungs, liver, and other organs use a fairly consistent amount of energy each day to keep you alive. Some runners believe they have a slow metabolic rate that causes them to gain weight easily. Not the case. Very few people have a “slow metabolism.”

2. Thermic effect of food. This refers to the energy needed to digest, absorb and either convert food into fuel for the muscles and organs or store the excess energy as body fat. The thermic effect of food increases ~14% with overfeeding, due to the added energy needed to process the excess food.

3. Purposeful exercise. This is what you burn during your workouts. This can vary considerably from day to day.

4. Non-exercise activity thermogenesis (NEAT). This refers to activities of daily living (brushing teeth, washing dishes, etc.), fidgeting, energy used to maintain posture when standing and sitting, and spontaneous muscle contractions that occur during the day apart from your purposeful exercise. People with high NEAT spontaneously putter around the house, fidget with pencils, use their hands when talking, and are animated and lively. NEAT is genetic and somewhat predictive of who stays lean throughout their lifespan.

People with low NEAT are good at sitting quietly. For example, obese people tend to sit 2.5 hours more a day than their peers and this can save them about 350 calories a day. Are they obese because they sit more? (Or do they sit more because they are obese?) Is NEAT the problem? 

What Happens with Overeating?

To better understand why some people lose or gain weight more easily than others, Dr. James Levine PhD of the Mayo Clinic in Minnesota designed a study to look at the biological mechanisms that hinder fat-gain. Dr. Levine studied 16 non-obese subjects (12 males and 4 females), ranging in age from 25 to 36 years. They volunteered to eat 1,000 excess calories a day (above what they needed to maintain weight) for 8 weeks. The subjects were healthy, did not do purposeful exercise more than twice a week, and maintained a stable weight. Prior to being overfed, the researchers monitored the subjects for two weeks to learn how much food they regularly consumed to maintain their weight.

During the study, the subjects lived at their homes but ate supervised meals at the research center. The food had been carefully prepared and measured in a metabolic kitchen. The weight-gain diet was high in protein (20% of total calories) and fat (40% of calories), and low in carbohydrate (40%). The researchers accounted for almost all of the excess 1,000-calories a day. On average, ~430 of the 1,000 calories were stored and ~530 were dissipated via increased energy expenditure. The researches even measured 3 days of poop before and at the end of the study to be sure the subjects did not excrete calories during overfeeding. Only 38 calories a day got flushed down the toilet during overfeeding — 13 calories more than during normal eating.

Here is the fate of the 1,000 excess calories the subjects ate:

-- Energy stored as fat ranged from 60-685 calories per day

-- Energy stored as muscle ranged from 15-80 calories per day

-- Additional calories burned by organs: about 80, on average

-- Additional calories used to digest the extra food: about 135, on average

-- Additional calories burned via NEAT ranged from none to 690.

The researchers used highly accurate methods to measure changes in body fat (DXA). Some of the subjects gained 10 times more fat than others, ranging from 0.8 to 9 lbs (0.36 - 4.23 kg). The overall weight gain ranged from 3 to 12 lbs (1.4 -5.5 kg), some of which was additional muscle and water. NEAT explained the big variation in weight gain that occurred with the subjects in this overfeeding study. The subjects who were good fidgeters and putterers gained less.

The average increase in NEAT was 336 calories a day, but this actually ranged from burning 98 calories less than baseline to burning 690 calories more than baseline. The subject who burned the most calories strolled around the research facility (or did equivalent movement) about 15 minutes more per hour than the other subjects.


Despite popular belief, the ease of gaining weight is unlikely due to having a “slow metabolism.”  Most often, runners who are easy gainers are mellow, sit calmly, and don't fidget. This contrasts to teammates that are bouncing around the locker room. If you are the mellow-type, blame your genetics — not a slow metabolism — for your ease of weight gain. And perhaps you can be grateful you can spend less money on food because you don’t eat as much?

Boston-area sports nutritionist Nancy Clark MS RD counsels both casual exercisers and competitive athletes at her private practice in Newton (617-795-1875). Her Sports Nutrition Guidebook and her food guides for runners, cyclists and soccer players are available at www.nancyclarkrd.com. Visit NutritionSportsExerciseCEUS.com for online education and information about upcoming live workshops.


Levine J, N Eberhardt, M Jensen. Role of Nonexercise Activity Thermogenesis in Resistance to Fat Gain in Humans. Science  283:212-214, 1999.

Levine J, Vander Weg M, Hill J, Klesges R. Non-exercise activity thermogenesis: the crouching tiger hidden dragon of societal weight gain. Arterioscler Thromb 26(4):729-36, 2006.

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