As the New Year dawned on January 2, The New York Times Well posted “Most Popular Health Questions You Asked This Year.” I was curious, of course, and particularly struck by a question addressing nutrition and exercise for weight loss, a topic I am particularly interested in.
The question: What is the best time of day to exercise if my goal is weight loss?
The answer by science writer Gretchen Reynolds: You might try setting your wake-up alarm earlier and exercising before breakfast.
In answering the question, Ms. Reynolds referred to a study published in 2010 by Karen Van Proeven and colleagues in the Journal of Physiology. The researchers found 28 healthy, young (average age 21) men and put them on a diet and exercise program for 6 weeks. The exercise protocol called for each subject to either run or cycle 4 times each week at a moderate intensity. But rather than giving them a diet designed to promote weight loss, these healthy young men were given high fat diets packing at least 30% calories more than they typically ate. In other words, they were told to stuff themselves. Subjects were divided into 3 groups: group 1 exercised before breakfast (fasting), group 2 exercised after breakfast (fed), and group 3 did not exercise (control).
After 6 weeks the control group gained about 3 kilograms (6.6 pounds), not unexpected as they were eating much more than usual without increasing activity. As might be expected, the exercise groups gained less. The group that exercised before eating gaining on average almost 1 kilogram and the group that exercised after eating gaining an average 1.4 kilogram. The researchers found that those who fasted before exercise burned more fat for fuel than those who ate before exercise. There is nothing surprising about this. Those who ate had a meal high in carbohydrate and were also given a carbohydrate beverage to drink during exercise so they had plenty of carbohydrate on hand to fuel their activity.
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Why this study doesn’t answer the question posed:
This was not a weight loss study! In fact, the study was designed so that subjects GAINED weight. Note that the subjects were normal weight young males. The metabolic response will likely be different in overweight, older individuals. Burning more fat for fuel during exercise will not translate into more body fat lost over time unless calories are reduced, which is the opposite of the scenario here.
Research has shown that exercising without eating can make it harder to have an effective workout and can also increase sensations of hunger, causing exercisers to eat more calories later in the day. For weight loss to occur, fewer calories must be eaten throughout the day than used for normal metabolism and exercise. Having a small meal or snack before exercise, such as yogurt, a piece of toast, or fruit, will provide the energy muscle cells need to perform and make it less likely that overeating will occur later in the day.
This was an odd choice of study to answer the question posed. While acknowledging that subjects did not lose weight, Ms. Reynolds quoted one of the authors as saying that their results were encouraging for those who want to be able to cheat on their diets without gaining weight. Huh?