By Gretchen Reynolds
The New York Times
From Our NYT Files: The Best Thing to Eat Before a Workout? Maybe Nothing at All
For those who can stomach it, working out before breakfast may be more beneficial for health than eating first, according to a study of meal timing and physical activity. Its results indicate that when we eat affects how much fat we burn during exercise and also alters molecular activity within fat cells, in ways that could have long-term implications for our physical well-being. Athletes and scientists have long known that meal timing affects performance. Most obviously, if you eat first, you have relatively high levels of blood sugar. Working muscles can readily use this sugar as fuel. If, on the other hand, you have fasted before working out, your muscles must rely primarily on the body’s skimpy supply of stored carbohydrates or its larger reservoirs of fat. Accessing this fat, however, requires extra metabolic steps to become available as energy, which makes it a relatively inefficient fuel source during times of strenuous exercise. As a general rule, the body tends to turn to fat as its primary fuel source when exercise is more moderate. Knowing this, many athletes experiment with meal timing, often training hard on an empty stomach, in hopes that this strategy will encourage their bodies to become more adept at using fat as a fuel.
But these efforts obviously have focused on sports performance. Far less has been known about how meal timing and exercise might affect general health. So for the new study, which was published this month in The American Journal of Physiology — Endocrinology and Metabolism, researchers from the University of Bath in England decided to home in on relatively average people and their fat cells. Most of us probably do not realize how busy and physiologically bossy our body fat can be. But in recent years, scientists have established that fat cells constantly make and excrete a wide variety of substances that influence other systems and organs in the body. The British scientists suspected that eating before exercise might affect the production of these substances. To find out, they first recruited 10 overweight and sedentary but otherwise healthy young men, whose lifestyles are, for better and worse, representative of those of most of us. (They did not recruit women because it is difficult to control for the effects of the menstrual cycle on metabolism; they hope to study women in the future.) They tested the men’s fitness and resting metabolic rates and took samples of their blood and fat tissue. Then, on two separate morning visits to the scientists’ lab, each man walked for an hour on a treadmill at a moderate pace that, in theory, should allow his body to rely principally on fat for fuel. Before one of these workouts, the men skipped breakfast, meaning that they exercised on a completely empty stomach, after a prolonged overnight fast.
On the other occasion, they ate a substantial, 600-calorie morning meal, supplied by the scientists, of toast, jam, cereal, milk and orange juice about two hours before they started walking. Just before and an hour after each workout, the scientists took additional samples of the men’s blood and fat tissue. Then they compared the samples. There were considerable differences. Most obviously, the men displayed lower blood sugar levels at the start of their workouts when they had skipped breakfast than when they had eaten. As a result, they burned more fat during walks on an empty stomach than when they had eaten first. On the other hand, they burned slightly more calories, on average, during the workout after breakfast than after fasting. But it was the impacts deep within the fat cells that may have been the most consequential, the researchers found. Multiple genes behaved differently, depending on whether someone had eaten or not before walking. Many of these genes produce proteins that can improve blood sugar regulation and insulin levels throughout the body and so are associated with improved metabolic health. These genes were much more active when the men had fasted before exercise than when they had breakfasted. The implication of these results is that to gain the greatest health benefits from exercise, it may be wise to skip eating first, said Dylan Thompson, the director of health research at the University of Bath and senior author of the study.
This was a very small, short-term study, though. It can’t tell us whether other types of meal timing such as, for instance, skipping lunch before an afternoon workout will produce similar effects or if the acute changes seen in fat burning and gene expression after fasting will necessarily translate into lingering health improvements over time. The results also do not suggest, I am sorry to say, that fasting before exercise will accelerate weight loss. In fact, in this study, eating before exercise resulted in the men burning more calories during their workout than fasting. In other words, many questions must still be investigated before scientists can offer recommendations about eating before exercise, Dr. Thompson said. But he will not be surprised, he said, if subsequent data reveal that working out on an empty stomach has advantages. “If we just think of this in evolutionary terms,” he said, “our ancestors would have had to expend a great deal of energy through physical activity in order to hunt and gather food. So, it would be perfectly normal for the exercise to come first, and the food to follow.”