You know that exercise is important, but there is a lot of conflicting advice out there concerning the “right” way to work out. One question that often arises is, “Will I lose more weight if I go running on an empty stomach?” Although in theory you may burn more fat this way, total calorie burn is about the same as eating a light snack before you exercise. Also, workouts on an empty stomach tend to be cut short because of hunger or lack of energy. The type of workout you do, the time of day you do it, and the length of time you spend at it are all important factors in choosing when to eat in relation to your workout.
Evidence for the Health Claim
Early-morning runners may not have time to eat before they exercise. As long as their workout is around 30 minutes long and does not include strength training, not eating beforehand could be beneficial. This is because when you wake up in the morning, your stomach is empty and your carbohydrate reserves are almost used up. Your body is thus more likely to burn calories stored as fat for energy.
A study published in the International Journal of Sports Medicine compared eight female subjects in four series of trials. In three of the trials, the subjects ate a small meal following an overnight fast, and then began exercising 30, 60, or 90 minutes after eating. In the fourth trial, the subjects did not eat before exercising. Results showed that the subjects burned more fat when they worked out on an empty stomach than when they exercised 60 or 90 minutes after eating.
A similar study found that people burned more calories from fat on the days that they did not eat breakfast before exercising than on the days that they ate a small snack before their workout. However, the difference was very small, and probably not meaningful.
Evidence Against the Health Claim
Many fitness experts do not believe that “running on empty” is the most efficient way to work out. Contrary to popular belief, working out on an empty stomach does not jumpstart your metabolism. After approximately 30 minutes of exercising without eating first, the body starts to use muscle as an energy source, which can lead to overexertion, dizziness, and dehydration. Also, exercising on an empty stomach lowers your blood sugar which leads to hunger, and people are then more likely to end up eating more after the workout than they would otherwise. Finally, weight training requires a lot of energy, and lifting weights without eating first does not work your muscles efficiently.
A study conducted by Maffucci and McCurry examined the exercise performance of eight female subjects three and six hours after eating a meal. Results showed that performance in moderate- to high-intensity workouts was improved by eating a high-carbohydrate, low-fat, low-protein meal three hours prior to exercising, as compared to performance after consuming the same meal six hours beforehand.
A study published in the 1999 Journal of Medicine and Science in Sports and Exercise found that people who ate a 400 calorie meal (breakfast) three hours before they exercised, were able to bike for approximately 30 minutes longer than when they did not eat beforehand.
Similar research has shown that although more fat calories may be burned by exercising on an empty stomach, the total amount of calories burned is comparable to the same workout after eating a light snack. Most importantly, fewer calories may be burned in the long run if you don’t eat before exercising since you may have less stamina and endurance to complete a full workout.
The best snack to have before you exercise is something light (100-300 calories) containing some carbohydrates and protein, such as fruit or yogurt. The more food you eat, the longer it will take for your body to digest, and if you work out too soon after eating you may get a stomach cramp. Some fitness experts suggest eating food in liquid form (like a fruit smoothie) to reduce the risk of getting a cramp while you exercise. Waiting to exercise for at least half an hour after eating will usually accomplish the same result.
Not eating before you work out is generally not supported by the evidence. If there is any benefit, it is likely to be minor. Also, it is important for people with various health conditions, such as diabetes and low blood pressure, to eat before they exercise.
If, however, you find that your workouts are more productive on an empty stomach, there is no reason to change your normal routine. If not, eating a light snack, such as a piece of fruit, before exercising will boost your energy for your workout and allow you to exercise long and hard enough to burn just as much, if not more, calories and fat.