Should I Fuel Up or Fast Before My Work Out?

Health Writer

If you’re an avid exerciser, then you probably have a specific fitness food plan. You either eat a small snack right before working out or eat a more substantial meal 60 to 90 minutes before exercising, or you make the conscious decision to exercise “on empty” and fuel up after exercise. Is there a right or wrong approach here? New research suggests that making the choice to eat or not eat before exercise significantly affects your fat stores.

Before we delve into the research and its implications, it’s important first to define exercise goals and what type of exerciser you are. Let’s describe some general exercise “types” and goals:

  • A “normal weight” or lean individual who exercises regularly to support health and optimal weight
  • An overweight or obese individual who is exercising to help support weight loss and better health
  • An exercise enthusiast or serious exerciser who engages in vigorous exercise on a regular (daily) basis for health and enjoyment and to improve performance.

Depending on which of these most closely describe you, the research has strong implications. How your body responds to exercise and the type and timing of meals may be very important, especially if you’re trying to shed fat. Fat and fat stores play an important role in fueling exercise. Is your body more likely to tap into fat stores and release fat if you eat or fast before your work out? That is the crux of this particular research study published in the March 2017 issue of the American Journal of Physiology-Endocrinology and Metabolism.

Researchers from the University of Bath, in the United Kingdom, recruited 10 healthy, overweight men with an average age of 25 and an average waist size of about 41 inches. The men walked for 60 minutes with a moderate-intensity effort, first on an empty stomach and then a second time two hours after having consumed a high-calorie, carbohydrate-rich breakfast. In both situations, researchers took a number of blood samples before and after exercising to measure different metabolic components. They also obtained samples representative of adipose or fat tissue behavior before and one hour after exercise.

The researchers focused on gene expression in the adipose tissue in the two trials. They noted that expression of two fat genes, PDK4 and HSL, increased significantly when the men had fasted, and decreased when they ate before exercise. Specifically, the rise in PDK4 levels correlates to fat stores being accessed as fuel during exercise, rather than tapping into carbohydrate byproducts that are released after a typical breakfast. HSL also rises when fat stores are being used to support increased activity.

So if the goal is to “burn fat stores and reduce weight,” then individuals who are overweight or obese would likely benefit from exercising on an empty stomach. If you eat and then exercise, your body will likely use the readily available energy in the bloodstream from the digested meal, and then tap the readily available stores of glycogen in the muscles, and not utilize the fat or adipose tissue as a main source of fuel. The researchers also suggest that there actually might be an overall health payoff for many of us in general, since exercising on empty may prime fat stores to release energy more easily.

My problem with the study is that it was a very small study. Another issue is the fact that if you engage in very vigorous exercise, you may not feel well if you are trying to sustain your effort on an empty tank. There’s a reason why marathoners are told to carb load before prolonged exercise (night before and morning of) in order to fuel the very long effort of a marathon. If you regularly exercise for an hour or more, then your performance and mental focus may be hampered by fasting.

The research may indeed have merit for individuals who are diagnosed as overweight or obese and who want to effectively shed fat pounds. This approach may also help an overweight individual who has hit a weight plateau. Of course, if you do choose to eat before exercise (and after), you do want to make the “best’ food choices. A pre-exercise carbohydrate and protein snack or meal will help to support your exercise effort. Stick with high-quality, complex carbohydrates like whole grains, oats, brown rice, sweet potatoes, fresh fruits, and vegetables. Stick with good-quality proteins like nuts and nut butter, and low-fat dairy products such as low-fat unsweetened Greek yogurt or cottage cheese. A glass of low-fat milk can also be just enough to fuel a workout without weighing you down.

Eating before a workout may not be a good fit for every exerciser, but fasting before a workout may also not work for everyone. If you have diabetes, you need to test your blood sugar levels before a workout and then follow the diet guidelines you and your diabetes-care team have set up, and always have a fast-acting glucose snack handy when exercising.

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