Cardio exercise requires energy -- energy that comes in the form of food. Although bodybuilders and some endurance athletes may fast before doing cardio, to purportedly improve their body's fat-burning capacity, research shows that not eating before cardio is largely detrimental to most fitness goals. You don't have to gorge, but a small snack before cardio may help you perform better and burn more total calories.
Myth Versus Reality
Fitness professionals and practitioners sometimes recommend not eating before cardio, especially first thing in the morning, because, they say, it forces the body to use fat stores, rather than the immediate food you consume, for energy. In theory, it sounds good; however, this process doesn't play out in reality. A meta-analysis published in the February 2011 issue of the "Strength and Conditioning Journal" found that you burn about the same amount of fat whether you eat before exercise or not. But, if you don't eat, you'll also burn more lean muscle mass -- the valuable tissue you want to keep for health and accelerated metabolism.
Dragging You Down
Not eating before a cardio workout can also result in diminished performance. Columbia Health points out that if you aren't adequately fueled for your cardio workout, you may prematurely fatigue -- so you don't work out as long, or as hard, as you could had you eaten. The results are fewer calories burned and the failure to reach your true fitness potential.
Hunger Attacks Later
You might regret not eating before your workout, especially when hunger strikes later. A workout done in the fasted state can result in decreased blood sugar levels, so you are susceptible to chowing down with abandon after exercise. A study published in a 2002 issue of the "Journal of Nutrition and Sport Metabolism" found that women who did not consume a snack containing 45 grams of carbohydrates prior to and during their workout ended up eating more all day long than women who did ingest the carbs. Your preworkout meal doesn't have to be large. Try a slice of whole grain bread with peanut butter or a banana and a small yogurt if you plan on working out more than 30 minutes and it's been more than two or three hours since your last meal.
Insulin Resistance Buffering
In certain cases, exercising in the fasted state may afford some benefits. If you find yourself indulging in high-fat, high-calorie meals, you may see some benefit to doing cardio before you eat. A study in the November 2010 issue of the "Journal of Physiology" found that participants who exercised in a fasted state following a diet that contained 50 percent more fat and 30 percent more calories than they normally consumed did not experience insulin resistance, a precursor to type 2 diabetes, and gained little to no weight after six weeks. The exercisers who ate a snack before their workout gained less weight than the nonexercising controls, but both groups experienced some degree of insulin resistance. If you are not eating a diet higher in fat or calories than you need, though, exercising on an empty stomach probably does more harm than good.