Fasting vs Fed Cardio

What’s the latest?

A new study examined the effects of exercising in a fasted state compared to a fed state, and how it relates to the body’s breakdown of adipose tissue, or body fat. The researchers had overweight subjects walk for one hour at a moderate intensity in a fasted state, then again a few weeks later two hours after eating. The data showed that the fasted group used more adipose tissue for energy production than the fed group. The authors state that as the body gets energy from the meal, less adipose tissue is broken down for fuel.

There are a few things worth noting about this study. The fed group was given a high-calorie, carb-heavy meal (650kcal, 120g carb, 13g fat, 21g protein) that contained a lot of sugar. It seems obvious that the body would use some of that excess sugar for fuel. I also noticed that although there is a significant difference between fasted and fed, both groups primarily used carbohydrates as a fuel source during exercise.

So should you exercise before or after breakfast?

That depends on how fasted exercise makes you feel. If you’re anything like me, then being hungry is detrimental to just about every situation. If you’re in a bad mood, then your workout will suffer. You may not have the motivation or focus to adequately push yourself, or you could end up cutting a workout short altogether due to feeling drained of energy. It’s more important to have an effective workout than to burn a little more fat.

However, some feel that any amount of food before cardio weighs them down and makes them feel sluggish. This could lead to postponing the exercise after eating in hopes that they’ll feel better soon. It often leads to workouts being rushed as they run out of the time they had set aside, and not very enjoyable due to the lingering discomfort from the previous meal.

If you’re somewhere in the middle —as most of us are— then you could benefit from having a light snack that is high-carbohydrate and low-fat before exercising. I like to aim for about 100kcal of items such as fruit, sports drinks, toast with jam, or a fruit smoothie. After your workout, have your normal meal. This is the option I personally choose and recommend for most people. It gives you a good amount of energy for your workout without the aforementioned discomfort.

Sources mentioned

American Journal of Physiology-Endocrinology and Metabolism accessed 041917

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