Should you eat before or after you hit the gym?

This is a question we are asked all the time, and here’s what we think …

First of all, let’s recap where our energy comes from – regardless of what we intend to use it for. Our body can use all macronutrients – carbohydrates, fat and protein – as energy sources, but it can store only two: carbohydrates and fat.

Running on Carbohydrates

Carbohydrates are stored in the liver and muscles in the form of a starch called glycogen. The liver stores about 400kcal worth of glycogen, the muscles approx. 1,600kcal. The body can access this energy source very quickly and easily, and with full glycogen stores, you can exercise vigorously for 90 to 120 minutes. But are your glycogen stores full if you head to the gym before breakfast?

Your body needs to be maintained, even while you are unconscious, and therefore calories will be burned while you are asleep. How many calories you burn while you sleep depends on your basal metabolic rate (BMR), which in turn is affected by your body composition: height, weight, body fat percentage and muscle mass. For example, an average person of 150lbs (70kg) might burn approx. 46kcal/hour, i.e. between 322 and 414kcal per night. Add to that perhaps three hours of wakefulness after dinner, which would have cost this average person about 72kcal/hour and they might wake up with only 1,370kcal of stored energy – provided that their dinner filled their carbohydrate stores (the typical Western dinner does). That’s enough for almost 4 hours of low-impact aerobics or on the cross trainer, or over 2 hours of running. Unless you are planning to run a marathon, you should typically be ok to go to the gym in a fasted state and have enough energy to exercise.

Running on Fat

Fat is stored in our fat cells, and while the storage capacity for carbohydrates is limited, we have almost unlimited capacity to store fat – a fact of which many of us are painfully aware. One pound of body fat contains about 3,500kcal, give or take. Even a very lean male still carries at least 5% body fat, meaning he carries a store of over 20,000kcal of energy. The trick lies in being able to access that almost bottomless supply of calories, or rather to access more of it – because we are using both carbs and fats for fuel at all times.

Will you perform better if you eat before hitting the gym?

A recent study review showed that short-duration performance was improved in a fasted state, whereas long-duration performance was better for those that had eaten. Other research found that doing HIIT exercises or weight training in fed or fasted states made no difference in terms of performance. Overall, it appears that people who are not elite athletes but use simply go to the gym for an hour or so to stay healthy are fine if they don’t eat before exercising, even if they train hard. Having said that, some people struggle with exercise if they haven’t eaten beforehand. Listen to your body and do what seems right for you.

That leaves one question:

Will you lose more weight if you train in a fasted state?

If you eat a carbohydrate-containing meal before exercise, your blood sugar levels will be up during training and your body will use that energy to fuel your training. If you don’t, it will use the stored glycogen (see above). However, running out of glycogen doesn’t mean that you necessarily have to stop moving. There is, after all, that vast store of calories in your fat cells. Only it’s not that easy to access it if you are not used to it.

While muscle glycogen is essential during exercise, we can’t rely on it solely, certainly not during endurance exercise. Also, you can’t transfer it from your arms to your legs; even if you have stored glycogen left in your arms, it won’t help you run for longer.

In our modern world, we have the opportunity to store up on carbs anytime we like, whereas our ancestors relied on fruit, roots, nuts and honey, and those weren’t always available. But they still had to be able to function and to hunt, so our bodies evolved to switch from carbs to fat as fuel, and we can still do that. If we keep topping up our carbs, however, those will be used before fat. Carbohydrates trigger insulin secretion, and insulin is the fat-storage hormone – which means that it will prevent fat burning. Following a low glycaemic diet (less starchy carbohydrates and plenty of healthy fats) keeps insulin at bay, allowing you to use fat as fuel and burn more fat.

Eat after the gym

Whether you eat before exercise or not, we recommend that you eat something after. Protein after exercise provides the amino acids needed to build and repair muscle, carbohydrates are used to replenish your glycogen stores. If you exercise in a fasted state, your body has just used the nutrients that were there, leaving less for recovery. Research found that muscle repair was more efficient if participants ate a protein-containing meal after exercise. For maximum benefit, eat as soon as possible after exercise.

Bottom Line:

Yes, you can go to the gym without having to eat first, but everyone is different and what matters is how you feel. If you can exercise in a fasted state: excellent. If you can’t: that’s fine, eat first. Either way, it is important to eat something – particularly protein – post-exercise.

If you do eat first, allow at least an hour for food to leave the stomach and avoid cramps  or regurgitation. 

And don’t forget your warm up and stretching!

For a referenced version of this blog, please email us on

Our Basic Sports Nutrition seminar covers the main energy pathways and top tips for what to eat for best performance, or we can arrange a seminar by a specialist sports nutritionist if you have more advanced needs. Call now on 07966 478974.

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