When you usually work out, do you eat beforehand or just refuel afterwards? For me, I usually aim to eat before and give myself an hour between eating and exercising, especially if I’m exercising in the morning. The reason why? Well, to be honest for me it wasn’t decided by looking at what research has found to be best. It was through my own trial and error…. I found that when I exercised without eating, I'd get light-headed and not be able to perform to my best and if I ate too soon before, I would feel sick and again not be able to push myself.
So, is there any research out there which determines if it's necessary?
Before we look at any studies, let’s talk about glucose and glycogen.
Glucose is used by the cells within your body for energy and can even be used during the absence of oxygen (anaerobic). We get it from the foods we eat, specifically carbohydrates.
Glycogen is a stored form for leftover glucose within our cells and is readily available to yield glucose when required (Berg et al., 2002). Most of our glycogen is stored in our liver cells and then a smaller amount is stored in our muscles. It is a readily available source of energy for when our blood glucose levels decrease and we require more for energy. The glycogen stored within our muscles is used as fuel during exercise and the glycogen in the liver and glucose in the blood can also be utilised. Our muscles will then replenish their glycogen stores post exercise (Diabetes UK, 2017). It’s important to note that if your body is short of glycogen, it may result in your body breaking down protein instead. As you may know, protein is a building block for our muscles – so there’s a risk it may actually cause you to lose muscle.
In one study conducted on overweight males, it was found that when they walked for 60 minutes on an empty stomach, stored fat (adipose) was used to fuel their metabolism whilst they were exercising.
In the case study where participants had eaten two hours before exercise, it was found that the adipose tissue was responding to the consumed carbohydrates in the meal and the carbs were used for energy instead of the adipose tissue being used as a fuel for energy, meaning that a fasted state, may lead to the desirable changes in adipose tissue (Chen et al., 2017).
It is, however, important to note that the meal which was given to the males in this study, was a high calorie carbohydrate-rich breakfast – it would be interesting to see what would happen if they tried a high protein and a high fat breakfast and saw how adipose tissue responded to these as well and even the impact of low, medium and high calorie breakfasts. Lots more variables to look at and their impacts!
Another research paper found that there is an advantage for body fat regulation and lipid metabolism in exercising before breakfast, in comparison to after. The breakfast provided 49% energy from carbohydrate 37% from fat and 14% from protein. This was conducted on 10 sedentary overweight men, walking for 60 minutes at 50% maximal O2 uptake (Farah and Gill, 2013). It may be that for those who are overweight and lead a sedentary lifestyle, lighter physical activity, such as walking, is more beneficial before eating, to achieve maximum benefits..
Going back to glycogen, what happens if say, you’re an athlete and you fail to consume enough carbohydrates that your glycogen stores become depleted, causing low levels of blood glucose?
As we discussed above, glucose is used for energy, so if you drop your blood glucose levels you are also dropping your energy stores. Athletes experience something called bonking, or hypoglycaemia (low blood glucose levels). Bonking leaves you feeling fatigued and a loss of energy. You can read more about bonking and fuelling for bike rides here.
In this situation your body (liver) will start to break down fat and protein to form glucose, and therefore a new source of energy. This means there’s a risk that if protein is being used to be broken down to produce glucose, it could lead to a decrease in muscle tissue, if the energy is required during exercise.
One study looked at the impact of carbohydrate depletion and carbohydrate loading on protein breakdown in subjects on a cycle ergometer for 1 hour at 61% VO2max (maximal oxygen uptake). They found that when subjects were in a state of carbohydrate depletion, protein equaled 10% of calorie burn in subjects. This means that protein was utilised to a greater extent in those in the carbohydrate depletion, than those who had carbohydrate loaded. This protein would have even come from their own muscle mass (Lemon and Mullin, 1980).
So, if you train in the evening or afternoon, you’ll most likely have eaten throughout the day, so it wouldn’t be a truly fasted state and you probably wouldn’t need to eat before exercising, as you will have some fuel.
If you are hungry you could have a banana or a light snack if you feel like your session may be more intense but you should be sufficiently fuelled from what you’ve already eaten that day, to carry out a workout. It’s important to make sure that you’re getting a good balance of complex carbohydrates, protein and healthy fats, including vegetables, throughout the day, not only to keep you going during the day but also to help whilst you exercise.
Also, make sure that you stay hydrated throughout the day, as feeling dehydrated can lead to you feeling lethargic and also a decrease in your performance when exercising.
When working out, it’s always good to refuel after exercising, consuming something that contains protein and carbohydrates, things like salmon, chicken, eggs, brown rice, sweet potatoes, porridge with nut butter, even a handful of nuts.
Your own individual experiences of how you feel either eating before or waiting until after exercise is a good guide to work on in knowing what works best for you. If you know that you cannot exercise successfully on an empty stomach, then make sure you eat before.
If you are going to eat beforehand, it’s important to make sure that you give your body and stomach enough time to start to digest, so that you don’t feel uncomfortable or sick during exercise, due to the food.
The type and intensity of the exercising you are taking part in will also influence when you eat. Something that is very intense may mean that you need to fuel before, over something of lighter activity. It’s best to work out what works for you and how you feel during exercise, it is individual to you. For me, that means if I’m going to the gym in the morning, I need some breakfast, or if it’s later on in the day, I may have a banana an hour before if I’m feeling hungry.
If you are going to exercise in a fasted state in the morning, you may find that you can only work at a moderate intensity and exercises overall may be reduced in intensity if trying to complete a high intensity session. However, it is all down to you and how you feel - if you feel better exercising in a fasted state then carry on with what you feel is best.
It is also important to remember that physical activity is an essential part of our lifestyle that we should try and include. Especially in a society where, for many people, we are no longer required to be as physically active throughout the day.
Berg, JM. Tymoczko, JL. Stryer, L. Biochemistry. (2002). 5th edition. New York: W H Freeman, Chapter 21, Glycogen Metabolism. Available here.
Chen, YC. Travers, RL. Walhim, JP. Gonzalez, JT. Koumanov, F. Betts, J. and Thompson, D. (2017). Feeding influences adipose tissue responses to exercise in overweight men. American Journal of Physiology. Available here.
Diabetes UK. (2017). Glycogen. Available here.
Farah, NM. And Gill, JM. (2013). Effects of exercise before or after meal ingestion on fat balance and postprandial metabolism in overweight men. Available here.
Lemon, PW. And Mullin, JP. (1980). Effect on initial muscle glycogen levels on protein catabolism during exercise. Available here.
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Daisy is a Registered Associate Nutritionist with a Master's Degree in Public Health Nutrition, which is Association for Nutrition (AFN) accredited. She, also, has a BSc degree in Psychology and Cognitive Neuroscience; and has completed an AFN accredited Diet Specialist Nutrition course and is currently studying for a PgDip in Eating Disorders and Clinical Nutrition.
Daisy has worked for an NHS funded project, the Diabetes Prevention Programme; and shadowed a nutritionist in Harley Street.
Daisy is Lucy's sister and is the Lucy Bee voice on all aspects of nutrition and its effect on the body.