Intermittent Fasting and Exercise: What Beginners Should Know

Intermittent fasting is one of the most talked-about ways of eating in the UK, and one of the most over-sold. If you have just started running, lifting or doing HIIT, you may be wondering whether you can train on an empty stomach, whether fasting will hold your progress back, and whether you should be doing it at all. This guide sticks to what the research can honestly support, and keeps the advice practical.

What is intermittent fasting?

Intermittent fasting is a pattern of eating that sets times when you eat and times when you do not. It does not tell you what to eat, only when. The most common versions are:

• 16:8. You fast for 16 hours, including sleep, and eat within an 8-hour window, for example midday to 8pm.
• 14:10 or 12:12. Gentler versions with a longer eating window. A 12-hour overnight gap is close to how many people already eat.
• 5:2. You eat as normal on five days and eat much less, often described as around 500 to 600 calories, on two non-consecutive days each week.

This article is about fasting as a chosen eating pattern. Religious fasting has its own rules and its own guidance, and is a separate topic.

What does the evidence actually say?

The honest summary is that intermittent fasting looks like one workable way to organise your eating, not a special one. When studies compare a set eating window against an ordinary pattern with the same amount of food, the results tend to come out about the same.

One of the larger trials, published in a leading medical journal in 2022, followed 139 adults living with obesity for 12 months. Everyone was asked to eat less, and half were also asked to eat only within an 8-hour window. Adding the window did not lead to a meaningfully better result than eating less on its own.

Research that pairs a set eating window with exercise is more limited. Recent reviews suggest that people who keep training, especially lifting weights, can generally hold on to muscle while following one, and some report slightly lower body fat. Most of those studies are small, last only a few weeks, and often involve people who already train. They cannot tell us much about the long term, or about complete beginners.

So what can we say? Some people find a set eating window simple and easy to stick to. Others find it leaves them hungry, distracted and flat in training. Both experiences are normal, and neither means you are doing it wrong.

Can you exercise on an empty stomach?

For most healthy adults, an easy session after an overnight fast is fine. A gentle run, a brisk walk, some mobility work or a short bodyweight session before breakfast is something plenty of people do without ever calling it fasting.

Longer and harder sessions are different. A 2018 review that pooled studies on fasted and fed exercise found that eating beforehand improved performance in aerobic sessions lasting more than about 60 minutes, but made no clear difference in shorter ones. That fits with what many runners notice: 30 easy minutes feels fine, while a long run with no food can fall apart.

Hard intervals, HIIT and heavy strength sessions vary from person to person. Some people feel sharp. Others feel weak or light-headed. If the quality of your hard sessions drops when you train fasted, that is useful information, and a good reason to move either the session or the meal.

Does fasted training burn more fat?

This is the claim you will see most often, and it needs care. Your body does tend to use a little more fat as fuel during a fasted session. But what you use in one session is not the same as what changes over weeks and months.

A small 2014 study put 20 volunteers on the same diet and had half do their cardio fasted and half do it after eating. After four weeks there was no significant difference between the two groups. A later review of similar studies reached much the same conclusion. The research is limited, but so far it does not show that fasted training gives you an advantage. Choose based on how you feel and what fits your day.

How should you time meals around your sessions?

The simplest rule is to line up your hardest sessions with your food, not against it. Try to train inside your eating window or shortly before it opens, so a proper meal follows soon after. The table below uses a midday to 8pm window as an example.

Two more things help. First, if you lift, aim for a meal with a good serving of protein within a few hours of the session. Sports nutrition guidance often suggests roughly 20 to 40g of protein per meal, spread across the day, with your total for the day mattering most. Second, do not finish a hard evening session and go to bed on nothing. If your window has already closed by then, the window is in the wrong place for your training.

What should you eat in your eating window?

A shorter window means fewer meals, so each one has to do more work. The most common problem for active people is simply not eating enough, which tends to show up as heavy legs, poor sleep, low mood and sessions that keep getting harder.

• Protein at every meal. Eggs, yoghurt, chicken, fish, tofu, beans or lentils.
• Carbohydrates around training. Oats, rice, potatoes, bread, pasta and fruit give you the fuel for harder sessions.
• Vegetables, fruit and healthy fats. These round out the plate and help you feel full.
• Plenty of fluid. Drink water through the day, including during the fast. Plain tea and black coffee are usually counted as fine in most fasting patterns.

If you cannot comfortably fit enough food into your window, make the window longer. A 10 or 12-hour window is still a routine, and it is far easier to train well on.

What are the most common mistakes?

• Starting fasting and a new training plan in the same week. Change one thing at a time so you can tell what is working.
• Putting your hardest session at the end of a long fast. Then wondering why it felt awful.
• Treating the window as a rule you cannot bend. Moving it for a long run, a race or a meal with friends is sensible, not failure.
• Ignoring warning signs. Dizziness, shakiness, feeling sick or a pounding headache mean stop, eat and drink, and rethink the plan.
• Expecting fasting to do the work. Regular training, enough sleep and enough food matter far more than the clock.

Who should speak to a GP first?

Fasting is not suitable for everyone, and for some people it carries real risk. UK health guidance generally advises speaking to your GP or a registered dietitian before trying it if any of these apply:

• You have diabetes, especially if you use insulin or other medication that lowers blood sugar.
• You take regular prescribed medication, or have a long-term health condition.
• You are pregnant or breastfeeding.
• You are under 18.
• You are underweight, or have lost weight without meaning to.
• You have, or have ever had, an eating disorder or a difficult relationship with food.

If fasting starts to make you anxious about food, or you find yourself thinking about eating for much of the day, that matters more than any plan. Stop, go back to regular meals and talk to your GP.

Do you need to fast to get fitter?

No. Nothing in the research suggests that beginners need a set eating window to get fitter, stronger or faster. What moves you forward is turning up for a sensible mix of running, strength and HIIT week after week, and eating enough to recover from it. If a fasting routine makes that easier, it can work. If it makes training feel worse, regular meals are just as valid a choice.

That is also how Edge approaches training. Running, strength, HIIT and mobility sit in one personalised plan, so you can see your hard and easy days and plan your meals around them. If you are unsure how a session fits your day, you can message a real coach anytime.

This article is general guidance, not medical advice. If you are new to exercise, returning from injury, pregnant, or managing a health condition, speak to your GP before starting, and see a physio for any niggle that does not settle.