You’ve probably heard the old adage that “breakfast is the most important meal of the day”. However, as more people try to hack their biology and control their weight, intermittent fasting has skyrocketed in popularity.

This lifestyle choice has seen breakfast fall out of favour, with many people opting to replace the first meal of the day with protein powders or bulletproof coffees and others choosing to skip the meal altogether and fast right through to lunch.

Some people report experiencing better mental clarity and focus, improved metabolism and more effective weight management after a period of intermittent fasting and some studies have backed up these effects.

How well intermittent fasting will work for you totally depends on your lifestyle, your physiology and your stress levels. However, when you add exercise and high strain into the mix, this is where things can become complex, even dangerous.

According to Stacy Sims, a fitness expert and nutritional science researcher specialising in the effects of biohacking on women’s bodies: “Intermittent fasting is useful. This is particularly true for the general population who are not very active and struggling with metabolic diseases.” However, Sims explains that, “you do not garner any additional benefits from layering intermittent fasting on top of exercising.”

She says: “If you’re already exercising, it’s not particularly helpful. And if you’re a woman who is adding athletics on top of intermittent fasting, it can be harmful to both your performance and your health.”

This is because high-intensity exercise puts strain on the body and if the body is not adequately fuelled with enough calories to provide the requisite energy for a workout, this can affect hormone regulation and muscle recovery.

Is it safe to fast and work out?

When it comes to whether fasting and exercise are compatible for everyone, there’s no simple answer. However, research does show that fasting and then putting strain on your body through exercise can be more detrimental for women’s bodies than it is for men’s.

“It comes down to kisspeptin,” explains Sims. Kisspeptin is a neuropeptide that’s responsible for sex hormones and endocrine and reproductive function. She explains that kisspeptin also plays a significant role in maintaining healthy glucose levels, appetite regulation and body composition.

“It’s also more sensitive in women than men. When it gets perturbed, our sex hormones aren’t produced and released the way we need them to be. Intermittent fasting and keto both disrupt kisspeptin production. When our brain perceives we have a deficiency in nutrients, especially carbohydrates, we have a marked reduction in kisspeptin stimulation, which not only increases our appetite but also reduces our sensitivity to insulin. This is why research shows intermittent fasting is more likely to cause impaired glucose intolerance in women, but not men,” says Sims.

But what about men’s bodies? Is fasting and then exercising bad for them too? Personal trainer and head of sport science at Torokhtiy Weightlifting, Sergii Putsov explains that, “fasting before exercise can be safe, but it mostly depends on one’s goals, body type and workout type.

“For some people, fasting before exercise can help boost fat metabolism, improve insulin sensitivity, and enhance endurance. It may also help improve oxidation, meaning that your body can use stored fat as the primary energy source. This is good for individuals aiming for fat loss or endurance athletes who want to enhance their metabolic flexibility.”

However, Putsov explains that despite these benefits, your performance in the gym might decrease if you don’t know what you’re doing when it comes to balancing fasting with exercise.

“Without sufficient glycogen stores, your body can feel fatigued, leading to an increase in the rate of muscle protein breakdown,” he notes.

When our brain perceives we have a deficiency in nutrients, especially carbohydrates this can affect our insulin sensitivity (Getty Images/iStockphoto)

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According to Putsov, if you’re going to fast and then work out, you should have a specific reason for doing so in mind and be aware of how your workout style is likely to impact your body and your recovery on an empty stomach.

Edwina Jenner, a PT and health coach agrees: “Not all workouts are created equally, so it depends on the demands that will be placed on your body. An hour of Pilates or yoga could be done in a fasted state, however, it would be different if you want to preserve or gain muscle.

“A strength training session requires fuel to help you optimise your training. Otherwise, you will be low on energy and fatigue more quickly. Your workouts become less effective, and you won’t have the reserves to push through your session, so it will take longer to achieve your goals.”

On the whole, unless you’re working towards a particular weight or metabolic target, training without fuel is rarely recommended by experts, and while intermittent fasting can offer some great benefits, it isn’t best to pair this with practice with a gruelling workout.

“Eating before training is very personal,” says Jenner, “however, 99 per cent of women I train who try fasting cannot stick to it, and it just leaves them feeling hungry, deflated, and as if they are failing because they can’t get through their workouts feeling hungry and tired.”

This echoes Sims’ research on fasting and exercise. “What happens when we layer exercise stress on top of the stress of denying our bodies an important fuel source? Stress hormones like cortisol rise even higher. As you keep increasing that stress, it keeps your sympathetic drive high and reduces your ability to relax. Your thyroid activity is depressed, which messes with your menstrual cycle.

“Your body also starts storing more belly fat. So now you’re looking at disrupted menstrual cycles, higher anxiety and stress, impaired performance and often weight gain – pretty much the opposite of what you’re looking for,” she explains.

So if you’re hoping to lose weight, exercising on top of a fast could actually have an adverse effect and result in hormonal imbalances and symptoms like cortisol face. Fasting can help with weight management, however, experts recommend not teaming it with intense exercise like weightlifting, especially if you’re not totally certain about how your body will react.

Experts advise waiting about an hour and a half to two hours before exercising after eating (Getty)

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What to eat before a workout

Jenner recommends skipping the fast and opting for a nutritious breakfast before a big exercise session. But, she says, there are rules for this too, especially if eating in the mornings generally makes you feel more sluggish and tired than not eating.

“You want to give your body a quick energy source but not overload your digestive system, leaving you feeling sluggish and nauseous when working out. So 20 or 30 minutes is an ideal time to go between eating and working out first thing,” she explains.

If you want to get more technical, the exact time you should wait will depend on the size and portion of your meal. “For small, easily digestible meals or snacks such as bananas, smoothies, or peanut butter toast, wait about one hour before you start your workout session,” says Putsov.

“For a larger meal such as oatmeal, full breakfast, or even eggs, it is good to wait about an hour and a half to two hours before exercising. This gives your body enough time to break down the food, which prevents digestive discomfort while exercising.”

Jenner also recommends balancing protein and slow-release carbs before exercise – something like overnight oats is perfect for this as it contains a mix of both nutrients, and you can add an additional scoop of protein. You might also want to consider a protein shake before you train to give you fuel for the session and keep you full.

In terms of which foods to avoid before exercise, Jenner says: “I would avoid sugary cereals, which will give you a sugar rush and leave you feeling hungry and low on energy. If you have just a piece of toast, which so many people do, add an egg to some smoked salmon for the extra protein source. It’s about eating foods that will give you a slow, steady release of energy rather than a quick spike and drop.”

“Besides what you eat, pay close attention to hydration,” adds Putsov. “Make sure you are adequately hydrated before exercising. Drinking plenty of water in the morning can help kickstart your metabolism and maintain energy levels. If you are engaged in high-intensity exercises, you can consider adding electrolytes to your water too.”

Though exercise can benefit everyone, the worst thing you can do for your body is push it to its physical limit without the support it needs. Without adequate food and water before an intense training session, you could end up feeling tired, lightheaded and nauseous in the short term, and in the long term, you could end up with bigger problems like fatigue, hormonal conditions and issues with mood.

Issues like low iron or menopause can also exacerbate low energy levels caused by fasting, so be sure to take your whole health into account and take steps to support yourself if you’re planning on hitting the gym hard.

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