What happens to your body when exercising during  fasting?

If you’re trying to lose weight whilst fasting and exercising be mindful of the type of exercise you choose so as not to upset your hormones. 

Fasting alters your hormone levels by lowering insulin and increasing your growth hormone,
together with boosting the release of the fat-burning hormone norepinephrine (noradrenaline).
Exercising in a fasted state can help to manage blood sugar levels, and may prevent the onset of
diabetes. What’s more, it boosts your metabolism to help you lose weight. Working out for just 30
minutes at high-intensity in a fasted state, when muscle glycogen (stored carbohydrates) are
depleted and energy levels are low,  induces stress on the body to promote autophagy; where
the body starts eating its cells and recycling the energy to make new stronger ones.

Before you sing hallelujah – don’t get too excited, as all good things in life have a flip side. 

The flip side is that working out at high-intensity in a fasted state your body may start breaking down
muscle, in addition to fat, to use protein for fuel, as there is no available glucose in your muscles
needed for fast-burning energy release to power your workout, which will then slow down your
metabolism and weight loss goals.


Be aware!  Fasting long-term for weight loss could slow down your metabolism further as your
body adapts, over time, to regular exercise and starts reserving its fat stores.


When is the best time to exercise when fasting?
Exercising on an empty stomach, when your glucose and energy is low is challenging.
To make your workout easier whilst fasting consider how you best perform exercising around your food timings. It helps to keep a food and activity diary to record your workout intensity, match this against your energy performance as this helps to determine whether you should exercise before, during, or after the fuelling window. Typically a fasting day would incorporate a fuelling window of 8 hours.

So if you choose to exercise during a fasted state go easy on yourself, and remember that exercise is generally best performed during the fuelling window, when glucose energy is available to
help you power through a workout and exert more effort into your exercise routine;
providing that you feed with the correct macro-nutrient balance and timings before and after

Exercising before the fuelling window is more suited to individuals that perform strong when
exercising on an empty stomach, such as endurance runners. 

It is vital to follow a post-workout nutrition plan to replenish glycogen stores fast. Try 
nutrient-dense carbohydrates with an adequate intake of protein to prevent free-radical damage from oxidative stress, aid muscle adaptation to exercise and support recovery.

Eat 400-500 calories, including 20-30g high-quality protein within 2 hours after exercising.

If you’re trying to build muscle then continue to consume protein every four hours within your feasting window. After this window you will miss the opportunity to use post-recovery fuelling. Simply listen to your body and choose the option best suited to you.


Does fasting affect the ability to perform?
During a fasted state, the muscles are depleted of glycogen and circulating blood glucose levels are
low. Exercising in a fasted state induces stress on the body, which switches to burn fat as a fuel to
sustain energy to exercise. At high-intensity the body’s preferred fuel is fast-energy glucose from
carbohydrates, as fat takes longer to break-down and burn into energy.

Over time, regular, but not sustained, intermittent-fasting enables our body to adapt to physical activity, encouraging our body to burn fat for fuel which enables us to sustain longer and more intense exercise – making us stronger and faster.


What’s the best way to fuel exercise when fasting?

The foundation to good health is all about the nutritional base, it is key to incorporate more whole-grains, vegetables, fruit, nuts and seeds that contain fibre, which helps to slow the rate at which energy is released into your blood stream, and this is key to help stabilise your blood glucose levels, so that you don’t get tired when you start to increase your physical activity level. Add plenty to your daily diet (but not immediately after exercise as you need to replenish fast and fibre will slow that down). Fibre will also help to support your gut health to produce a good diversity of digestive enzymes to help facilitate in the uptake of nutrients to support your health. These foods also contain lots of vitamins and minerals to provide you with a nutrient-dense diet to optimise your health.














Vegetables and fruits in particular contain lots of antioxidants, and these are needed more when you begin to exercise. Antioxidants such as carotenoids like beta-carotene, found in sweet potato and some other orange fruits and vegetables , and lycopene a phytochemical found in tomatoes and some other red fruits and vegetables, and vitamin C abundant in citrus fruit.

Antioxidants play an important role in fighting free radical damage from oxidative stress generated from exercise. A simple way to incorporate more antioxidants into your diet is to add herbs and spices to your meals such as, garlic, turmeric, basil and ginger. Incorporate dark leafy greens too into your meals as these are packed with vitamins and minerals including; folate, iron and calcium and vitamin B12. Folate and B12 work together to make red blood cells to supply O2 to our muscles – to fire your workouts faster.


Key nutrients:
Omega-3 Fatty Acids
Eat oily fish, such as sardines, mackerel and salmon (boosting vitamin D too) twice weekly to get
beneficial omega-3s, or consume flax or walnuts as plant-based sources of omega-3 fatty acids.
Ensuring adequate levels of Omega 3 will help to nourish your red blood cells and reduce
inflammation for a faster recovery post-exercise.


Vitamin C
It is a very powerful antioxidant that helps to combat free radicals created post-exercise, to promote faster healing and aid recovery. Foods including citrus fruit, strawberries, kiwi fruit, mango, oranges, tomatoes, broccoli and red pepper are particularly high in vitamin C.


Calcium plays an important role maintaining normal muscle contraction, it works by depositing
calcium to individual muscle fibres to improve its efficiency which helps when adapting to exercise.
Food sources naturally high in calcium include dairy products like milk, yogurt and cheese, in
addition to steamed tofu, sardines and whitebait. Dark green leafy vegetables, such as broccoli, bok
choy, spinach and kale are also good sources.


My top tip to train while fasting
 On fasting days stick to low-intensity exercise, such as restorative yoga, hiking and cycling.
Reserve HIIT and RET training exercises for non-fasting days. Alternate working on different
muscles groups daily, to allow time for muscle adaptation to exercise and avoid injury.


If you’d like support to reach your nutrition and lifestyle goals through a more personalised approach then take a peep at my weight loss programmes which can be tailored to fit you. Fancy a chat, pop me a message!




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