So here’s a question that comes up regularly in my clinic: should you be training in a fasted state or do you need to ‘fuel’ before you train?
Let me start by making it clear that the answer depends on many factors such as your goals, stress levels, body composition, your biological gender, hormonal balance, insulin sensitivity etc.
There’s no one size fits all, so here’s some guidance as to whether fasted exercise is suitable for you:
When is it beneficial to train fasted:
If your goal is to reduce body fat (you will be burning fat stores in a fasted state), particularly belly fat which is a potential symptom of insulin resistance (reduced ability for cells to take up glucose, so excess glucose is converted to triglycerides and stored as body fat, particularly around the abdomen).
In the case of insulin resistance (or pre-diabetes/type 2 diabetes) fasted exercise first thing in the morning is a great way to burn off excess glucose produced by the liver overnight. Even a fasted walk can be beneficial for reducing these morning glucose levels. Remember active muscles can use glucose without the need for insulin so exercise is key for reducing glucose levels without requiring spikes in insulin.
We’ll come onto this in more detail below, but factors which are conducive to fasted exercise include: being male (biologically), having minimal life stressors, being post-menopausal and having a healthy metabolism (particulalry a healthy thyroid function).
The timing of fasted exercise is also something to consider. Fasted sessions are most suitable for those who can train first thing in the morning, it’s not so practical for those who train in the evenings and will have had to skip two meals before training. This may negatively affect your available energy for performance and you’re also likely to be in a calorie deficit which would be detrimental if your goal is to gain muscle. Over time you would have insufficient macronutrients to build muscle mass (essentially you’d be in a catabolic state, breaking down muscle tissue).
With this in mind, don’t be a hero and try to continue fasting after your morning workout, you want to break your fast in your post workout window, as exercise (particularly high intensity) will raise your cortisol levels (stress hormones). Cortisol is catabolic, you’ll need glucose to bring this back down, ensuring optimal muscle recover. For example a good post workout option would be a shake containing protein powder (whey isolate/collagen/plant protein), coconut water (electrolytes and carbs) and kiwi fruit/pineapple (carbs and bromelain which aids in muscle recovery).
When might it be beneficial to train in a fed state:
As mentioned above, I tend to tread more carefully with recommending fasted exercise for the following people:
Women of menstruating age: anything that might hinder procreation (the ability to get pregnant) has the potential to down regulate hormones including your metabolism (thyroid function). This is so that the body can hang onto calories and nutrients needed for pregnancy. Fasted exercise can therefore negatively impact sex hormones, thyroid hormones and encourage fat storage. Some women have a higher tolerance for this but if you’ve historically played about with a lot of fad diets, overtraining and chronic calorie deficits, you may not respond well to fasted exercise.
People with high levels of perceived stress: I talk about this a lot, the ‘hurry, worry, money’ culture has meant that many of us are in chronic states of low level stress, adding fasted exercise on top of other life stressors can again down regulate sex and thyroid hormones (and therefore metabolism).
Those whose goal is to gain weight or muscle: you’d need to make sure you were still hitting your overall macronutrient goals and adequate calories if you wanted to train in a fasted state.
People who suffer with hypoglycemia (low blood sugar): obviously training in a fasted state (where you will be depleting glucose levels) may exacerbate this hypoglycemia. This doesn’t necessarily mean you need to load up on carbs before exercise, a simple shake may be all you need (combine fat, protein and carbs to keep your blood sugar levels throughout your workout).
Those with osteopenia: when we sweat, we lose minerals, including calcium, so exercising in a fasted state may exacerbate the loss of calcium. Again this is probably more of a case of managing your overall intake of nutrients across the course of day and making sure you are consuming adequate sources of calcium, vitamin D and Vitamin K (the three work in synergy to maintain bone density)
Outside of the factors mentioned above, the type, frequency, duration and intensity of exercise is obviously also important to consider, for example the difference in energy requirements between a fasted 30 min walk and a fasted marathon.
I hope this has been helpful, like anything with nutritional recommendations, the detail is where you’ll find the best results for you.
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The content in this article is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding your health.