Is fasting bad for women's metabolism? Nutrition expert reveals 'when women fast it increases their…’

A doctor explained how oxidative fibres allow women to exercise long and slow by using free fatty acids. However, fasting can increase stress and cortisol.

Fasting is a huge part of many cultures, including India. Meanwhile, women often follow popular diet trends to get fit and healthy and starve their bodies of essential nutrients to get in that ‘perfect shape’. But how does fasting affect women?

In a YouTube video shared by the channel Huberman Lab Clips on March 29, Andrew Huberman, a neurobiology and ophthalmology professor with an Instagram following of 7 million, sat down with Dr Stacy Sims, PhD, to delve deeper into the ‘truths about fasting for women’. Dr Stacy is an exercise physiologist, nutrition scientist, and expert in female-specific nutrition.

Why fasting is non-ideal for women's metabolism?

Per Dr Stacy, women have more oxidative fibres in their bodies. “So, we hear all the things about fasting improving our metabolic flexibility, telomere length, [and] parasympathetic activation, but by the nature of women having more oxidative fibres, we are already metabolically more flexible than men,” she explained.

What are oxidative fibres, and how do they relate to metabolic flexibility?

Explaining what oxidative fibres are, Dr Stacy said that ‘they are muscle fibres that have more aerobic capacity, so those are the ones that you can go long and slow for a very long period of time because it uses a lot of free fatty acids’.

She further explained, “You need a little bit of glucose in order to activate those free fatty acids. So, we look at when a woman starts to exercise; she goes through blood glucose first and then gets into free fatty acid use. She doesn't tap so much into liver and muscle glycogen, which is another misconception that happens.”

What happens when you fast and why you should avoid it

When women fast or do ‘fasted workouts’ (fasting while working out), trying to improve that metabolic flexibility, it “increases the stress”. Per Dr Stacy, when one considers the overall stress, it leads to a cortisol increase. “They can't hit intensities high enough with no fuel to invoke the post-exercise responses of growth hormone and testosterone, which then drop cortisol. So, from an overall stress perspective, that fast didn't work out and holding that fast for a long period of time increases cortisol,” she explained.

Per the National Institutes of Health (NIH), symptoms of excess cortisol include weight gain (especially in the face and abdomen), fatty deposits between the shoulder blades, diabetes, hypertension, hirsutism in women, proximal muscle weakness, and osteoporosis.

Why shouldn't you fast?

Lastly, Dr Stacy explained though many women will say that fasting has been working for them for many years, the other side of the question is, how much better would they be if they were to actually pay attention to the circadian rhythm and fuel according to the stress at hand?

“…Knowing that you're going to garner less stress that way and if we're really tying in nutrition according to that profile, instead of following a fast, we see better brain improvements as well we see more cognitive function, we see less thyroid dysfunction, and overall a woman does much better when we are not in that fast state,” she added, in the end.

Note to readers: This article is for informational purposes only and not a substitute for professional medical advice. Always seek the advice of your doctor with any questions about a medical condition.