Intermittent fasting (IF) promises better health and weight loss, just by narrowing the hours in the day when we eat. However, there are valid concerns, especially for women. Skipping meals, especially breakfast, is something many women have learned to do anyway to minimise calorie intake and manage their time deficit. 

There is quite a lot of evidence reported to show that IF can help to improve insulin resistance, lower blood glucose and triglycerides, improve immunity, improve brain health, normalise fat metabolism and improve longevity. One of the problems is that most of the studies done have been done on men, and very few are women-specific. Those that are, point to women’s metabolisms suffering with IF in ways that are not often discussed. This article goes into this in more depth, but below I discuss the reasons I spend more time discouraging women from IF, than encouraging them. 

What is IF?

16:8 You fast for 16 hours and eat during an 8 hour window, such as between 12midday and 8pm. 

12:12 fasting is the most gentle approach of fasting 12 hours a day, eating in a 12 hour window. 

5:2 In this way of eating you eat normally 5 days a week, and you eat only 500 calories on 2 days a week. 

Alternate day fasting is where you may follow a 16:8 or even a 20:4 fasting regime, but only on alternative days. Some people fast completely on alternative days. 

Another way of fasting is to just eat one large meal a day. 

Does IF work well for insulin sensitivity in women?

In the literature, while men experience increased insulin sensitivity,  women do not tend to experience increased insulin sensitivity and in fact experience the opposite, a decrease in glucose tolerance. 

Downregulation of reproductive hormones

It is well known (but rarely mentioned when discussing IF) that fasting is not good for women’s reproductive systems. In rats, when fasting the female body tends to down-regulate its reproductive capacity, and become more masculinised. This also means becoming more aroused, hyper-alert and energetic- but less fertile. Ovaries shrank, cycles became less regular, some lost their cycles altogether. This is probably a survival mechanism developed in times of scarcity, which makes sense. However, this is also associated with a heightened stress response, irregular or cessation of cycles, and increased infertility. 

Fasting should always be avoided by women who are pregnant, and also those who are seeking to become pregnant. But any woman can also consider this issue. 

IF and Menopause

Since our hormones become more stable after menopause, and we tend to put on some weight at this time (which is probably a natural phenomenon), many women take up IF at this time. However, the other factors I discuss here are still potential issues for women at this time. It is very individual. 

IF and Libido

There are no studies on this in women, however it has been shown to lower libido in men. It is probably that it would also lower libido in women because it interferes with normal reproductive hormones, lowering oestrogen and progesterone levels. However, for some women, the benefits of IF may outweigh the potential disadvantages, as feeling better about our bodies is strongly associated with better libido. 

Reduced thyroid function

Studies done on women in the Muslim month of Ramadan when fasting during daylight hours is done, show that thyroid hormones are negatively affected. For many normal healthy people this is not too much of a problem and they do return to normal after fasting, but for the millions of women with thyroid disease, diagnosed or subclinical, this could present a problem. Especially if they intend to continue IF indefinitely. More studies need to be done in this area. 

Increased Eating Disorder Behaviour

This heightened survival mechanism when fasting for women may also be an underlying factor for the next point I want to bring up. Which is, it may be an underlying factor in women’s vulnerability to anorexia nervosa. 

Women are used to starving themselves, and I spend more time in my practice trying to encourage women to eat enough and in a balanced way, than encouraging them to eat less. Disordered eating is so rife and normalised amongst women, and we tend to consider it completely normal to undereat and skip meals, even while preparing nutritious meals for loved ones. 

So IF can trigger or worsen disordered eating behaviour in the endless quest for weight loss. However, it is also unlikely to be a sustainable long-term approach, and there is a definite increased risk of developing an eating disorder such as anorexia nervosa or binge eating disorder. 

Increase in Stress Hormones

Going too long without food can induce a sort of euphoric state due to the extra stress hormones released to help balance blood sugar. So it feels like it must be good because it feels good for a while- this is a survival mechanism useful for periods of actual starvation when increased mental clarity can help find food. Women’s bodies are particularly sensitive to increased stress levels, and it is one of the underlying reasons for so many hormonal imbalances. Reproductive hormones are less important when your survival is at stake. 

I find most women are already living in chronic stress, and IF and calorie restriction adds yet another stressor. 

Not all women will have problems with this, but enough will that it needs to be discussed openly. 

We can handle stress, and it can give us extra clarity and drive- until we burn out, because those mechanisms in the body are not meant to be turned on for months or years at a time. Burnout is a very common phenomenon. 

Nutritional Imbalances

I have spoken with women who feel guilty because they have tried IF and found themselves bingeing on unhealthy foods during their eating window. This is a response to calorie deprivation and blood sugar drops. It has been shown that women who skip breakfast are more likely to binge on high calorie junk foods in the afternoon. That’s why we develop all this weird guilt around food because we feel able to control our eating sometimes, such as in the morning, but not other times such as mid-afternoon or late evening. That is because we cannot override thousands of years of evolutionary mechanisms with our meagre willpower. When the body feels there is starvation, it will reach for any high calorie, low nutritional value food that is available- which is often cakes, biscuits, icecream, sweets and chocolate. 

Any sort of calorie restrictive diet can lead to nutritional deficiencies, as it takes as least 1600 calories, and probably more, of good quality food, to meet our nutritional needs as women. Long term calorie restriction, including IF, which is what many women do, can lead to nutritional deficiencies such as protein, B12, calcium, iron and other essential nutrients. 

Ignoring our Intuition

Another aspect of IF for women is that as with following any strict program, it can teach us to ignore listening to the cues from our own body, especially hunger cues. Many of us are so attuned to following the food rules we have made for ourselves, we have lost touch with the cues our body sends us to look after its needs. We lose trust in our body, and in the sensation of hunger. We feel they betray us. But it is our thinking that is betraying us. Not listening to our hunger cues, such as outside our window of eating, can lead to a lot of confusion. 

Eating small meals more often has been shown to increase our metabolic rate, and is often a more natural way for women to eat. But it takes time to undo years of diet conditioning. 

My recommendation

If you feel you still want to try IF, what I recommend is to do a gentle version such as 12:12. That just means fasting for 12 hours overnight and eating in a 12 hour window. For example, from after dinner at say 7pm, until 7am the next morning. This is unlikely to do too much damage, while still providing benefits of giving your body a break from digestion. 

However, if you are pregnant, trying to get pregnant, under a lot of stress or burned out, have thyroid issues, susceptible to eating disorders, or have a long history of yoyo dieting, I would not recommend IF at all. 

IF affects us all differently, and women differently to men. There are women who find it works well for them. By all means, experiment if it calls you. 

However, any weight loss approach that doesn’t involve getting MORE in touch with your body and its signals, rather than less, is unlikely to be sustainable in the long run.