The differences between men and women go beyond X and Y chromosomes. Nutrition needs differ with gender and with age, for example. At the core, a healthy balanced diet is similar, yet the details differ. And the details matter. Take fasting, for example. More and more research now shows the effects of intermittent fasting differ in men and women. Say what? I know, I know. Not fair, right? Dietitian Linia Patel delves deeper into the world of intermittent fasting.
Let’s find out more….
Basics of intermittent fasting
Intermittent fasting (IF) is a style of eating where you cycle between periods of severe or complete calorie restriction. The length and time of these periods of calorie restriction and periods of healthy eating vary based on the type of intermittent fasting you choose to do.
In the popular 5:2 diet, you eat normally (i.e., balanced) for five days and then you restrict your caloric intake to approximately 500 calories (women) or 600 calories (men) on the other two days.
In the 16/8 approach, you fast for 16 hours each day and then eat during the other eight hours of the day. Commonly in this approach people optimise the overnight fast and skip breakfast, making lunch your first meal of the day.
Benefits of fasting
There is mounting evidence that, when done properly, intermittent fasting may help with regulating blood glucose, controlling blood lipids and managing your bodyweight and, therefore, helping reduce your risk of diseases like cardiovascular disease and cancer1,2,3.
However, what is really interesting and perhaps less well known is that research on intermittent fasting demonstrates gender inequality!1,2 When you sift through the data on women in a fasted state there are two interesting observations. The first is that there is very little data available. The second is something quite fascinating: women don’t respond to fasting like men do1,2,4,5.
Intermittent fasting and gender
During short periods of fasting, one study showed that men’s metabolism increases up to 14%. In addition, another study reports that the male body improves testosterone utilisation and growth hormone production3. These are all key physiological changes for ripped abs and reduced disease risk. Women, however, do not respond to intermittent fasting like men do1,2,4.
Frustratingly, there are no controlled human studies for us to draw clear conclusions about how intermittent fasting affects women. The little research we do have, however, does suggest that women should be cautious with intermittent fasting.
A recent study looking at rats tells us why. The subjects of this study included 10 male and 10 female normal-sized rats. Half the rats ate whenever they wanted. The other half only ate every second day and they fasted (i.e., food was removed) in between. This went on for 12 weeks, which extrapolated for humans is about 10 years. At the end of the 12 weeks, the fasting female rats had lost 19% of their bodyweight, yet their ovaries had shrunk and adrenal gland size had increased, which indicates hormone dysfunction and exposure to chronic stress5. The take-home message from this study and other similar research is that key functions like hormone production and appetite control are incredibly sensitive to energy intake5,6,7.
When looking at human studies, one study found that, while IF improved insulin sensitivity in male subjects, female subjects saw no such improvement. In fact, for women, the glucose tolerance actually worsened. Other research looking at the effect of IF on blood lipids found that men’s ‘good’ cholesterol remained stable and their triglycerides decreased, yet women improved their ‘good’ cholesterol but their triglycerides remained stable. However, it’s also important to note that there have been studies that show the same positive effect in men and women (in terms of body fat, bodyweight, blood pressure and blood lipid levels). However, the catch to this research is that subjects in these studies were obese and post-menopausal, therefore the results may not be applicable to leaner people or women in the perimenopausal window.
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It’s the hormones….
So why do women respond so differently? There are a couple of theories.
Firstly, the hypothalamic-pituitary-gonadal (HPG) axis, which is responsible for regulating key hormones that are involved in functions like appetite control and reproduction, is incredibly sensitive to energy intake5,6,7. The effect of a negative energy balance is thought to work through kisspeptin. Kisspeptin is a protein-like molecule that neurons use to communicate with each other to get important stuff done. Research suggests that women have more kisspeptin than males, meaning greater sensitivity to changes in energy balance5,6,7.
Secondly, it also seems that our livers are different when it comes to energy metabolism. Research looking into livers of mice following IF indicates livers of male mice stop producing energy storage molecules, whereas livers of females make use of all available resources in an effort to ‘stock’ the energy necessary to maintain the reproductive capacities. This means that, if needed, the woman’s body is more likely to tap into lean muscle mass as an energy source8.
Negative energy balance is most likely to blame for the hormone domino effect. However, it’s not just about how much you eat. It’s about the other stressors in life (i.e., too little sleep or life stress) that could also trigger a negative cascade effect, suggesting that particularly for women there is a time and place for fasting1,4,9.
Take home message
Research on the science of intermittent fasting, especially for women, still has a long way to go. Women wanting to dabble with intermittent fasting should do so with caution and should most definitely speak to a dietitian or registered nutritionist.
- Pradeep M et al (2016), Role of therapeutic fasting in women’s health: an overview, Midlife Health, 7(2): 61-4.
- Yeoh E et al (2015), Fasting during Ramadan and associated changes in glycaemia, caloric intake and body composition with gender differences in Singapore, Observational Study, 44(6): 202-6.
- Trepanowski J et al (2011), Impact of caloric and dietary restriction regimens on markers of health and longevity in humans and animals: a summary of available findings, J., 10: 107.
- Horowitz J (1999), Effect of short-term fasting on lipid kinetics in lean and obese women, Cell Metabolism, 276 (2).
- Freire T et al (2020), Sex-specific metabolic responses to 6 hours fasting during the active phase in young mice, Physiol., 10.
- Bazhan N et al (2019), Sex differences in liver, adipose tissue and muscle transcriptional response to fasting and refeeding in mice, Am J. Physiol., 8(12): 1,529.
- Kumar S et al (2013), Intermittent fasting dietary restriction regimen negatively influences reproduction in young rats: a study of pypothalamo-hypophysial-gonal axis, PLoS One., 8(1).
- Torre S (2018), Short-term fasting reveals amino acid metabolism as a major sex discriminating factor in the liver, Cell Metabolism, 28(2): 256-267.
- Torre D et al (2011), Amino acid dependant activation of liver estrogenic receptor alpha integrates metabolic and reproductive functions via IGF-1, Cell Metabolism, 13: 205.
About the author
Linia Patel has a BSc degree in biochemistry and physiology. Since graduating in 2006, Linia has become a leading dietitian and sports nutritionist. She is currently a PhD candidate in public health. Her passion is translating nutritional science into easy-to-digest and practical advice. @liniapatelnutiriton
Where to next? Why not find out the difference between hunger and appetite HERE