Updated: Apr 1, 2020
The Pros & Cons of Intermittent Fasting
>Reduces chronic disease risk, including cardiovascular disease, obesity and diabetes¹
>Benefits gut health²
>Aids weight loss³
>Improves cognition, learning and memory¹
>Slows ageing and promotes longevity⁴
>Better stress response⁵
>Can lead to hormonal imbalances and even infertility for some women⁸
Intermittent fasting (IF), where eating is restricted to a limited period during the day, has been practised since ancient times however it has become a topic of increasing interest over recent years, particularly with regards to its associated health benefits. There are several forms, including 16/8, which involves fasting every day for at least 16 hours; 5:2, where eating is restricted on 2 days of the week; and whole day fasts, undertaken once or twice a week.
Since most of us already fast overnight, a lot of people choose to simply extend this by skipping breakfast to achieve the 16 hour window, however my preference is to instead eat your last meal of the day at mid-afternoon, read why below!
Several studies have demonstrated that IF is able to increase general health, benefit the gut microbiome, and prevent obesity and chronic disease². Current research suggests that IF, particularly involving prolonged nightly fasting, may be used as an effective non-pharmacological approach to achieving weight loss, reducing chronic disease and improving overall health².
IF has been shown to induce physiological responses similar to those observed from regular aerobic exercise leading to improved health indicators such as lower insulin resistance, blood pressure, body fat and inflammation¹. By altering levels of important hormones involved in metabolism and hunger (including insulin, ghrelin and leptin), and creating a fat-burning state (ketogenesis³), IF can stimulate significant positive changes in the body¹, including enhanced fat metabolism, removal of damaged cells and inhibition of the mTOR pathway, which regulates cell metabolism, growth and survival⁴.
By reducing inflammation as well as oxidative stress throughout the body, IF has shown benefits for several bodily systems including the brain and nervous system, enhancing cognition, learning and memory, as well as sensory and motor function¹. IF can also increase the body’s ability to deal with stress, boost immunity, slow ageing¹’⁵ and instigate changes in the gut flora that helps to protect against the development of chronic health conditions¹.
Studies have shown that fasting, particularly overnight, can positively affect the diversity of the gut microbiome (associated with better health) as well as improve the permeability and integrity of the gut lining by allowing it more chance to heal during the overnight fast when the body is rejuvenating⁷.
Night-time IF has also been shown to improve sleep and circadian rhythms⁶ so the timing of food intake appears to be important due to the link between blood glucose levels, insulin and cortisol (the stress hormone which peaks during the day and opposes our sleep hormone, melatonin)⁷. Several research studies have confirmed that consuming food earlier in the day and reducing/eliminating food consumption in the evening can aid weight loss and improve health by significantly lowering glucose levels and inflammatory markers ⁷. In contrast, eating at night can lead to elevated blood glucose levels, increasing the risk of developing diabetes, and disrupt sleep⁷.
If you are considering giving IF a try, it’s important to begin gradually by slowly increasing the period of fasting, and preferably under the guidance of a professional Nutritionist to ensure that you are still getting all the nutrients your body needs. They can also help with the often arduous but all-important meal planning, which really is essential for successful IF!
Cautions for Women...
Adopting this type of fasting regime can have several detrimental effects on a woman’s health however, leading to hormonal imbalances and ultimately to fertility issues⁸. I’ve set out below my recommendations for women who wish to benefit from the positive aspects of IF without potentially risking their health.
Unlike men, women can be extremely sensitive to the signals of potential starvation, with the body viewing it as a stressor⁹. So periods of fasting can really imbalance a woman’s hormones, with studies indicating that as well as stopping ovulation from occurring and causing absent menstrual cycles, a woman’s ovaries may even shrink and insomnia may be experienced⁸.
The reproductive system is very closely linked to our metabolism via several hormonal pathways, such as the hypothalamic-pituitary-gonadal (HPG) axis, which regulates the production of hormones involved in ovulation, including oestrogen and progesterone¹º. The female body has evolved throughout time to ensure that reproduction takes place during times of food abundance and is inhibited when food is scarce⁹. The HPG axis is therefore very sensitive to environmental factors such as stress and fasting, with the stress hormone cortisol able to suppress the ovaries’ production of both oestrogen and progesterone¹º.
Note that oestrogen isn’t only required for reproduction, it is also an important regulator of metabolism¹º. (I’m going to follow this post with one about why it’s essential that women ovulate in case you are thinking that could be an option!)
Other factors that can affect a woman’s hormonal balance include:
- insufficient food intake
- nutrient deficiencies
- excessive exercise
- insufficient rest and recovery
- high stress
- illness, infection and chronic inflammation
So instead of following a strict IF regime with consecutive days of long fasts, I recommend that women adopt a modified approach, which involves limiting IF to 2-3 days of the week and preferably to non-consecutive days. This more gentle approach should help to prevent any significant hormonal swings, enabling women to reap all the benefits of IF without risking their health. In addition, it’s important to ensure that any nutritional deficiencies are properly addressed before experimenting with fasts so that they begin with a strong nutritional foundation.
Also note that IF is not advised during pregnancy or in cases of chronic stress, insomnia and eating disorders⁸.
¹ Longo, V. D., & Mattson, M. P. (2014). Fasting: Molecular Mechanisms and Clinical Applications. Cell Metabolism, 19(2), 181–192.
² Mindikoglu, A. L., Opekun, A. R., Gagan, S. K., & Devaraj, S. (2017). Impact of Time-Restricted Feeding and Dawn-to-Sunset Fasting on Circadian Rhythm, Obesity, Metabolic Syndrome, and Nonalcoholic Fatty Liver Disease. Gastroenterology Research and Practice, 2017, 3932491.
³ Dillon, K. Gupta, S. (2019) Biochemistry, Ketogenesis. Updated 21 April 2019. StatPearls Publishing LLC, Treasure Island.
⁴ Laplante, M. Sabatini, D. (2009) The mTOR Signaling at a Glance. Journal of Cell Science 2009 122: 3589-3594;
⁵ Soni et al, 2011. Ghrelin, Leptin, Adiponectin, and Insulin Levels and Concurrent and Future Weight Change in Overweight Postmenopausal Women. Menopause. 2011 Mar; 18(3): 296–301.
⁶ Longo, V. D., & Panda, S. (2016). Fasting, circadian rhythms, and time restricted feeding in healthy
lifespan. Cell Metabolism, 23(6), 1048–1059.
⁷ Patterson, R. Sears, D. (2017) Metabolic Effects of Intermittent Fasting. Annual Review of Nutrition 2017. August 2017. 37:1, 371-393.
⁸ Kumar, S., & Kaur, G. (2013). Intermittent fasting dietary restriction regimen negatively influences reproduction in young rats: a study of hypothalamo-hypophysial-gonadal axis. PloS one, 8(1), e52416.
⁹ Fontana, R., & Della Torre, S. (2016). The Deep Correlation between Energy Metabolism and Reproduction: A View on the Effects of Nutrition for Women Fertility. Nutrients, 8(2), 87.
¹º Rettberg, J. R., Yao, J., & Brinton, R. D. (2014). Estrogen: a master regulator of bioenergetic systems in the brain and body. Frontiers in neuroendocrinology, 35(1), 8–30.