Intermittent Fasting

The Intermittent Fasting trend looks like it’s here to stay and has captured the attention of both the general public and researchers. Some swear by it for weight loss and others claim that it boosts their brain health- but what does the evidence actually say? And is it safe?

What is Intermittent Fasting (IF)?

IF differs from other diets as it focuses on when you eat rather than what you eat.

There are three main types of IF

  • 16:8: Fasting window of 16 hours and eating window of 8 hours, the most popular IF method as people find it easiest to stick to
  • 5:2: Eat normally for 5 days with 2 non-consecutive fasting or low-calorie (<500-600kcals) days
  • Alternate day fasting: Fast every other day, some allow for 500 kcals during fasting days

We do not have clear evidence to say which protocol is best. It is important to note that a longer fast e.g., 36 and 72-hours, are not necessarily better and can be medically risky.  Always check with your doctor before starting any IF regime.

Weight loss

Research has found that IF can be a safe and effective tool for weight loss. Several human studies have found that IF resulted in a reduction in body weight, body fat and waist circumference. The main reason people lose weight on IF is that by having less time to eat, you tend to eat less food (that is, fewer calories) which results in weight loss. There are some claims that intermittent fasting delivers special metabolic benefits that assist weight loss, however, the evidence behind this is relatively minimal.

For a program to be truly effective, however, it should help individuals to lose weight and keep it off long-term. Yet, there is very little research on the effects of IF beyond the 1-year mark. Long-term studies with larger participant groups would strengthen the evidence behind IF.

Intermittent fasting vs continuous calorie restriction

Research in humans has found time and time again that for weight loss, IF not any more effective than continuous calorie restriction, that is, it is no more effective than just eating fewer calories throughout the day (Welton et al., 2020) (Cioffi et al., 2018). The reason why someone might prefer IF over regular calorie restriction is that they find IF easier as they don’t have to consciously be thinking of reducing calories, there are fewer meals to prepare and provides structure to your day of eating. IF also provides you with more flexibility in your food choices as you don’t have to cut out any foods.

Fat loss

When you lose weight, you lose fat but also some muscle. For better health, you want to focus on fat loss and hold onto as much muscle as possible. So far research comparing fat loss results between IF and regular calorie restriction is mixed. Some studies suggest IF is better (Harvie and Howell, 2017) while other studies say the opposite (Templeman et al, 2021). Regardless of whether you are IF or just reducing calories, eating enough protein and incorporating resistance training (e.g., lifting weights) helps you hold onto muscle.

Overall, IF might help you lose weight. There is nothing special about IF, some people just find it easier to follow. Just like any other diet, people lose weight on IF because they are eating fewer calories than they are burning. However, if you don’t eat fewer calories then IF is not going to help you lose weight. Going for long periods without food can of course make you very hungry which can cause you to overeat in your eating window. Consistently overeating in your eating window will result in weight gain.

Metabolic risk factors- blood sugar, cholesterol, and blood pressure

IF may improve your insulin sensitivity and blood sugar control as demonstrated by a 2019 systematic review (Cho et al., 2019). Other systematic reviews found that IF improved cholesterol levels (Meng et al., 2020).

Brain benefits

Some research suggests the potential brain benefits of IF. For example, a review of studies of in mice found that IF was associated with increased activity-dependent brain-derived neurotrophic factor (BDNF) which has emerged as a key regulator of cognitive performance and brain health (Seidler & Barrow., 2021). However, more research is required to assess the impact on brain health in humans.


Doesn’t focus on the quality of diet

A downside of IF is that it doesn’t focus on diet quality. This means you can be eating all the ultra-processed food and still be IF. This would also put you at risk of developing nutrient deficiencies. Regardless of whether you are following IF or not, it is important to eat a balanced and nutritious diet to keep you feeling your best. Rather than straightaway jumping on the IF bandwagon, think about ways to improve your current meals e.g., could you add more veggies to your meals, swap processed and fatty meats for lean meats and plant sources of protein, and use healthier sources of fat? Could you try to eat more mindfully?

Women’s Health

Some evidence suggests that IF might be less beneficial for women compared to men.

Animal studies found that female rats experienced excessive weight loss, reproductive problems, and intermitted in. However, we still require human research to observe these issues in women. One study found that while it improved insulin sensitivity in nonobese men, it worsened blood sugar control in nonobese women (Heilbronn et al., 2005). However, another study of women in the overweight and obese BMI category found that IF improved insulin sensitivity (Harvie et al., 2013).

Those with hormonal issues or reproductive issues should definitely seek the advice of a professional e.g., a dietitian or registered nutritionist. For these individuals, IF may not be suitable. If you start fasting and lose your period, stop fasting immediately.


Dinners with friends, pizza night, Christmas dinners, and pretty much any social event that involves food can make it harder to stick to if they occur outside your eating window. Allowing for some flexibility in your IF regime flexibility can make it more sustainable in the longer term.

Who shouldn’t do IF

There are some groups of people who should avoid IF as it would be medically risky for them.  They are:

  • History of eating disorders
  • Pregnant and breastfeeding women
  • Children and adolescents
  • Underweight individuals
  • Any immunosuppressed
  • Anyone who has been a transplant patient
  • CKD patients
  • Advanced Heart Disease (heart failure)
  • Poorly controlled diabetes
  • Type 2 diabetics on insulin medication
  • Type 1 diabetes, unless it’s very monitored by a dietitian and doctor
  • Elderly

Then there are also some people, that while it would be safe for them to do IF, it might not be best suited to them or their lifestyle

  • Frequently feeling very restricted during fasting windows and bingeing during eating windows
  • Those who are trying to gain muscle


The research behind IF is still growing and we require longer-term studies with larger participant groups to draw firm conclusions in many areas such as which is the best regime to follow. If you are considering IF, know that it can be a helpful tool for some people if they find it sustainable and enjoyable and are able to meet their nutrient requirements. However, if IF does not work with your lifestyle and you don’t like the idea of fasting, then IF is not for you- and that is perfectly OK! Regardless of whether or not you practice IF, eating balanced nourishing meals, hydration, sleep, and physical activity are the most important pillars of physical health.

Always consult with your doctor before starting IF. Working with a dietitian or registered nutritionist with expertise in IF is highly recommended to optimize your nutrition and assess if Intermittent Fasting is appropriate for you.

Photos by Unsplash


Lee, B.-W. (2019). The Effectiveness of Intermittent Fasting to Reduce Body Mass Index and Glucose Metabolism: A Systematic Review and Meta-Analysis. Journal of Clinical Medicine, 8(10), 1645.

Cioffi, I., Evangelista, A., Ponzo, V., Ciccone, G., Soldati, L., Santarpia, L., Contaldo, F., Pasanisi, F., Ghigo, E., & Bo, S. (2018). Intermittent versus continuous energy restriction on weight loss and cardiometabolic outcomes: a systematic review and meta-analysis of randomized controlled trials. Journal of translational medicine, 16(1), 371-371.

Harvie, M., & Howell, A. (2017). Potential Benefits and Harms of Intermittent Energy Restriction and Intermittent Fasting Amongst Obese, Overweight and Normal Weight Subjects-A Narrative Review of Human and Animal Evidence. Behav Sci (Basel), 7(1).

Harvie, M., Wright, C., Pegington, M., McMullan, D., Mitchell, E., Martin, B., Cutler, R. G., Evans, G., Whiteside, S., Maudsley, S., Camandola, S., Wang, R., Carlson, O. D., Egan, J. M., Mattson, M. P., & Howell, A. (2013). The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women. British Journal of Nutrition, 110(8), 1534-1547.

Heilbronn, L. K., Civitarese, A. E., Bogacka, I., Smith, S. R., Hulver, M., & Ravussin, E. (2005). Glucose tolerance and skeletal muscle gene expression in response to alternate day fasting. Obes Res, 13(3), 574-581.

Meng, H., Zhu, L., Kord-Varkaneh, H., O Santos, H., Tinsley, G. M., & Fu, P. (2020). Effects of intermittent fasting and energy-restricted diets on lipid profile: A systematic review and meta-analysis. Nutrition, 77, 110801.

Seidler, K., & Barrow, M. (2022). Intermittent fasting and cognitive performance – Targeting BDNF as potential strategy to optimise brain health. Frontiers in Neuroendocrinology, 65, 100971.

Templeman, I., Smith, H. A., Chowdhury, E., Chen, Y.-C., Carroll, H., Johnson-Bonson, D., Hengist, A., Smith, R., Creighton, J., Clayton, D., Varley, I., Karagounis, L. G., Wilhelmsen, A., Tsintzas, K., Reeves, S., Walhin, J.-P., Gonzalez, J. T., Thompson, D., & Betts, J. A. (2021). A randomized controlled trial to isolate the effects of fasting and energy restriction on weight loss and metabolic health in lean adults. Science Translational Medicine, 13(598), eabd8034.

Last reviewed: 22/06/2022

Last modified: June 21, 2022