Intermittent fasting — by Kirstin Kadé
What is intermittent fasting?
There has been increased interest in a concept known as Intermittent Fasting (IF) in both popular media and the scientific world over the years as a potential strategy for weight-management and improving metabolic health. There are two main variants of IF, intermittent energy restriction (IER) and time-restricted feeding (TRF), which involve periods of complete or partial abstinence from food (fasting) alternated with periods of free, habitual food intake (feeding). Fasting has taken place in one form or another for thousands of years, having an important role to play in the Christian faith and other religions, and even as a consequence of natural disasters and lack of food during times of war.
IF results in overall chronic calorie restriction with periods of no food intake that can ultimately lead to reduced body fat and weight loss over time. Research has shown that humans do not compensate for the energy deficit of fast days, and thus over time less calories are consumed when one follows an IF eating pattern.
From a metabolic perspective, IF allows us to extend what is known as the fasted metabolic state, during which the body relies on fat as a principal fuel source. To better understand this let’s take a look at the different metabolic states that we switch between after eating a meal:
This period lasts from when you eat up until approximately 4 hours after the meal. This is the time when your body digests, absorbs, stores, and utilises the food that you’ve eaten. Insulin is secreted in the presence of carbohydrates, driving the storage of glucose as glycogen in the liver and muscles (which provide limited reserves) and as fat in adipose tissue.
This period follows the postprandial phase and lasts for about 6 hours after a meal. During this time, your body starts to break down stored carbohydrate (glycogen) into glucose, releasing it into circulation as glucose for body cells to use. The body also begins to utilise some fat from adipose tissue as fuel.
This phase only really kicks in after 10 or more hours after a meal. It is during the fasted state that the body relies on the breakdown of fat stored in adipose tissue as its main fuel for the proper functioning of our cells and tissues.
Considering a typical daily eating pattern, where we eat breakfast, lunch, dinner, and snacks in between, most of us rarely experience being in the true fasted state for more than 4 hours each day. Thus, IF provides the body with a bit more time to utilise its fat stores and limits the time it has to store excess glucose in adipose tissue.
What does the evidence show?
Alterations in meal timing and the timespan between meals has been shown to beneficially influence glucose and lipid metabolism independent of weight-loss. Compared to traditional energy restriction (i.e. pretty much all diets), variants of IF have the potential to make it easier to achieve a calorie deficit over time. Complete and partial fasting approaches have demonstrated short-term weight loss and positive metabolic health benefits in both animal and human studies, and it has also been shown to be effective for weight-maintenance for periods of up to 1 year. Rodent studies have shown that IF has the potential to protect against heart disease, type 2 diabetes, cancer, and Alzheimer’s disease, however it’s worth noting that results from animal studies cannot be automatically applied to humans.
Although the evidence supporting the potential benefits of variants of IF on short-to-medium term weight loss, weight maintenance, and metabolic health, there is still a lot that we have to learn and understand about IF in humans, particularly with regards to its health effects and overall sustainability in the long-term. Most studies conducted up until this point have been performed in animals, or have been conducted in humans for short periods of time without very long-term follow-ups.
The Pros and Cons of IF
IF has the potential to be an effective weight-loss and weight-maintenance strategy
It may also provide health benefits that include reducing inflammation, improving insulin sensitivity and blood lipids, reducing cardiovascular risk, and as an effective weight-loss strategy
Different variants of IF are relatively simple to understand and follow, as they dictate when to eat rather than what to eat
Fasting for a couple of days a week or for a few more hours a day than normal is more achievable for some people than other diets and eating plans
Using IF as a long-term weight-loss strategy does make it a diet, and just like many other diets out there, research has shown that in the long-term, they just don’t work (give this article a read if you want to learn more about this)
Although it is not likely to be harmful for most individuals, the long-term effectiveness and effects of IF are largely unknown at this point.
Reducing how much you eat is likely to result in hunger, irritability, poor concentration, tiredness, fatigue, and dizziness. From a practical perspective, following an IF regimen could affect work and other daily tasks.
Although some evidence has shown that individuals do not fully compensate for fasting days on days of free eating, it can be argued that there is a likely risk of IF promoting a restrict/binge mentality and an unhealthy relationship with food.
IF is not for everyone, and it’s a good idea to consult with a qualified healthcare provider before trying it out. It should be avoided during pregnancy and breastfeeding, during stressful periods, and for anyone on long-term medical conditions (e.g. diabetics)
My Point of View
IF definitely has potential metabolic benefits, but there is a lot we still need to learn and understand before it can be used for therapeutic interventions. However, I think that it is wise to first assess your intentions and what’s important to you before giving something like this a try. For example, starting the day off with a cup of coffee and a warm bowl of oats and sharing an enjoyable meal with my husband after a long and busy day is an important part of my daily routine. Although life doesn’t revolve around food, the daily ritual of sharing moments around meals is important for my heart, mental health, and overall wellbeing. Figuring out what works for your body and for your body clock is also important. I know that although there are times where it isn’t always practical, my digestive system treats me better when I allow for a good 14-15 hours between dinner and breakfast the next day. I am an advocate for finding balance , learning to eat intuitively and developing a healthy, guilt-free relationship with food, and believe that although IF has its benefits, just like any ‘diet’ it has the potential to promote unhealthy restrict/binge cycles that are not constructive in the long-term.
1 — Antoni R, Johnston KL, Collins AL, Robertson D. The Effects of Intermittent Energy Restriction on Indices of Cardiometabolic Health. Research in Endocrinology. 2014:1-24.
2 — Antoni R, Johnston KL, Collins AL, Robertson D. The Effects of Intermittent Energy Restriction on Glucose and Lipid Metabolism. Proceedings of The Nutrition Society. 2017 Jan.
3 — Antoni R, Johnston KL, Collins AL, Robertson D. The Effects of Intermittent Energy Restriction on Postprandial Substrate Metabolism. Proceedings of The Nutrition Society. 2016 Jan. Available from: https://www.researchgate.net/publication/291553419_Investigation_into_the_acute_effects_of_intermittent_energy_restriction_on_postprandial_substrate_metabolism
4 — Harvie M, Wright C, Pegington M et al. The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women. Br J Nutr. 2013;110:1534-47.
5 — Wegman M, Guo M, Bennion D et al. Practicality of intermittent fasting in humans and its effect on oxidative stress and genes related to aging and metabolism. Rejuvenation Res. 2015:18;162-72.
6 — Mann T, Tomiyama AJ, Westling E, Lew A, Samuels B, Chatman J. Diets Are Not the Answer. American Psychologist. 2007 Apr;62(3):220-33.
7 — Chris Kresser [Internet]. 2015 Dec 3 [cited 2018 Feb 3]. Could You Benefit From Intermittent Fasting?
Kirstin is a qualified Food Scientist and is currently studying a MSc Human Nutrition at the University of Surrey in the U.K. Her blog, Taste & See, is a space where she shares nutrition-related knowledge and wholesome recipes that aim to make good health through food accessible to others.