Why has intermittent fasting (IF) become so popular in the past few years?
One of the main reasons is simplicity. There are a million and one diets that involve specific foods or nutrients, but IF skirts all those details. Let’s see what the main types of IF are, and what the evidence shows about weight loss and other health effects.
Also known as time-restricted feeding, IF alternates periods of normal food intake with extended periods (usually 16–48 hours) of low-to-no food intake. This approach lends itself to different variants, including those:
Alternate-Day Fasting (also known as Alternate-Day Modified Fasting). This diet can take different forms: you can eat over 12 hours then fast for 36 hours; you can eat over 24 hours then fast for 24 hours; or you can eat normally over 24 hours then eat very little (about 500 kcal) over the next 24 hours.
Eat-Stop-Eat. You fast or severely restrict calories for 24 hours, either at regular intervals (two days per week in the 5:2 Diet) or just from time to time.
Random Meal Skipping. You skip meals at random throughout the week.
Feeding Window. You can only eat during a set period of time every day (from 10 a.m. to 6 p.m, for instance).
Should you decide to try intermittent fasting, pick a variant you think you can stick with for at least a few weeks.
Yes, IF works, though to very different degrees in different studies. This variability could be explained by different factors, among which is the specific IF variant studied. For instance, simply skipping breakfast led to some weight loss in one study but not in another. In each of those two studies, the control group was provided with a standard breakfast, such as oatmeal, but neither group was restricted in what they could eat the rest of the day.
It’s also possible that people with more weight to lose may benefit more from an IF approach, but one thing is sure: if you compensate for the meals you skipped by eating more later in the day, or the next day, or the next, you won’t lose weight. The weight-loss equation is simple: you need to ingest less calories than you burn. IF is just one way to make this happen, but it is a way that some people find easier than the more common “eat smaller meals” approach.
To lose weight, you need to burn more than you eat — or, conversely, to eat less than you burn. Some people find this goal easier to reach through intermittent fasting than through the traditional “smaller meals” approach.
The body of evidence on IF in humans is still relatively small, but a number of studies have reported improvements in various health markers aside from weight, notably lipids. It is still an active area of debate if there are any unique metabolic benefits to IF over chronic caloric restriction (CCR, the traditional “eat smaller meals” approach).
The most intriguing of these debated benefits is a prolonged life. It has long been known that caloric restriction in general (i.e., CCR and IF both) can slow the aging process and extend lifespan in many animal models, due in part to its kick-starting some regenerative processes. Keep in mind, however, that those animals were either fed low-calorie diets or rotated through periods of fasting for most of their lives. It is still not known if IF can reliably produce greater life-extension over CCR in animal models, let alone in humans, and if it can, which variant is best and how many weeks, months, or years are required to make a difference.
Assessing the potential metabolic benefits of CCR and IF is a long-term endeavor, and IF is still very new. It may provide unique metabolic benefits over CCR, as well as mood benefits, but solid research is still sparse, so that “further research in humans is needed before the use of fasting as a health intervention can be recommended.”
Preliminary evidence suggests that fasting could have unique metabolic benefits, including life extension, but those need to be confirmed by further research in humans.
Depending on the length of your fast, you may experience stress, headaches, constipation, or dehydration. Staying hydrated is particularly important; it’ll also help mitigate any headaches or constipation.
Some preliminary evidence suggests that a periodic reduction of caloric intake may produce physiological benefits similar to those of fasting. A fast-mimicking diet is a strategy where, instead of completely forgoing food, you simply consume a low-calorie diet for 5 days straight each month. The typical protocol entails eating 1,090 kcal (10% protein, 56% fat, 34% carb) on the first day, then 725 kcal (9% protein, 44% fat, 47% carb) on each of the next four days.
Keep in mind that IF is not for everyone. People with impaired glycemic control should avoid fasting, as it causes poorer glucose response. Also, if you’re pregnant, underweight, younger than 18, or have a history of disordered eating, IF is probably not for you.
Intermittent fasting is a viable strategy to slim down or maintain a healthy weight. In that respect, however, continual caloric restriction (the traditional “eat smaller meals” diet) offers similar benefits. What matters most is consistency. People who find eating less often easier than eating less could especially benefit from trying IF. As for unique metabolic health benefits of fasting … the debate is still ongoing.
- Fasting or keto: possible treatment for multiple sclerosis
- Do you need to eat six times a day to keep your metabolism high?
- Is it really that bad to skip breakfast?
- Could fasting help treat MS symptoms?
- Low-fat vs. low-carb? Major study concludes: it doesn’t matter for weight loss
- Can hypothyroidism lead to fat gain?
- Does aspartame increase appetite?
- How do I stay out of "starvation mode?"
- Measuring body fat percentage: It's an accuracy thing
- Is my “slow metabolism” stalling my weight loss?
- Does eating at night make it more likely to gain weight?
- I'm not too tired to stuff my face
- Does diet soda inhibit fat loss?
- Will eating breakfast keep you lean?
- Do you need to detox?
- Can you lose weight by turning down the heater?
- Does daily weighing help you lose weight?
- How do I get a six-pack?
- Will my breasts shrink with weight loss?
- How does protein affect weight loss?
- What should you eat for weight loss?
- Exploring chia seeds for weight loss
- Can food have negative calories?
- 5 little-known facts about protein
- Will lifting weights convert my fat into muscle?
- How do I lose fat around my belly?
- 3 Science-based steps to curbing your appetite
- Does high-protein intake help when dieting?
- Whey vs soy protein: which is better when losing weight?
- How important is sleep?
- How to minimize fat gain during the holidays
- I have lost significant weight and now have loose skin. How can I tighten up my skin?
- Stepping up weight loss: Can walking help dieters shed fat?
- Impact of intermittent fasting on health and disease processes. Ageing Res Rev. (2016) Mattson MP, Longo VD, Harvie M.
- Effects of intermittent fasting on body composition and clinical health markers in humans. Nutr Rev. (2015) Tinsley GM, La Bounty PM.
- Skipping breakfast leads to weight loss but also elevated cholesterol compared with consuming daily breakfasts of oat porridge or frosted cornflakes in overweight individuals: a randomised controlled trial. J Nutr Sci. (2014) Geliebter A, et al.
- The causal role of breakfast in energy balance and health: a randomized controlled trial in lean adults. Am J Clin Nutr. (2014) Betts JA, et al.
- Intermittent Fasting: The Choice for a Healthier Lifestyle. Cureus. (2018) Ganesan K, Habboush Y, Sultan S.
- Caloric restriction. Mol Aspects Med. (2011) Speakman JR, Mitchell SE.
- Of mice and men: the benefits of caloric restriction, exercise, and mimetics. Ageing Res Rev. (2012) Mercken EM, et al.
- Interventions to Slow Aging in Humans: Are We Ready?. Aging Cell. (2015) Longo VD, et al.
- Efficacy of fasting and calorie restriction (FCR) on mood and depression among ageing men. J Nutr Health Aging. (2013) Hussin NM, et al.
- Health effects of intermittent fasting: hormesis or harm? A systematic review. Am J Clin Nutr. (2015) Horne BD, Muhlestein JB, Anderson JL.
- Fasting-mimicking diet and markers/risk factors for aging, diabetes, cancer, and cardiovascular disease. Sci Transl Med. (2017) Wei M, et al.
- Fasting until noon triggers increased postprandial hyperglycemia and impaired insulin response after lunch and dinner in individuals with type 2 diabetes: a randomized clinical trial. Diabetes Care. (2015) Jakubowicz D, et al.