Greatist’s #WTF is series looks at new trends in health and fitness to explain what the heck they are, why people care, and if they live up to the hype.

An empty stomach is perfectly natural. Fasting is something we all do while we’re asleep and whenever we don’t have food in our mouths — you’re probably fasting right now. Due in part to a recent spate of bestsellers and documentaries, “intermittent fasting” (or “IF,” typically defined as consuming nothing but water for 16 to 36 hours) has gained tremendous popularity in health and fitness circles as a way to lose fat, live longer, and even build muscle.

But taking a break from food can be incredibly controversial, particularly among those who are still sold on the old doctrine of eating six small meals per day. So how could IF be one of the simplest and safest tools for managing your health?

What’s the Deal?

We’ve gone over this before, but to refresh: The human metabolism does not grind to a halt if you skip a meal (or three). For it to slow down by even ten percent, one would need to fast for 72 hours straight (don’t worry, no one’s recommending giving up food for three days) Increased meal frequency does not promote greater weight loss in subjects who were prescribed an 8-week equi-energetic energy-restricted diet. Cameron JD, Cyr, MJ, et al. Behavioural and Metabolic Research Unit, School of Human Kinetics, University of Ottawa, Ottawa, Ontario, Canada. British Journal of Nutrition, 2010 Apr;103(8):1098-101. Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. Zauner C, Schneeweiss B, et al. Intensive Care Unit, the Department of Internal Medicine IV, University of Vienna, Vienna, Austria. American Journal of Clinical Nutrition, 2000 Jun;71(6):1511-5. Leucine, glucose, and energy metabolism after 3 days of fasting in healthy human subjects. Nair KS, Woolf PD, et al. Department of Medicine, University of Rochester School of Medicine and Dentistry, NY, USA. American Journal of Clinical Nutrition, 1987 Oct;46(4):557-62. Importance of blood glucose concentration in regulating lipolysis during fasting in humans. Klein S, Holland OB, et al. Department of Internal Medicine, University of Texas Medical Branch, Galveston, TX, USA. American Journal of Physiology, 1990 Jan;258(1 Pt 1):E32-9.. In fact, even 48 hour fasts have been shown to have no negative effect on metabolism, cognitive performance, or fatigue A double-blind, placebo-controlled test of 2 d of calorie deprivation: effects on cognition, activity, sleep, and interstitial glucose concentrations. Lieberman HR, Caruso CM, et al. Military Nutrition Division, US Army Research Institute of Environmental Medicine, Natick, MA, USA. American Journal of Clinical Nutrition, 2008 Sep;88(3):667-76. Lack of effect of short-term fasting on cognitive function. Green, MW, Elliman NA, et al. Consumer Sciences Department, Institute of Food Research, Reading, U.K. Journal of Psychiatric Research, 1995 May-Jun;29(3):245-53.. That’s not to say fasting can’t be a little uncomfortable — we’ll get to that later.

But why would anybody want to fast? For starters, IF shares many of the benefits of following a low calorie diet, such as a lower risk of stroke and cardiovascular diseases Caloric restriction: implications for human cardiometabolic health. Bales CW, Kraus WE. Division of Geriatrics, Department of Medicine, Duke University Medical Center, Durham, NC, USA. Journal of Cardiopulmonary Rehabilitation and Prevention, 2013 Jul-Aug;33(4):201-8. Beneficial effects of intermittent fasting and caloric restriction on the cardiovascular and cerebrovascular systems. Mattson MP, Wan R. Laboratory of Neurosciences, National Institute on Aging Intramural Research Program, Baltimore, MD, USA. The Journal of Nutritional Biochemistry, 2005 Mar;16(3):129-37. Alternate-day fasting and chronic disease prevention: a review of human and animal trials. Varady KA, Hellerstein MK. Department of Nutritional Sciences and Toxicology, University of California at Berkeley, Berkeley, CA, USA. American Journal of Clinical Nutrition, 2007 Jul;86(1):7-13.. Fasting’s effect on the heart is especially interesting: One study concluded just one day without food per month can potentially halve the risk of developing coronary artery disease Usefulness of routine periodic fasting to lower risk of coronary artery disease in patients undergoing coronary angiography. Horne BD, May HT, et al. Cardiovascular Department, Intermountain Medical Center, Murray, Utah, USA. American Journal of Cardiology, 2008 Oct 1;102(7):814-819..

But there’s another effect of periodic fasting that both reduces the risk of chronic disease and improves the body’s digestion of carbohydrates (as in, they become less likely to make you fat). It’s all about insulin, a hormone responsible for the uptake of nutrients into the liver, muscles, and fat cells. Because the body releases insulin when carbohydrates are consumed, eating too much and too often can make us less sensitive to it. (Makes sense, right?) Unfortunately, an abundance of food (as well as other factors, like insufficient sleep and exercise) has made poor insulin sensitivity fairly commonplace Effect of intermittent fasting and refeeding on insulin action in healthy men. Halberg N, Henriksen M, et al. Dept. of Muscle Research Centre, The Panum Institute, University of Copenhagen, Denmark. Journal of Applied Physiology, 2005 Dec;99(6):2128-36.. That’s bad. Not just because it makes it more difficult to lose fat and absorb nutrients, but also because it increases the risk of diabetes and several kinds of cancer Insulin, glucose, insulin resistance, and pancreatic cancer in male smokers. Stolzenberg-Solomon RZ, Graubard BI, et al. National Cancer Institute, Department of Health and Human Services, Rockville, MD, USA. Journal of the American Medical Association, 2005 Dec 14;294(22):2872-8. The correlation between metabolic syndrome and prostatic diseases. De Nunzio C, Aronson W, et al. Department of Urology, Sant'Andrea Hospital, University La Sapienza, Rome, Italy. European Urology, 2012 Mar;61(3):560-70. Insulin resistance and breast-cancer risk. Bruning PF, Bonfrèr JM, et al. The Netherlands Cancer Institute, Amsterdam, The Netherlands. Bruning PF, Bonfrèr JM, et al. The Netherlands Cancer Institute, Amsterdam, The Netherlands. International Journal of Cancer, 1992 Oct 21;52(4):511-6.. Fortunately, the problem can be improved by essentially doing nothing — not eating increases insulin sensitivity, meaning regular fasts allow you to eat more carbs, get less sick, and burn more fat Effect of intermittent fasting and refeeding on insulin action in healthy men. Halberg N, Henriksen M, et al. Dept. of Muscle Research Centre, The Panum Institute, University of Copenhagen, Denmark. Journal of Applied Physiology, 2005 Dec;99(6):2128-36. Usefulness of routine periodic fasting to lower risk of coronary artery disease in patients undergoing coronary angiography. Horne BD, May HT, et al. Cardiovascular Department, Intermountain Medical Center, Murray, Utah, USA. American Journal of Cardiology, 2008 Oct 1;102(7):814-819..

Twenty-four hour fasts have also been shown to increase the brain’s production of growth hormone by up to 2,000 percent in men and 1,300 percent in women (the effect ends when the fast does). This is good news for anybody looking to slow the aging process: Growth hormone isn’t just awesome at lowering body fat while preserving muscle (weightlifters, rejoice!) but it improves physical function, bone quality, and longevity Growth hormone in health and disease: Long-term GH therapy--benefits and unanswered questions. Clemmons D. Nature Reviews Endocrinology, 2013 Jun;9(6):317-8. Adult growth hormone deficiency - benefits, side effects, and risks of growth hormone replacement. Reed ML, Merriam GR. Geriatrics and Extended Care, VA Puget Sound Health Care System, Madigan Health Care System, Tacoma, WA, USA. Frontiers in Endocrinology (Lausanne), 2013 Jun 4;4:64. Basal growth hormone concentration increased following a weight loss focused dietary intervention in older overweight and obese women. Miller GD, Nicklas BJ, et al. Department Health and Exercise Science, Wake Forest University, Winston-Salem, NC, USA. The Journal of Nutrition Health and Aging, 2012 Feb;16(2):169-74..

So Fasting Affects Men and Women Differently?

It just might, but exactly why or how much is still a point of debate. While there are some assertions that women are more sensitive to the stress of going without food, many have great success with it. More research is needed, but it’s important to remember that as with all diets, IF works for some people and doesn’t for others. Feel it out and see what works for you.

Won’t I Get Hungry?

We hear ya. While on a fast, it’s a good idea to drink plenty of water, plain tea, black coffee, and other very low-calorie drinks to keep the stomach from feeling too empty — even diet soda isn’t considered a fast breaker. But it might relax you to know the initial hunger probably isn’t because the body requires food, but because of a hormone called ghrelin.

Ghrelin is an appetite stimulant the body learns to secrete based off your meal patterns, so it makes you hungry when you would normally be eating. (This has earned it the nickname, “hunger’s timekeeper.”) That’s why eating throughout the day keeps you hungry, and it can also make IF uncomfortable at first The influence of higher protein intake and greater eating frequency on appetite control in overweight and obese men. Leidy HJ, Armstrong CL, et al. Department of Dietetics & Nutrition, University of Kansas Medical Center, Kansas City, Kansas, USA. Obesity (Silver Spring). 2010 Sep;18(9):1725-32.. After a few fasts, however, the body learns to produce less ghrelin, and you get more control over when you eat.

But those first few fasts can be jarring, and for some, struggling with ghrelin is not worth the adaptation period — and that’s fine! Eating is personal, and if the discomfort is severe, there’s no obligation to continue. The takeaway here is that you have more control over when you eat than you might think. Like most things, it just takes a little practice.

So Should I Try It Out?

The science behind IF is pretty solid, but that doesn’t mean it’s the only path to better health. If fighting through ghrelin surges or skipping meals with loved ones become insurmountable problems — or if you’re hypoglycaemic, diabetic, or have a history of eating disorders — IF might be worth avoiding. As always, it’s wise to speak with a physician before changing the way you eat.

It’s also important to remember that no matter when we eat, what and how much we eat is always important — the improved hormones and smaller eating windows of IF are not a carte blanche to consume thousands of extra calories!

Of course, ideas about exactly when to eat and when to fast can vary from one person to the next. There are many different IF protocols: Some swear by a daily fast of 16 hours, and others prefer 24-hour fasts once or twice a week. Keep track of how you feel, and again, if the fast is too hard, just break it. Try again another time, perhaps after a particularly big meal. If it’s unbearable, stop bearing it. While a heightened awareness of food intake is a great side effect, the best part of intermittent fasting — at least according to its proponents — is the flexibility and simplicity it brings to the eating process. Eat when hungry, don’t when not. Six meals per day or one big dinner, the research shows that you can eat when you feel like eating. The metabolism won’t shut down, muscles won’t atrophy, and the sun will rise in the morning. It takes a load off, don’t it?

There’s no harm in giving it a try. Intermittent fasting is rapidly becoming a popular and powerful method for fighting disease, improving body composition, and taking some of the stress out of dieting and meal planning, but remember: if it doesn’t work for you, it’s not the only way to get results

Special thanks to Dr. John Berardi, founder of Precision Nutrition, for his help with this article.

Got something to say? Let us know in the comments below, or tweet the author @ncjms.


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