Intermittent fasting, or the practice of going without food for extended periods of time, has emerged as one of the trendiest (and most science-backed) diet approaches to lose weight, gain lean muscle, balance blood sugar and activate powerful anti-aging genes.
Skipping meals likely sounds counterintuitive to some. For decades, nutritionists and trainers have touted the idea of eating mini-meals to stoke the metabolism. Experts have also cautioned against restricting calories too severely, lest you kick your body into “starvation mode,” where it begins storing fat in case of a famine. But more recently, scientists have learned that fasting does not trigger fat storage. Instead, it does the opposite: Fasting forces the body to burn up fat stores. The stress of burning fat for fuel actually causes cells to adapt and become more resilient in a way that benefits health and longevity.
Even better, intermittent fasting is perhaps the easiest plan to follow — you simply don’t eat for a specific amount of time each day. No counting calories, no tracking carbs, no hitting protein marks. Just block out the time window to stop eating and don’t eat.
So what can you expect if you want to give intermittent fasting a try? In this article, we’ve spoken to a weight loss-focused physician and looked over dozens of studies to explore the benefits, the best approaches, and strategies for success.
- What is Intermittent Fasting?
- The Benefits of Intermittent Fasting
- Different Types of Intermittent Fasting
- Strategies to Make Intermittent Fasting Work
- Coffee and Fasting
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In short, intermittent fasting is voluntarily going without food for a prescribed period of time, with periods of normal eating between fasts. A fast starts the moment that you put your fork down and stop eating. You break the fast as soon as you start eating again. Fasts can be as short as a few hours and as long as days.
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Experts believe that intermittent fasting has been part of human history since the beginning. Our hunger-gather ancestors likely didn’t have consistent access to food, so the body adapted to be able to use food when it was abundantly available (like when they caught big game) and to conserve energy while still fueling the high levels of mental and physical performance needed to continue hunting and gathering during the lean times.
Over time, fasting evolved into a spiritual practice. In the bible, both Moses and Jesus are said to have fasted in the desert for 40 days. Today, just about every culture and religion throughout the world still calls for periods of fasting. For instance, during the holy celebration of Ramadan, Muslims are called to fast from sunrise to sunset for 30 days. Similarly, Jews abstain from eating and drinking on Yom Kippur, and Christians may fast on Good Friday or throughout Lent.
It was the Ancient Greeks that introduced the idea of fasting as a medicinal therapy. Hippocrates, the father of modern medicine, famously wrote, “To eat when you are sick is to feed your illness.” Paracelsus, an ancient healer in the Western tradition, echoed Hippocrates centuries later, writing, “Fasting is the greatest remedy, the physician within.”
To understand why you should consider skipping food for a specified amount of time, let’s look at what happens in the body during a fast.
- Fasting naturally reduces overall calorie intake, resulting in a calorie deficit that can lead to weight loss.
- Fasting shifts the body into ketosis, activating fat burning.
Consuming fewer calories is arguably the most fundamental part of losing weight. However, many find it difficult to stick to light, less filling meals a day. IF allows you to lower your overall calorie intake by skipping a meal or two per day or by forgoing a full day of eating twice per week. Many people are better able to maintain diet compliance since they don’t have to worry about counting calories and can eat with complete freedom on non-fasting days.
Fasting also stimulates fat burning. Within six to eight hours of starting a fast, the body burns through all available glucose in the bloodstream. It burns through its glycogen stored (bundles of glucose stored in the liver). Eight to twelve hours into a fast, cells are forced to switch over to burning fat. In this process (called lipolysis), fat cells release fatty acids. These fatty acids are ferried to the liver to be converted into ketones, a source of quick-burning fuel that can actually make people feel more energetic and alert while fasting. (1)
- Abstaining from food means you don’t secrete insulin.
- This may increase insulin sensitivity, which could lower the risk of diabetes and metabolic syndrome.
Along with an increase in growth hormone — more on that in the next section — intermittent fasting gets a lot of attention for its effects on insulin. Insulin is the hormone responsible for shuttling glucose to the muscles and organs to be used for fuel and transporting any leftover glucose to fat cells where it is stored as fat for energy needs.
If you eat a lot, especially if you eat many refined carbs, especially if you have many unhealthy lifestyle habits like smoking, poor sleep, and inactivity, you can become insulin resistant. This means the body becomes less sensitive to the effects of insulin and requires more and more of it to do the same job of sending nutrients where they need to go. Insulin resistance is closely associated with higher levels of body fat, inflammation, and higher diabetes risk.
“Insulin typically goes down during fasting, and the effect can be seen in as soon as twenty-four hours or so,” says Dr. Aastha Kalra, a New York-based physician who specializes in weight loss. “Therefore, fasting is an acceptable technique to improve insulin sensitivity. Keeping insulin levels low can be a missing piece in the weight loss puzzle.”
Not eating means you’re not releasing insulin, which is why research suggests that fasting may improve insulin sensitivity, which is a key component of losing fat and improving nutrient absorption. (2)(3) In fact, some studies show that fasting can prevent and, in some cases, even reverse type 2 diabetes within a matter of months.
Human Growth Hormone Production
- Fasting may stimulate growth hormone, which helps preserve muscle mass.
While it may be true that a continuous stream of amino acids is more anabolic than going without food, fasting isn’t as catabolic as previously thought. In other words, fasting might be better for fat loss than for muscle gain. (4) (It makes it easier to eat less and harder to eat more, after all.)
But skipping meals doesn’t seem to break down your muscle. For example, a randomized study published in 2016 found that obese people fasting every other day for eight weeks lost the same amount of weight and retained the same muscle as a control group. (5)
“Think about it from an evolution standpoint: we wouldn’t evolve well if we started burning muscle in the absence of food,” says Dr. Kalra. “We retain muscle due to a hormone called human growth hormone, which is crucial for the maintenance of muscle and the bone.”
While most of the studies showing this have been on multi-day fasts, there’s still evidence that one-day fasts may effectively increase your body’s growth hormone production. (6)(7)(8)(9)(10)(11) It’s a stretch to say fasting is anabolic, but it doesn’t appear to be truly catabolic so long as your overall calories are in check.
- Fasting may increase autophagy, a process by which the body eliminates diseased cells.
Autophagy means “self-eating,” It refers to how the body consumes its own dead and diseased cells and recycles them into new parts. A lot of people have never heard of autophagy, but it’s an important component of longevity. It’s triggered by exercise and appears to be switched off by insulin spikes, meaning that fasting could be a great way to prolong it.
“By digesting its own parts, the body does two things: first, it gets rid of unnecessary proteins that might be damaging or malfunctioning, and then it recycles the amino acids into new cellular components,” says Dr. Kalra. “So what happens with fasting is these pathways are turned on or off, and then we see autophagy or cellular regeneration happen.”
There are a few schools of thought on achieving the benefits of fasting, but they all revolve around not eating for a fixed period of time. Here are a few of the more popular fasting methods.
This method is also called time-restricted feeding, followed by everyone from Hugh Jackman to Chris Pratt to Jennifer Aniston. Originally popularized by the Swedish strength coach Martin Berkhan in his LeanGains Method, this protocol calls for fasting for sixteen hours and restricting all food to an eight-hour window.
[See the results of a study that used the 16/8 protocol combined with heavy lifting]
You can set your schedule however you prefer, but most people set it so they’re sleeping through at least half of their fasting window. Usually, this means you skip breakfast and just eat lunch and dinner between 11 a.m. to 7 p.m. or 12 p.m. 8 p.m. Or, if you’d rather skip dinner, you can move the feeding window to whenever you like (7 a.m. to 3 p.m. and 2 p.m. to 10 p.m. are other popular eating windows). Berkhan typically recommends eating a small meal before working out and eating your largest meal post-workout.
OMAD (One Meal A Day)
A more extreme version of the 16/8 method, OMAD shortens the feeding window to just one to two hours a day. Followers eat just one huge meal, usually around the same time each day. To get enough calories in one meal, followers might include a palate-stimulating snack and dessert. Many people like the simplicity of having only to plan and cook one meal each day.
Eat Stop Eat
Pioneered by nutrition writer Brad Pilon, Eat Stop Eat advocates one or two 24-hour fasts per week. It’s no more complicated than that — Pilon is a student of zen philosophy and sees fasting as a way to eat with fewer rules, not more. His goal is to use fasting as a means to understand that “we do not have to eat all the time. Therefore we are free to choose when we eat.”
Alternate Day Fasting
Also called ADF, this is a protocol often used in clinical studies of fasting, including many cited in this article. The idea is to fast for a full 24 hours every other day simply. Depending on your experience, going without food for 24 hours may be more difficult than limiting food intake to a specified time. Still, a potential upside is that ADF usually takes off weight faster than other protocols. It also doesn’t come with any calorie limits on non-fast days. For those who find 24 hours of complete fasting too intense, a modified ADF method allows 500 calories for women and 600 calories for men to be consumed on fasting days.
Popularized by British journalist Dr. Michael Mosley and his documentary Eat, Fast, and Live Longer, this plan calls for fasting two days a week and eating normally the other five days. Like the modified alternate-day fasting method, Dr. Moseley doesn’t actually require complete abstention from food. Instead, he recommends consuming 25 percent of the number of calories you would normally eat (about 500 calories for women and 600 calories for men) on fast days, focusing on lean meat and leafy green vegetables.
No matter which intermittent fasting protocol you choose, you may experience hunger or struggle with cravings during your fasting periods, especially as your body adjusts. To help combat hunger pangs, experts advise staying optimally hydrated. Aim to drink at least half your body weight in ounces of water daily. Most fasting advocates also say you can drink unlimited amounts of zero-calorie beverages, including tea and coffee (but see the debate on coffee intake below), as well as unsweetened sparkling water on fasting days.
[Read more: The complete guide to naturally boosting growth hormone]
Scheduling fasts during your busiest times (like during your peak work hours or the days of the week you run all your errands) can also be a good way to keep your mind off food. After all, if you’re tied up with spreadsheets or sales calls, you’re less likely to have time to daydream about what to make for dinner.
Exercise can stimulate hunger, so it’s important to align your workouts with your fasting schedule. Figuring out when the best time to exercise is for you may take some experimentation. As a general rule, doing cardio in a fasted state can help you burn more fat and leave you starving, so try to push cardio to the end of your fasting window so you can eat right after. Resistance workouts are also typically best when done either right at the end of your fasting window or within your eating window so you can properly refuel with protein afterward.
Coffee does trigger a physiological response when you drink it (the caffeine spikes cortisol levels, and the liver has to metabolize it), so does it count against your fast? According to Dr. Satchin Panda, a researcher widely considered an authority on fasting, yes. He once told Dr. Rhonda Patrick on her podcast,
No, fasting is kind of slightly over. You may not be in 100% fast, but in 40% or 50% fast. So that’s where things become murky.
To his point, coffee does seem to impact insulin, but the amount it does increase is pretty insignificant, at least for those who don’t have diabetes. Some studies find that coffee improves insulin sensitivity. Others dispute this. (15)(16) As far as the acute effect of a cup of coffee on your insulin levels, the evidence is also mixed. (17)(18)(19)
What coffee does have going for it is it doesn’t seem to inhibit ketosis — indeed, it might actually increase your ketone production — and there’s some limited evidence that the polyphenols in coffee might increase autophagy rather than hinder it. (20)(21)(22)
[Read more in our guide to artificial sweeteners and your health]
What about diet soda? Unfortunately, the evidence here is also mixed. One study of seventeen obese people with diabetes found that sucralose (a.k.a. Splenda®) increased insulin by about 18 percent, but other studies have found no effect at all. (23)(24)(25)(26) For what it’s worth, Krista Varady, Ph.D., who conducts human studies on an alternate day and 16/8 fasting, allows participants to consume up to two diet sodas during their fasting windows, and the artificial sweetners don’t seem to impact their results. Participants still saw significant insulin sensitivity improvements and weight loss.
Bottom line, if you’re fasting because you want to keep calories low and lose weight, then sipping a few cups of black coffee or tea and even diet soda probably won’t make much of a difference. “If your main goal is to reduce calories, then you might even be fine with having some cream in your coffee or with drinking diet soda,” says Dr. Kalra. “But if your priority is health benefits like improved insulin sensitivity, then you may want to eliminate everything but water and plain coffee or tea.”
A growing body of research suggests intermittent fasting is at least equal if not better for fat loss than following a more traditional calorie-restricted diet plan. It is easier for many people to follow. What’s more, this way of eating seems to offer a host of health and longevity benefits, including lower blood sugar, reduced inflammation, and higher human growth hormone levels. There’s no need to do it if you try it and hate it. Still, if preparing fewer meals and spending less time eating seems like an attractive option, it doesn’t look like eating fewer meals will negatively affect your muscle gains. It may just help you lower your body fat and achieve high levels of physical and mental performance. Just make sure your overall calories and macronutrients are in check.
- Matteson, M, et al. Impact of intermittent fasting on health and disease processes. Aging Res Rev. 2017 Oct; 39: 46–58. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5411330/figure/F1/
- Halberg N, et al. Effect of intermittent fasting and refeeding on insulin action in healthy men. J Appl Physiol (1985). 2005 Dec;99(6):2128-36.
- Horne BD, et al. Usefulness of routine periodic fasting to lower risk of coronary artery disease in patients undergoing coronary angiography. Am J Cardiol. 2008 Oct 1;102(7):814-819.
- Tinsley GM, et al. Time-restricted feeding in young men performing resistance training: A randomized controlled trial. Eur J Sport Sci. 2017 Mar;17(2):200-207.
- Catenacci VA, et al. A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity. Obesity (Silver Spring). 2016 Sep;24(9):1874-83.
- Ho KY, et al. Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. J Clin Invest. 1988 Apr;81(4):968-75.
- Moller L, et al. Impact of fasting on growth hormone signaling and action in muscle and fat. J Clin Endocrinol Metab. 2009 Mar;94(3):965-72.
- Darzy KH, et al. The impact of short-term fasting on the dynamics of 24-hour growth hormone (GH) secretion in patients with severe radiation-induced GH deficiency. J Clin Endocrinol Metab. 2006 Mar;91(3):987-94.
- Vendelbo MH, et al. Exercise and fasting activate growth hormone-dependent myocellular signal transducer and activator of transcription-5b phosphorylation and insulin-like growth factor-I messenger ribonucleic acid expression in humans. J Clin Endocrinol Metab. 2010 Sep;95(9):E64-8.
- Lanzi R, et al. Elevated insulin levels contribute to the reduced growth hormone (GH) response to GH-releasing hormone in obese subjects. Metabolism. 1999 Sep;48(9):1152-6.
- Greenwood FC, et al. The plasma sugar, free fatty acid, cortisol, and growth hormone response to insulin. I. In control subjects. J Clin Invest. 1966 Apr;45(4):429-36.
- Alirezaei M, et al. Short-term fasting induces profound neuronal autophagy. Autophagy. 2010 Aug;6(6):702-10.
- Li L, et al. Chronic intermittent fasting improves cognitive functions and brain structures in mice. PLoS One. 2013 Jun 3;8(6):e66069.
- Singh R, et al. Late-onset intermittent fasting dietary restriction as a potential intervention to retard age-associated brain function impairments in male rats. Age (Dordr). 2012 Aug;34(4):917-33.
- Uchiyama Y, et al. Autophagic neuron death. Methods Enzymol. 2009;453:33-51.
- Matsuda Y, et al. Coffee and caffeine improve insulin sensitivity and glucose tolerance in C57BL/6J mice fed a high-fat diet. Biosci Biotechnol Biochem. 2011;75(12):2309-15.
- Pham NM, et al. Coffee and green tea consumption is associated with insulin resistance in Japanese adults. Metabolism. 2014 Mar;63(3):400-8.
- Dewar L, et al. The effect of acute caffeine intake on insulin sensitivity and glycemic control in people with diabetes. Diabetes Metab Syndr. 2017 Dec;11 Suppl 2:S631-S635.
- van Dam RM, et al. Effects of coffee consumption on fasting blood glucose and insulin concentrations: randomized controlled trials in healthy volunteers. Diabetes Care. 2004 Dec;27(12):2990-2.
- Rakvaag E, et al. Acute effects of light and dark roasted coffee on glucose tolerance: a randomized, controlled crossover trial in healthy volunteers. Eur J Nutr. 2016 Oct;55(7):2221-30.
- Vandenberghe C, et al. Caffeine intake increases plasma ketones: an acute metabolic study in humans. Can J Physiol Pharmacol. 2017 Apr;95(4):455-458.
- Pietrocola F, et al. Pro-autophagic polyphenols reduce the acetylation of cytoplasmic proteins. Cell Cycle. 2012 Oct 15;11(20):3851-60.
- Pietrocola F, et al. Coffee induces autophagy in vivo. Cell Cycle. 2014;13(12):1987-94.
- Pepino MY, et al. Sucralose affects glycemic and hormonal responses to an oral glucose load. Diabetes Care. 2013 Sep;36(9):2530-5.
- Ma J, et al. Effect of the artificial sweetener, sucralose, on gastric emptying and incretin hormone release in healthy subjects. Am J Physiol Gastrointest Liver Physiol. 2009 Apr;296(4):G735-9.
- Ma J, et al. Effect of the artificial sweetener, sucralose, on small intestinal glucose absorption in healthy human subjects. Br J Nutr. 2010 Sep;104(6):803-6.
- Ford HE, et al. Effects of oral ingestion of sucralose on gut hormone response and appetite in healthy normal-weight subjects. Eur J Clin Nutr. 2011 Apr;65(4):508-13.
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