How does intermittent fasting work? (And 5 types of intermittent fasting)

Written by Robb Wolf (opens in a new tab)

There’s a boatload of content on intermittent fasting these days. Unfortunately, most of this content is clickbait: “Lose X Pounds on the One Meal A Day Diet!”The result is widespread confusion about how intermittent fasting works with regard to weight loss. The focus—even of well-intended articles—is almost always on fat-burning and rarely on calories.Sure, fasting can help you burn body fat. I’m not disputing that. What I am disputing is that this increased fat burning drives weight loss. It drives other benefits (mostly related to ketosis) and it can facilitate fat loss, but running on fat for energy doesn’t guarantee weight loss.Why? It’s possible for your body to run on fatty acids even when you’re still overeating total calories. (Mmm… bacon.) Simply put, when more energy (calories) comes into the system than gets used, energy must be stored. And conversely, when more energy is spent than enters the system, weight will be lost. If you’re considering fasting (or anything else) to lose weight, this is an important concept to remember.I’ll dig more into how fasting works and explain various intermittent protocols in a minute, but let’s first cover the <a href="https://drinklmnt.com/blogs/health/how-does-intermittent-fasting-work?utm_source=klaviyo&utm_medium=email&utm_campaign=&utm_term=how-does-intermittent-fasting-work&utm_content=blog">basics of intermittent fasting</a>.<h2>What Is Intermittent Fasting?</h2>When you practice intermittent fasting, you take regular, periodic breaks from calories. Depending on the protocol, these breaks can span anywhere from 12 to 36 hours.An intermittent fast doesn’t have to be a zero-calorie fast. It can be a limited calorie fast and still have <a href="https://www.cureus.com/articles/12903-intermittent-fasting-the-choice-for-a-healthier-lifestyle" target="_blank" rel="noopener noreferrer">positive effects</a> on body weight, blood sugar, etc.Intermittent fasting is based on a simple principle: that organisms evolved to oscillate between periods of fasting and periods of feeding. This oscillation promotes optimal health.In limited quantities, fasting can be a beneficial stress to the human body. It upregulates pathways—like AMPK—that get us burning fat, <a href="https://link.springer.com/article/10.1007/s11033-020-05900-x" target="_blank" rel="noopener noreferrer">repairing DNA</a>, and making ketones.Then we refeed. And feeding upregulates other pathways—like mTOR—that help us build muscle, heal wounds, and generally grow.Both cycles are important, but modern culture only encourages one of them. I’m thinking of those “fourth meal” commercials that celebrate the downfall of America’s metabolic health.We feed constantly. We overeat. And then we end up with a crisis of obesity and diabetes.I believe <a href="https://drinklmnt.com/blogs/health/can-intermittent-fasting-help-reverse-type-2-diabetes" target="_blank" rel="noopener noreferrer">intermittent fasting may help reverse this trend</a>. But not for the reasons most people cite.<h2>How Does Intermittent Fasting Work for Weight Loss?</h2>Intermittent fasting has been <a href="https://www.cureus.com/articles/12903-intermittent-fasting-the-choice-for-a-healthier-lifestyle" target="_blank" rel="noopener noreferrer">shown</a> in multiple studies to promote weight loss in obese and non-obese people. The reason why is obvious: these folks are eating less.When you compress your daily feeding window or <a href="https://drinklmnt.com/blogs/health/intermittent-fasting-vs-calorie-restriction" target="_blank" rel="noopener noreferrer">restrict calories</a> multiple days per week, you eat fewer calories overall. This often leads to a state of negative energy balance.<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6003580/" target="_blank" rel="noopener noreferrer">Negative energy balance</a> means that you consume fewer calories than you expend through resting metabolic rate (breathing, heartbeat, digestion, etc.), exercise, and fidgeting. In other words, calories out exceed calories in.Maintaining this caloric deficit is essential for weight loss. For Paleo. For keto. For intermittent fasting. For any diet.The unique thing about intermittent fasting (and to a lesser extent, <a href="https://drinklmnt.com/blogs/health/ketogenic-diet-what-you-need-to-know" target="_blank" rel="noopener noreferrer">keto</a>) is that people don’t consciously restrict calories. It happens naturally.This makes sense. If you go from three meals per day to one meal per day, your total caloric intake will likely drop, even if you strive to keep it the same. The stomach can only hold so much food comfortably.It also explains why folks lose weight in fasting <a href="https://www.cureus.com/articles/12903-intermittent-fasting-the-choice-for-a-healthier-lifestyle" target="_blank" rel="noopener noreferrer">studies</a> that allow ad libitum (as much as you like) feeding. They eat their fill, but they still eat less overall.In my view, calorie reduction explains the lion’s share of fasting-related weight loss. What about the metabolic stuff though?<h2>Fasting and Metabolic Health</h2>In the past fifty years, rates of type 2 diabetes have increased by a factor of seven in the United States. More than 34 million Americans <a href="https://www.cdc.gov/diabetes/basics/type2.html" target="_blank" rel="noopener noreferrer">have</a> type 2 diabetes, and many more have prediabetes.At its core, type 2 diabetes is a problem with the hormone insulin. This problem is called <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1204764/" target="_blank" rel="noopener noreferrer">insulin resistance</a>.When someone is insulin resistant, they’re unable to effectively store blood sugar in muscle and liver cells. As a result, blood sugar stays dangerously high, driving many complications of diabetes.To compensate, the insulin-resistant person makes more insulin to handle that blood sugar. Although insulin plays many roles, the effects on appetite might be most important in this discussion. Insulin-resistant individuals, and/or folks who experience significant blood sugar swings, tend to experience more hunger.Insulin resistance is driven by diet and lifestyle factors—(for instance, a <a href="https://drinklmnt.com/blogs/health/how-sugar-is-making-us-sick" target="_blank" rel="noopener noreferrer">high-sugar American diet</a>)—and it can be reversed with diet and lifestyle factors. Fasting appears to be one of these factors.One <a href="https://pubmed.ncbi.nlm.nih.gov/30646030/" target="_blank" rel="noopener noreferrer">study</a> found that 12 months of 5:2 fasting (I’ll cover 5:2 soon) caused significant drops in average blood sugar in type 2 diabetics. This suggests an improvement in insulin function.But was the protocol the key, or the reduction in calories? Since the calorie-restricted group (who didn’t fast) saw similar improvements, I think we have our answer.<h2>Fasting and Circadian Rhythm</h2>The form of intermittent fasting I recommend is called time-restricted feeding. It entails eating all your daily calories in a compressed time frame.The benefits of time-restricted feeding go beyond weight loss. When you don’t snack overnight, you align the 24-hour wake-sleep cycle known as circadian rhythm.Circadian rhythm <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5388543/" target="_blank" rel="noopener noreferrer">regulates</a> a hefty chunk of the human genome. Genes that mediate melatonin production, insulin function, immune factors, the list goes on.It’s common knowledge that light plays a large role in circadian rhythm. Bright light in the morning wakes you up and darkness at night promotes melatonin secretion to make you sleepy.Food plays a similar yet <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5388543/" target="_blank" rel="noopener noreferrer">lesser-known role</a>. Feeding stimulates circadian clocks in your muscle and liver tissue, and fasting overnight tells these clocks it’s time for bed.The best circadian protocols fall between 12 hours and 18 hours of fasting per day. I’ve found that anything longer increases the risk of sleep disruption.<h2>5 Types of Intermittent Fasting</h2>Intermittent fasting protocols can be lumped into two main buckets:<ol><li style="font-weight: 400">Daily fasts</li><li style="font-weight: 400">Weekly fasts</li></ol>I prefer daily fasts because they’re easier to stick with and (generally) promote healthier circadian rhythm. But weekly fasts have been <a href="https://www.cureus.com/articles/12903-intermittent-fasting-the-choice-for-a-healthier-lifestyle" target="_blank" rel="noopener noreferrer">shown</a> to have benefits too. Let’s review these protocols.<h3>Daily Fasts</h3><h3>#1: 12/12</h3>Also called overnight fasting, 12/12 is the ideal place to start with intermittent fasting. Most people can go 12 hours between dinner and breakfast comfortably.When you don’t snack overnight, you:<ul><li style="font-weight: 400">Consume fewer calories overall</li><li style="font-weight: 400">Promote restful sleep</li><li style="font-weight: 400">Keep blood sugar and insulin levels lower</li><li style="font-weight: 400">Save on the grocery bill</li></ul>The takeaway? Overnight fasting is a quick win in the fasting game, and it’s doable for almost everyone.<h3>#2: 16:8</h3>I practice closer to the likes of 16:8. I eat my first meal around 9 AM and finish my second meal around 4-5 PM. (An eight-hour feeding window). Then I fast for 16 hours.This protocol works best for me on many levels. It helps me stay productive, make a few extra ketones, and sustain my active lifestyle. I doubt I could squeeze enough calories into one meal to fuel my day.Fasting for 16 or 18 hours per day isn’t <a href="https://drinklmnt.com/blogs/health/should-you-train-fasted-benefits-downsides-and-when-to-do-it" target="_blank" rel="noopener noreferrer">optimal for muscle gains</a>, but research has shown that lean mass and strength can be maintained on this protocol, assuming adequate protein and a resistance training program.<h3>#3: OMAD</h3>The protocol for one meal a day (OMAD) is simple: Shove all your daily calories into one meal.Drastically compressing your feeding window usually entails a reduction in overall calories, which often leads to weight loss. I already beat that horse to death.But what if calories are held constant in an OMAD vs. 3-meal-per-day experimental design? Well, according to a <a href="https://pubmed.ncbi.nlm.nih.gov/17413096/" target="_blank" rel="noopener noreferrer">study</a> from the American Journal of Clinical Nutrition, weight-maintenance OMAD improves body composition in normal-weight people.But get this. During the OMAD condition, these folks also saw significant increases in blood pressure and LDL cholesterol—both undesirable effects from the perspective of <a href="https://drinklmnt.com/blogs/health/electrolytes-and-heart-health" target="_blank" rel="noopener noreferrer">heart health</a>. This is possibly due to the stress associated with OMAD. Some folks thrive on OMAD, while others tank. Just keep that in mind.<h3>Weekly Fasts</h3><h3>#4: 5:2</h3>To practice 5:2 intermittent fasting, you eat 0-25% of your normal calories on 2 non-consecutive days per week. The other five days you eat normally.<a href="https://www.cureus.com/articles/12903-intermittent-fasting-the-choice-for-a-healthier-lifestyle" target="_blank" rel="noopener noreferrer">Research</a> has shown that 5:2 can help with weight loss. But again, I believe calorie restriction is driving that train.<h3>#5: ADF</h3>Alternate-day fasting (ADF) follows the same rules as 5:2, except that you fast every other day instead of twice per week. It’s the longest of the protocols at 36 hours per session, and I don’t recommend it for most people. The potential for sleep disruption, stress, and other side effects is too high.<h2>Who Should Practice Intermittent Fasting?</h2>I believe that almost anyone would benefit from a 12 or 13 hour overnight fasting practice. Anything that promotes better sleep promotes better health.But the longer forms of intermittent fasting aren’t for everyone. For starters, fasting isn’t a good idea for children, pregnant or nursing women, those with eating disorders, and underweight people. These groups need more nutrients, not less.What about type 2 diabetics? The research <a href="https://drinklmnt.com/blogs/health/can-intermittent-fasting-help-reverse-type-2-diabetes" target="_blank" rel="noopener noreferrer">suggests</a> fasting can help, but only with medical supervision. This will prevent complications (like low blood sugar) from occurring.And for everyone else, I advise going slow with fasting. Get the overnight fast under your belt, then try 13, 14, or 16 hours.Do what’s comfortable. And when it becomes uncomfortable or unpleasant, back off to an easier fast. Follow that system and you’ll find your sweet spot.