I started writing about intermittent fasting in 2005. As I saw it, following a periodic fasting schedule was a valuable way for folks to pump the brakes on over-indulgent eating habits. Eat less frequently and you naturally consume less food. Eat less food in a shorter window and—especially for those with obesity and diabetes—your metabolic health can shift toward the better. It made sense to me.
It still makes sense, but I’m no longer one of the few people writing about it. A chorus of voices has arisen to serve the public’s growing interest in fasting. Unfortunately, many of these sources portray fasting as a panacea for all people all the time. Let’s be clear: Intermittent fasting isn’t always beneficial. The longer-duration fasting schedules, in particular, have the potential to do more harm than good, especially when practiced by lean, metabolically healthy people.
While folks embark on fasting for a variety of reasons, one of the largest draws involves the notion that fasting will forestall aging and prevent cancer. People think these results are driven by the suppression of various growth-promoting signals, including mTOR and insulin-like growth factors (IGF’s). There’s a lot of misguided thinking around this topic that fuels our tendency to focus on less likely risks.
Meanwhile, folks ignore guaranteed issues like sarcopenia, the loss of muscle mass as we age. Sarcopenia is a guaranteed process with highly predictable parameters, and we can prevent its effects by strength training and eating adequate protein. To that last point, is it possible to absorb enough protein in one daily meal to maintain muscle? I’m not sure. And at the least I think eating one meal per day is far from optimal for muscle mass maintenance.
What about the already-lean person? Should they abstain from calories every other day via alternate day fasting? I don’t think so. Eat enough meals per day to stay lean and strong. Then drop in a bit of fasting or time restricted eating if it helps you perform better.
Essentially, more is not always more. That’s a taste of my fasting philosophy. What follows is the four-course meal. You’ll learn how intermittent fasting works, the benefits of intermittent fasting, five fasting protocols, and three criteria to decide which protocol to choose. By the end, you’ll be in an excellent position to choose the intermittent fasting schedule that works best for you.
Intermittent Fasting 101
I define intermittent fasting as scheduled breaks in caloric consumption of between 12 to 36 hours. Extended fasting is when the breaks exceed 36 hours.
What do I mean by “breaks in caloric consumption”? I mean consuming significantly fewer calories than one would typically consume. For instance, eating 0 to 500 calories in one 24-hour period, then eating 2000 calories in the next 24-hour period.
Wait, doesn’t fasting have to be a zero calorie affair? Not necessarily. The research suggests that the benefits of fasting—weight loss, metabolic improvements, etc.—still result if a person consumes less than 25% of total daily calories during the fasting interval. And similarly, there are benefits to calorie restriction between 5-10% below calorie expenditure. Most reputable weight loss approaches within the last 100 years have functioned this way.
My position is that, whether we fast or just figure out a way to spontaneously reduce calorie intake (i.e. higher protein diets are great for satiety), the end result is largely the same.
Some fasting schedules entail fasting on certain days of the week, while others entail fasting for a certain amount of time every day. The weekly schedules allow limited calories on fasting days, while the daily schedules do not.
The Machinery of Fasting
You don’t need any special tools to start fasting. You already have everything you need.
Like us, our ancestors were well-adapted to temporary calorie restriction. They had body fat—a storehouse of latent energy, thousands of calories strong—ready to be burned during times of scarcity.
As a byproduct of this fat burning, their livers produced molecules called ketones to fuel their hungry brains. Being in ketosis helped them stay sharp and therefore helped them survive.
All this machinery remains today. We still burn fat during a fast. We still produce ketones. We still get that mental edge in a reduced-calorie state. But since fasting is no longer a Darwinian necessity, most people don’t do it.
Instead, most people spend the greater part of the day (and night) munching, nibbling, and grazing. That’s another aspect of our genetic programming: see food, eat food. That instinct kept cavemen alive. However, in times of plenty, that same heuristic becomes maladaptive. We overindulge (especially on sweets), and we succumb to obesity, diabetes, and chronic disease.
Intermittent fasting is a deliberate practice that prevents overeating. Most of its benefits, I believe, follow from this point. And let me be clear about this: if you are lean and metabolically healthy, I’m not sure there is much (if any) benefit to intermittent fasting other than that it may simplify your life (cooking fewer meals per day) or serving as THE tool that helps you avoid overeating.
Benefits of Intermittent Fasting
The biggest reason people fast is to lose weight. It’s the big selling point. There’s compelling evidence that people lose weight on various intermittent fasting schedules.
It makes sense: If you compress your feeding window, you’ll naturally consume fewer calories. And when you consume fewer calories than you expend, you lose weight.
Also, being in a ketogenic state helps with appetite management. And ketosis has other benefits related to inflammation, mental health, and energy levels. But you don’t need ketosis to explain fasting-related weight loss.
Beyond weight loss, I group the benefits of fasting into two buckets:
- Metabolic benefits
- Circadian rhythm benefits
By metabolic benefits, I mean the benefits for blood sugar regulation and fat metabolism. (Lower blood sugar, lower insulin, increased fat burning.) And when you look at the evidence, intermittent fasting looks to be a promising therapy for type 2 diabetes.
The circadian rhythm benefits are crucial too. By fasting overnight, you promote the conditions necessary for restful sleep. Why? Because food (like light) activates the part of your brain that controls your metabolism, appetite, sleep cycles, and more. Food during the day wakes you up, fasting at night shuts you down.
5 Intermittent Fasting Schedules
Here I’ll review the five most common fasting schedules, starting with daily programs—my preference—and ending with weekly programs.
#1: 12/12 (Overnight fasting)
Overnight fasting is the simplest form of time-restricted feeding (TRF). TRF is a species of intermittent fasting that entails compressing your daily feeding window to 12 hours or fewer. The research suggests that TRF provides metabolic and circadian benefits across many populations.
If you’re new to intermittent fasting, 12/12 fasting is the ideal place to start. Go between dinner and breakfast without munching (12 hours between meals) and you’ll reap the rewards.
16/8 is the next level of time-restricted feeding. It entails eating all your daily calories within an 8-hour window each day.
There’s no need to consciously restrict calories on 16/8. Rather, you may want to consciously feast to get enough nutrients (like protein) in the compressed window.
When calories are held constant, the research suggests 16/8 is perfectly compatible with fat loss and muscle maintenance. Skipping breakfast is also compatible with enhanced productivity.
In case you were wondering, I practice something like 16/8. It’s just enough fasting to keep me lean and just enough feeding to keep me energized. I tend to front load more calories early in the day and make my dinner a lighter meal, but that’s all preference.
When you practice OMAD, you eat “One Meal a Day.” All of your calories come from that single feeding.
In one study, normal-weight people doing OMAD did lose fat compared to 3-meal-per-day controls (calories between groups held constant) but they also showed elevations in blood pressure and LDL cholesterol. The “why” isn’t clear, but understand that fasting is a stressor for your body. More isn’t always better.
Another issue I have with OMAD is that it often leads to unintentional calorie restriction. That’s okay for obese and type 2 diabetic folks, but I’m not crazy about it for people already at a healthy weight, especially not a hard-charging athlete. If you get super busy and can only fit in one meal that day, that’s okay. It shows you are metabolically flexible and resilient. But train hard and eat OMAD for a long period of time, and I can almost guarantee you’ll overtrain, lose muscle, lower performance, damage your hormonal function, and hurt your sleep.
I’ve worked with a LOT of people in the 23 years I’ve been doing this. Back in the early 2000’s I was super excited about various forms of fasting for athletes. But it did not take me long to discover that aggressive fasting (like OMAD) could break an athlete in a spectacular way.
I should also mention that for those with diabetes, it’s important that your fast is medically supervised. Always talk to your doc before diving headfirst into a program that will significantly affect your blood glucose levels.
5:2 weekly fasting entails consuming 0-25% of normal calories on two non-consecutive days per week. For example, on a 2000-calorie diet, one would consume 0-500 calories on Monday and Thursday, and then 2000 calories on the other five days.
Unsurprisingly, 5:2 can help with weight loss. That’s what happens when you eat fewer calories overall. I could see a schedule like this working for some athletes if, on lower calorie days, the meals are predominately protein (to help minimize muscle loss) and the training is of lower intensity.
Alternate-Day Fasting (ADF) is one of the most aggressive intermittent fasting schedules. You consume 0-25% of your regular calories every other day.
ADF is often used for therapeutic purposes. For instance, it seems that ADF can have pretty profound (and relatively rapid) effects for folks with obesity and diabetes (with medical supervision, of course).
But it’s not easy. You’re asking folks to sacrifice three-to-four days per week to the fasting gods.
ADF may be worth it as an attempt to kick diabetes, but I don’t recommend it for widespread use. The potential for muscle loss is too high, and the sustainability factor is too low.
How To Pick a Fasting Schedule
To pick a fasting schedule, I suggest using three criteria:
- Your schedule
- Your enjoyment
- Your goals
Let’s review how each can guide your decision-making framework.
How would you structure your perfect day? Your answer will inform your choice of fasting schedule.
If you enjoy doing compositional work in the morning (as I do), you might skip breakfast and pursue 16/8. But if family breakfast is a sacrament, perhaps a simple overnight fast would work better.
In general, the weekly fasting schedules are more disruptive than the daily schedules. It’s tough to find a rhythm when you’re fasting every other day.
I enjoy my fasting schedule tremendously. After 16 hours of fasting, that first meal back is a joy. Minimizing your meals can maximize your eating pleasure.
Well, up to a point, that is. Intermittent fasting shouldn’t feel like a chore. Hunger is normal, but you shouldn’t feel irritable, cold, or tired. These are signs you’re fasting too aggressively. Back off to a shorter fast.
Ideally, you should start with an overnight fast and work your way up the fasting ladder. Most folks tend to feel best in the 13 to 18-hour range. More is NOT inherently better. If you get overly hungry, just eat. There is no medal waiting for those who overdo their fast.
Are you fasting to lose weight? For health and longevity? To keep muscle and lose fat? It’s up to you to decide what an appropriate goal is, but typically I see the latter.
Knowing your goals helps you choose the appropriate fasting schedule. For example, more extended regimens are probably useful for weight loss, but not so great for muscle maintenance.
I view weekly regimens like 5:2 and ADF as the power tools of the fasting tool shed. They’re useful for breaking through stubborn cases of obesity and type 2 diabetes, but too forceful for regular use.
As you’ve probably noticed, I prefer daily regimens. And within those, I tend to prefer 12/12 or 16/8. They offer the benefits of intermittent fasting without acting as an excessive stressor on your body. Go beyond that, and you run the risk that your practice becomes unenjoyable or unsustainable.
Sustainability is the key. And on that note, I’ll leave you with one last tip: when folks don’t dial in their electrolytes (specifically sodium) on a fasting regimen, they tend to feel like hell and quit. Check out my article on electrolytes and fasting to learn more. (Hint: Drink LMNT, our tasty electrolyte drink mix that won’t break your fast.)
To sum it all up, the trick is to find a schedule that slots nicely into your lifestyle. Once fasting becomes a habit, you won’t even need to think about it.