Intermittent fasting: A science-based guide for beginners

From the desk of
Robb Wolf
ScienceIntermittent fasting: A science-based guide for beginners

If you want to start practicing intermittent fasting (IF), but you’re feeling overwhelmed by the seemingly never ending options around the topic, I’m here to help. I mean seriously, why are we making not eating so difficult? 

Intermittent fasting should be, as one of my Russian friends once said, “easy as fall off turnip truck, no?” And he’s right! One of the lesser appreciated benefits of fasting is that it should save you time and energy. Just don’t eat for some reasonable period of time, and then… well… EAT! You’re welcome, that will be four easy installments of $199.99.

Kidding, of course. I’m here to answer all of the frequently asked questions about fasting:

  • How long should one fast?
  • What time of day is best to eat?
  • How many meals should one eat per day?
  • What about macronutrient and calorie targets?
  • Drink water or dry fasting?
  • Why am I getting headaches?
  • Should I fast intermittently every day?
  • Should I train fasted?

I’ve been talking about intermittent fasting since 2005, long before it became trendy. And although that may seem like a comprehensive list of questions, it’s just scratching the surface. Folks manage to spin up a remarkable amount of inquiry on this topic.

I’ll do my best to tackle most of these questions in one piece, but my main goal is to help you begin this whole process with a solid fundamental understanding of intermittent fasting—NOT to give you the answers to a slew of disjointed questions, which is, unfortunately, how people tend to tackle diet and lifestyle changes.

With that, I encourage you to focus on these two questions:

  1. What is your PRIMARY goal?
  2. What is your UNIQUE context?

Oftentimes, folks aren’t entirely sure why they are contemplating IF. Often it is to lose weight or lean out, which is fine, but without context it’s hard to say if intermittent fasting is a good tool or a disaster waiting to happen.

IF does have benefits, but it isn’t always the best tool for everyone. It can help folks lose fat (if used properly) and can fast-track entering ketosis. It can help build resilience and mental toughness. Overnight fasts are beneficial for your 24-hour genetic clock—the circadian rhythm—that regulates a large chunk of your genome. Practicing some kind of IF while traveling across multiple time zones seems to accelerate the process to reset your circadian clock to your new location.

But it’s not all rainbows and autophagic unicorns. For example, am I a big fan of fasted training? Not really. In 2005, I thought fasted training was the bee’s knees. Now, more than 15 years later, working with thousands of people from elite members of the military to balding has-been athletes (like myself) I’d rate it above a parlor trick, but a good bit below Chuck Norris Tears.

But perhaps I’m getting ahead of myself. Let’s start by defining intermittent fasting, then work our way into when it is (and is not) a solid tool.

What Is Intermittent Fasting?

I define intermittent fasting—also called time-restricted feeding—as any fast between 12 and 36 hours. Over 36 hours and you’re in extended fasting territory, where medical supervision is a good idea. Some people put that outer limit at 24 hrs. Wars, mainly imagined, rage over arbitrary definitions such as this. But I’ll leave it to you to decide the timeframe that makes the most sense.

What do I mean by a fast? Broadly speaking, it’s a period of macronutrient (calorie) deprivation.

A fast doesn’t have to be zero calories. A variety of fasting regimens allow for reduced calories on “fasting days” and they appear to have similar benefits. I’ll talk more about them later, but this is an important initial takeaway: ANY method that prevents overeating appears to offer significant benefits. Folks will often acknowledge and then immediately forget this point when contemplating a fast.

Nutrient deprivation (aka, fasting) sends a powerful signal. It tells your body to access its most durable energy source: body fat.

This was highly useful during homo sapiens’ long stint as hunter-gatherers. They didn’t always succeed at hunting and gathering, so they needed reserves to last through periods of privation. Those reserves lined their body as adipose tissue.

Our ancestors were lean, but they nonetheless had tens of thousands of calories of fat on their frames. And they stayed lean because they accessed and burned this fat regularly… while also not overeating. (Let’s not forget that side of the energy balance equation!)

This isn’t the case in modern society. By feeding constantly, we never access our fat reserves. Instead, we keep adding to them.

Fat metabolism is largely governed by the hormone insulin. Feed too much and too frequently and insulin stays perpetually high. And when this energy storage hormone stays perpetually high, one enters a metabolic spiral that leads to insulin resistance, high blood sugar, obesity, and type 2 diabetes.

One can accomplish this process by eating high carb, or low carb, but it appears that the combination of fat and carbs, as they occur in modern processed foods, is a recipe for weight gain and many of the problems we associate with modern chronic disease.

Intermittent fasting drops calorie intake, keeping insulin low, which in turn allows your cells to break down and burn body fat for energy. That’s the metabolic shift fostered by IF.

Benefits of Intermittent Fasting

The benefits of IF flow mostly from the metabolic shift I just described. Take these potential benefits with a grain of salt, because they don’t apply to everyone.

#1: Weight loss

Most people practice intermittent fasting to lose weight, lose fat, feel better, or all of the above.

The evidence suggests that various forms of IF can fulfill this promise, both in obese and non-obese populations.

Weight loss is driven, of course, by the reduction in calorie intake. When people compress their feeding window, they tend to eat less food.

IF can be a decent tool for folks who are unwilling to change the composition of what they eat, but who are game to press “pause” on their eating at certain times. That being said, what you eat does matter, of course. The overconsumption of processed foods and sugar are making us sick and unhealthy.

The big picture? So long as the strategy moves on closer to the desired goals, it’s all good.

#2: Metabolic health

Nothing lowers blood sugar and insulin levels like eating nothing. Because of this, intermittent fasting is a promising therapy for type 2 diabetes and a possible preventative measure for folks at risk for metabolic issues.

To be clear, I don’t think folks with diabetes should jump into fasting head-first. Along with medical supervision, a low-carb diet may be a gentler approach to treat the problem. Low carb+time restricted eating can work absolute miracles for various insulin resistance situations. This is a vastly underutilized set of tools in the battle against metabolic disease.

#3: More stable energy

When you switch to fat as fuel, what are you switching from? Mostly, you’re switching from glucose.

We need glucose to live, but when you rely on glucose exclusively for energy, you’re buying a ticket for the blood sugar rollercoaster. As your blood sugar goes up and down, your energy goes up and down along with it.

Flipping the metabolic switch with fasting or a keto diet gets you off the ride. The more you run on fat, the more stable your energy becomes.

Metabolically flexible individuals can make this shift from carb burning to fat burning fairly seamlessly. Losing fat, lifting weights, and doing cardio all improve our metabolic flexibility, but there are remarkable differences in this capacity from person to person. For example, try as I might, I am unlikely to ever be as metabolically flexible as my wife.

#4: Mental sharpness

Intermittent fasting promotes a metabolic state called ketosis in which you burn fat and produce ketones. Ketones then shoulder a portion of the brain’s ravenous energy requirements.

I feel my mental best in ketosis, and many others report likewise. It’s worth playing with.

#5: Circadian rhythm

Last but not least is how fasting affects your circadian rhythm. This rhythm is regulated by two main external factors:

  1. Light
  2. Food

Bright light and food (especially protein) in the first part of the day help wake you up. Avoiding blue light and food at night helps you wind down and produce your sleep hormone, melatonin.

Point in case: Folks who eat late, get little natural daytime light, and have far too much artificial light in the evening often struggle chronically with body composition. Get your sunshine and get some rest, folks!

Types of Intermittent Fasting

Before covering practical tips, I’ll list the most popular fasting protocols. You don’t have to follow any protocol to the letter, but they provide useful guide rails for what’s on offer.

Popular Intermittent Fasting Protocols:

  • 12/12. Also known as overnight fasting, 12/12 entails going from dinner to breakfast without food. I believe overnight fasting is broadly beneficial because it prevents unnecessary snacking and enhances the circadian rhythm.
  • 16/8. Practicing 16/8 means fasting for 16 hours on a daily basis and squeezing your meals into an 8-hour window.
  • OMAD. One meal a day (OMAD) is self-explanatory. I recommend eating your meal before the sun goes down to keep your circadian clocks tuned.
  • 5:2. This regimen involves five days of regular eating punctuated by two non-consecutive fasting days per week. On fasting days, you can consume up to 25% of your normal calories.
  • ADF. Alternate day fasting (ADF) entails fasting every other day. As with 5:2, you can consume up to 25% of your normal calories on fasting days.

How To Choose A Fasting Protocol

Each of the above protocols has evidence behind it. Which one should you choose?

If you’re new to fasting, I recommend starting with an overnight fast. This is achievable for most folks at a high level of comfort. Plus it’s a great health habit. To this end, I’d also recommend eating more calories early in the day and fewer as the day goes on. BIG breakfast and/or lunch, with just a token dinner.

If you were to skip a meal entirely, I’d suggest that be dinner—but this can be tough to do socially (it definitely wouldn’t fly in my house, as we have kids and dinner time is family time). It’s also important to note that this can disrupt sleep for some folks. If your eating is messing with your sleep, that simply will not do. Sleep is just as, if not even more important to your goals as diet and exercise are.

If you want to extend your fast from there, proceed one hour at a time. By this I mean to add one hour of fasting to your protocol, sit on that for a week, then repeat if all is still well. See how your body reacts at 13, 14, and 15 hours of fasting instead of jumping straight to, say, OMAD (“one meal a day”), which could be a shock to your system.

Also, get clear on why you’re fasting. To lose 20 pounds? To stay lean and in ketosis? To have more time for work?

If you want to lose weight, the longer fasts may get you there faster. But only if your body and schedule can tolerate them. If it’s no fun, there’s no shame in backing off.

If you want to maintain (or gain) muscle, any fast longer than 16/8 is dicey. It will be hard to consume sufficient protein and calories to support lean mass gains. I’d FAR prefer folks do something like 16/8 and lift weights 3x per week vs doing OMAD and not having the energy to do that amount of training.

Keep in mind, folks often “feel great” with a new fasting program… until they don’t. A few weeks in, sleep can get squirrely. This is a case to pump the fasting brakes, not double down on it. Remember, longer fasts aren’t necessarily “better” or “healthier”. Longer fasts can be detrimental, especially for certain groups. I can’t stress enough how important it is to listen to your body.

Groups That Should Be Careful With Intermittent Fasting

Beyond the above considerations, certain people should be cautious with any fast longer than 12 or 13 hours, or simply avoid it altogether. These groups include:

Though early evidence suggests fasting can be therapeutic for diabetes, type 2 diabetics should be careful too. They shouldn’t fast without medical supervision, especially if they’re on blood sugar lowering medications like insulin or metformin. A clinician is necessary to guide and supervise this process.

Tips for Intermittent Fasting Success

When it comes to IF, there’s plenty of advice out there, and not all of it is helpful. So I mean it when I say that these five tips should help you stay ahead of the game.

#1: Take electrolytes

Fasting increases the risk of electrolyte imbalances for two reasons:

  1. You aren’t eating electrolyte-rich foods
  2. You’re excreting more sodium and potassium through urine

Using a zero-sugar electrolyte drink mix like LMNT can help prevent the headaches, muscle cramps, and tiredness that often come along with fasting. It can also help with hunger between meals.

#2: Drink to thirst

You often hear advice to drink more water on a fast. Unfortunately, if you drink too much plain water, you’ll dilute blood sodium levels and along with it your energy. The solution is to take electrolytes along with your water and drink to thirst. It’s a solution in that it will solve a problem, and it’s a solution because it’s water and electrolytes!

#3: When you eat, eat well

When you’re eating one or two meals a day, those meals should be as nutrient-dense as possible. Get plenty of protein, vitamins, minerals, fatty acids, and other essential compounds by eating meat, fish, eggs, and vegetables during feeding windows. Something like Paleo or keto tends to work well.

If your meals are so large you feel bloated, redistribute food into an additional meal. There is more magic in finding a process you can live with long-term than there is in hammering your body with a process that does not work for you.

#4: Go keto first

Before progressing beyond an overnight fast, consider shifting to a low-carb diet. Eating a low-carb diet keeps insulin low, helping you access body fat for energy. This “fat adaptation” makes fasting easier.

#5: Don’t try to be a hero

People are fairly gung ho about intermittent fasting these days. They get competitive with others and themselves.

But if a 24, 20, or 16-hour fast doesn’t feel right to you, don’t pressure yourself to power through. Maybe 12 or 13 hours is your sweet spot.

In the end, it’s your body. You’re the one who reaps the benefits or consequences of the actions you take.

And how you feel—moment to moment—is the most important data point. How’s your energy? How’s your mood? How are you sleeping? Are you feeling strong?

Intermittent fasting should enhance your wellbeing in all of these areas and should move you towards your GOAL. If it’s not, consider backing off to a lighter regimen.

For me, IF isn’t about pushing through discomfort. It’s about an improved system for living—a system that I enjoy and benefit from on a subjective basis. That’s how you pursue intermittent fasting sustainably.

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