Chapter 03 Science or fiction? Exploring the benefits of intermittent fasting

Chapter 3

Science or fiction? Exploring the benefits of intermittent fasting

Intermittent fasting (IF) is the name some nutrition experts give to the practice of occasionally going for extended periods without eating. This fancy name implies that IF is the exclusive domain of the nutritional elite. It’s not. In fact, we all do some form of IF every single day, except we don’t call it that. We call it sleeping.

The time from your last meal at night until your first meal the next day (assuming a typical sleep-wake cycle) makes up your “fasting” interval. And the time from your first meal of the day until your last meal makes up your “feeding” interval.

Put in simple terms, if you typically eat dinner by 8 PM and breakfast at 8 AM the next day, you’re fasting for 12 hours and feeding for 12 hours. Some people refer to this as a 12/12 fast. I know it’s weird to give complicated names and numbers to normal patterns of eating and sleeping, but trust me, these will come in handy in a second.

For now, I’d like to talk about why IF is getting so much press.

When history meets research

Intermittent fasting is nothing new. Humans have fasted for most of their history, whether it’s during the typical overnight period, during more extended periods of food scarcity, or for religious reasons.

What is new is that clinical research on IF’s benefits for health and longevity is beginning to catch up.

Data show that IF, when done properly, might help extend life, regulate blood glucose, control blood lipids, manage body weight, gain (or maintain) lean mass, and more.

Rather than something we’re forced to endure – a result of poor food availability or cultural expectations – IF is becoming something that health and physique-oriented people are seeking out in order to keep their bodies in top shape.

The proposed benefits of IF in animals and humans read like a laundry list of “look better,” “feel better,” “live longer” physiological changes. These include:


  • blood lipids (including decreased triglycerides and LDL cholesterol)
  • blood pressure (perhaps through changes in sympathetic/parasympathetic activity)
  • markers of inflammation (including CRP<, IL-6, TNF, BDNF, and more)
  • oxidative stress (using markers of protein, lipid, and DNA damage)
  • risk of cancer (through a host of proposed mechanisms; we’ll save them for another review)


  • cellular turnover and repair (called autophagocytosis)
  • fat burning (increase in fatty acid oxidation later in the fast)
  • growth hormone release later in the fast (hormonally mediated)
  • metabolic rate later in the fast (stimulated by epinephrine and norepinephrine release)


  • appetite control (perhaps through changes in PPY and ghrelin)
  • blood sugar control (by lowering blood glucose and increasing insulin sensitivity)
  • cardiovascular function (by offering protection against ischemic injury to the heart)
  • effectiveness of chemotherapy (by allowing for higher doses more frequently)
  • neurogenesis and neuronal plasticity (by offering protection against neurotoxins)

With this list of benefits, IF appears to be an amazing cure-all. So why isn’t everyone doing it?

Well, as I’ve said, everyone is doing it! In most cases, people are fasting for 12 hours every single day. Unless you’re waking up at night and raiding the fridge, you’re probably already enjoying some of the benefits of IF. You just didn’t know it.

However, current research is showing that some of these benefits may only be realized after longer periods of fasting – around 20-24 hours, depending on your activity levels. For example, if you’re fairly sedentary during the fast, you may need the full 20-24 hours without food to realize the benefits. However, if you’re very active, or you exercise purposefully during the fasted state, you may be able to enjoy the same benefits after only 16-20 hours without food.

This brings up an important point: I strongly recommend you follow an exercise program regardless of whether you’re experimenting with IF. Although exercise and IF share some of the same benefits, many researchers believe their combined impact on energy balance and cellular adaptation enhances the benefits of both interventions. However, in the absence of clear research data, this could just be wishful thinking.

Not so fast! (Sorry.)

Research in favour of IF looks compelling. So, shouldn’t you get started right away? Maybe, maybe not. Sometimes looks can be deceiving.

Before we get all gung-ho and skip meals or entire days of eating, I think it’s important to mention a few things you’re not going to read on most pro-IF web sites.

Problem 1:
Most of the research to date has been done using animal models.

Although animals (like rats and monkeys) are convenient test subjects, they’re not perfect models for predicting human response patterns. So, all the animal data suggesting strong benefits with IF aren’t necessarily helpful in predicting what will happen when humans try it.

When we look to the human data, we find – disappointingly – that experiments using IF are very limited. Also, those experiments that have been done often use poor experimental control groups. This makes their descriptive and predictive power limited.

(There are some excellent reviews on this and I’ll point to them in the resources section in case you’re interested.)

I know this is annoying. I wish science were done perfectly every time, too. But right now, based on the available research, we’re left with far more questions about IF than answers. Nothing is definitive.

As a side note, none of this is surprising. Human subjects are notoriously hard to recruit for research projects, unless they’re well-paid, especially for projects that seem inconvenient or uncomfortable.

With IF, you’ve got a double-whammy. First, IF studies don’t sound all that attractive. (“Come to our lab so we can starve you for a day.”) Second, there aren’t many big-dollar companies lining up to fund studies that support not eating.

Note from Krista:
How Ramadan got me interested in Intermittent Fasting

I first became interested in IF many years ago, when a client contacted me to ask about Ramadan and muscle loss. During the holy month of Ramadan, for about 30 days, observant Muslims fast every day from sunrise to sunset. The fast-breaking evening meal is often a big dinner. Seems like the perfect scenario for muscle loss and fat gain, right?

Well, I’m glad I checked the clinical research before smugly assuming that Ramadan fasters’ muscles were dissolving and their bellies expanding. Turns out, even though many folks were chowing down on large meals every evening, they were sometimes healthier during Ramadan than the rest of the year. In particular, heart disease and markers of inflammation decreased.

Given the concerns over how well animal research applies to humans, observant Muslims make up a useful human study group for intermittent fasting. They’re highly motivated to be compliant, there are lots of them, and researchers can follow them for a month every year. Plus, more traditionally observant Muslims (also Mormons, another common fasting study population) typically avoid foods – such as alcohol and junk food – that might skew a study’s results.

About this time, new research also suggested that eating less (aka caloric restriction, or CR) could also improve longevity. But daily CR – for the rest of one’s life – seemed horrible. CR advocates looked like walking skeletons, and the chronic daily restriction slowed their metabolism (including hormone production) to a crawl. Sure, they’d live forever, but in that state, who’d want to?

Thus, fasting periodically seemed like a good way to combine the longevity benefits of CR with the lowered inflammation and other health benefits of fasting, while still maintaining hormonal health and lean mass.

This research got my attention. And like JB, I decided to experiment.

Problem 2:
IF is often compared with “normal” eating.

Whether that’s standard rat-chow (in the case of our furry little friends) or the North American diet (for our slightly larger friends), neither diet is best for health, body composition, or performance. In comparing study participants using IF strategies to those using suboptimal dietary intakes without fasting, we are actually “stacking the deck” in favour of IF.

How so? To start, the standard North American diet is often hyper-energetic – we eat more than we burn – which leads to weight gain over time. Since IF protocols often lead to a negative energy balance – burning more than we eat – the comparison isn’t exactly fasting vs. non-fasting. It’s more like a comparison between under-eating and over-eating. And that limits what such studies can actually tell us about intermittent fasting.

You see, almost all controlled calorie studies – not just the IF ones – show improvements in a wide spectrum of health and body composition markers, especially when body weight and body fat are lost in the process.

So maybe it’s not the IF protocol that’s leading to all the benefits described above. Maybe it’s just burning more than we eat that makes all the difference.

Inconclusive… but interesting

Beyond calorie control, the typical North American diet is full of highly processed macronutrients, chemical additives, and environmental pollutants. By asking subjects in the IF studies to abstain from food for extended periods, perhaps we’re not only tricking them into eating fewer calories, we’re also limiting their intake of health-degrading chemicals.

Of course, you might argue, that’s one of the main points of fasting. However, IF isn’t required to reduce our intake of processed food, additives, and pollutants. Maybe we could just stop eating processed foods, additives, and pollutants and experience the same benefits.

Beyond this speculation, there are many other reasons why the IF-related research is inconclusive. But I don’t want to bog this book down with too much exercise and nutritional science.

In the end, I’m not trying to argue for or against the benefits of IF. I actually think IF can be a cool approach to solving a few health- and body composition-related problems. However, as a trained scientist, I am trying to keep it real. While IF research does look promising, this area hasn’t yet evolved to the point where we can say with certainty that the benefits come exclusively from fasting.

Right now, it’s equally plausible that:

  1. eating fewer calories than you burn; and
  2. eating a diet lower in processed foods, chemicals, and pollutants

… may offer most of the same benefits as IF. Add in a good exercise program and you might be able to match benefit for benefit.

Is it the fasting or is it the negative energy balance?

My good friend Alwyn Cosgrove – a well-known training expert and gym owner – illustrates how hard it is to draw conclusions:

“99% of the beginner fat loss clients at my gym come to us constantly skipping breakfast. They don’t eat between 8 PM and noon or 1 PM every day. So they end up fasting between 16 and 18 hours most days, just like a lot of the fasting advocates recommend. Sure, their diets aren’t very good to start with. But they’re fasting and not getting leaner. In fact, many of them are gaining fat.

“When we add in a healthy breakfast within 15 minutes of waking up, we see big differences right away. I don’t know if eating breakfast helps them control hunger, leading to fewer total calories eaten later in the day. I’m not completely sure. Maybe there are other metabolic or nutritional differences that help here too. All I know is that stopping the fast first thing in the morning kicks off a host of positive changes for these clients. It works every time. In the real world.”

Of course, in Alwyn’s example, a few things change simultaneously. His clients start exercising regularly, which makes a big difference. They also start eating an extra meal each day (breakfast). Those are the direct changes.

Indirectly, I’d guess that adding breakfast affected their meals later in the day, causing them to eat less with each one. Because of the breakfast, they just weren’t as hungry at lunch or dinner.

I also bet their new commitment to fitness not only led them to join the gym and start eating breakfast, but to also change the types of food they ate, even if they weren’t told explicitly to do so.

That’s why I suspect Alwyn’s clients benefit from not fasting. The sum total of their two main changes (adding exercise and adding a healthy breakfast) and their two secondary changes (improving food type and amount later in the day) led to the most important requirement for weight loss: a negative energy balance.

Simply put, their energy burn began to exceed calories eaten. They lost weight, got healthier, and improved their lives by not fasting.

But this isn’t an argument for or against breakfast (or fasting). I suspect that if all else is equal – a decent amount of exercise, controlled total calorie intake, appropriate food selection, and proper meal timing – it doesn’t matter all that much whether clients eat breakfast (a shorter fast) or skip breakfast (a longer fast).

There’s only one problem: it’s really hard to make sure all else is equal.

Exercise and eating decisions don’t operate in a vacuum. One decision influences the next, and so on. This happens on both the conscious and unconscious level. And there’s an interesting cross-talk between the body (physiology) and the brain (psychology).

Thus, people skipping breakfast without a plan usually overeat later in the day. In fact, evening over-eating is one of the biggest problems for our fat loss clients too. This results in more body fat, a higher risk of diabetes, and a host of other health problems. That’s why many coaches recommend that people new to eating well and exercising should start eating breakfast.

In other words: It’s not the breakfast, but what happens after the breakfast that’s important.

However, it seems like those who have a good plan for controlling calories later in the day and stick with it can get away with skipping breakfast without any negative consequences.

So really, breakfast only matters when it’s factored into your eating decisions for the entire day.

That’s why self-experimentation and lifestyle matching are really important. If you want to maximize your results, you’ll have to figure out how you respond to eating breakfast or skipping it. Things like your schedule, your unique physiology, and your own self-discipline will play a big role here. But more on all that later.

I will say this: I’m paying attention. As the IF research program continues, I’ll be watching closely to see what happens when calorie-controlled, nutrient-dense, healthy IF diets are compared to calorie-controlled, nutrient-dense, healthy diets that don’t use extended fasts. Only when these studies are done will we really know whether the magic is in the IF – or in improving food amount and food selection.

[In fact, at Precision Nutrition we’re now putting together just such a pilot study with our own clients.]

That research could take a long time, though. Here’s my advice: Don’t wait until these studies are done to start living better. Control your food intake and quality right now, start an exercise program right now, and you’ll get many, if not all, of the benefits above.

Of course, that’s easier said than done. Fitness professionals have been telling people to “eat less and exercise more” for years now. And that hasn’t been working so well.

That’s why I prefer to actually give people strategies and best practices for improving their eating and exercise. To learn more about them, you can check out the resources chapter at the end of this book. I’ll link you to a free 5-day course that will help you build a strong foundation for improved body composition and health.

You should only try more advanced protocols like IF after you’ve built this strong foundation. A one-time, one-day fast, as proposed in the introduction? That’s cool. Leaping – as I did – into more elaborate plans? Save that sort of self-experimentation for later.

As they say in grade-school grammar: you have to learn the rules before you can break them. A guy like me – who’s been exercising and eating well for nearly 20 years – has mastered the essentials.

Now it’s time to break some rules.

Chapter 4
Why do intermittent fasting?
There are many reasons to explore IF. Here's what piqued my curiosity.
Chapter 2
Hi, My name is John. And I haven't eaten in 24 hours.
I tried not eating for 24 hours. I learned a lot, and so can you.

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