Intermittent fasting is one of the most popular health and fitness trends at the moment.
Intermittent fasting not a diet. It does not specify what foods you should be eating, but rather when you should be eating. It is a pattern of eating that involves cycling between periods of feeding and fasting.
Fasting has been a practice throughout human evolution. Our ancestral hunter-gatherers didn’t have access to the abundance of food that we do today. They had to hunt down animals or scavenge plants, and those resources weren’t always available every day. As a result, humans have evolved to be able to function without food for extended periods of time.
There aren’t many things in this world that are absolute binaries, but this is one of them. Your body is always in one of two states: fed or fasted. Your body is in the fed state when it is digesting and absorbing food. This begins the moment you eat, and lasts for up to around 4 hours. During this period of time, the breakdown of food and subsequent rise in blood glucose stimulates the release of insulin to help shuttle that glucose into your liver, muscle and fat cells for storage. Our bodies shift into the fasted state once the food has been digested, absorbed, and stored. In this state, insulin levels are low which allow us to tap into our energy stores (liver glycogen and adipose tissue). During an overnight fast, aka sleeping, we burn through a large portion of our liver glycogen. This means that our bodies are primed to utilize fat for fuel in the morning upon waking.
Many people think that our bodies selectively burn one fuel source at a time, but this is actually a misconception. High intensity exercise is very glycolytic (burns more carbs), whereas lower intensity aerobic training is more ‘fat-burning’ in nature. However, regardless of the type of activity being performed, the body burns carbs and fats simultaneously — it’s just a matter of which one is being utilized to a greater extent. The body can also be trained to become metabolically flexible — increased efficiency in swapping between fuel sources for energy — but that’s a topic for another day.
The unfortunate reality in our world is that we’re never giving our bodies a break from food. How many times have you heard that breakfast is the most important meal of the day, or that eating 6 small meals a day is necessary in order to keep your metabolism elevated? As it turns out, there are many benefits to eating in a similar pattern as that of our ancestors. Here’s how you can begin to implement that pattern.
In my opinion, intermittent fasting can be broken down into two distinct categories: time-restricted feeding and extended fasts.
Time-restricted eating involves keeping your feeding window to a certain portion of the day. Everyone is technically practicing some form of time-restricted feeding since our body is in the fasted state while sleeping, but we’re going to consider ‘time-restriction’ to be longer than 12 hours.These are some of the most popular methods:
- 16:8 — This is the most popular protocol because it’s sustainable and fits our daily schedules. It involves eating within an 8 hour window, say 10 am to 6 pm, and fasting for the remaining 16 hours.
- 5:2 — This is a more aggressive method where you select 2 non-consecutive days of the week and limit your caloric intake to about ¼ of your total daily energy expenditure (the number of calories you burn in a day). You eat normally on the other 5 days.
- One Meal a Day (OMAD): This is the most extreme version of time-restricted feeding. You fast for 23 hours of the day and eat one meal within a 1 hour window.
An extended fast, on the other hand, simply involves going at least a full day without food. If you eat dinner on Sunday night, you wouldn’t eat again until Tuesday morning at the earliest.
This is the most common reason for people to try intermittent fasting. The idea is that, by reducing the number of meals you eat in a day, you automatically reduce the number of calories consumed. As I mentioned above, fasting also depletes your body’s glycogen stores. Without this primary fuel source, you switch over to burning fat for fuel in a process called ketosis.
A 2012 study (2) found that mice who were restricted to eating within an 8 hour window gained less weight and had improved protection against metabolic diseases than mice who were allowed to eat ad libitum, even when total calories were the same. A more recent 2016 study (4) found that an 8 hour feeding window enabled resistance-trained males to maintain more muscle mass and lose more fat mass.
Fasting brings about several hormonal changes in your body as well that can benefit weight loss and overall health:
- Insulin: Fasting improves insulin sensitivity, which is an important marker of overall health. The more insulin sensitive you are, the more efficiently your body can shuttle glucose into your cells, the less blood glucose there is to ultimately move into fat cells. People who have metabolic diseases like type 2 diabetes are insulin resistant — the cells don’t respond well to insulin, resulting in chronically elevated blood sugar levels. Thus, fasting can help protect against some of these diseases.
- Human Growth Hormone (HGH): HGH is a key hormone that regulates growth and metabolism. HGH deficiency can lead to loss of muscle mass, increase in fat mass, fatigue, and more. Levels of HGH have been shown to increase 5 fold in something as short as a two day fast (1). You may have heard that fasting puts you into a catabolic state and muscle falls off your body. This is a myth — HGH plays a huge role in muscle preservation during a fast!
- Norepinephrine: Norepinephrine, in addition to adrenaline and cortisol, are the three major stress hormones in our body. Norepinephrine levels are elevated during a fast, resulting in increased energy expenditure as well as lipolysis (breakdown of fat into fatty acids). (6, 9).
The combination of calorie reduction and hormonal changes seems to benefit body composition. However, keep in mind that weight loss is still ultimately governed by energy balance. If you binge during your feeding window and grossly over consume, you will still gain fat.
Autophagy represents a cells adaptation to starvation. “Auto” means self, and “phagy” means to eat. This word literally means “self-eating”… Spooky right? It’s the body’s mechanism of getting rid of all the broken down, old cell machinery (organelles, proteins and cell membranes) when there’s no longer enough energy to sustain it.
As we go about our daily lives, we’re exposed to an extraordinary amount of toxins and stress through our environment. This increases oxidative stress in our bodies, a harmful state that is incredibly damaging to our cells. I’ve discussed how antioxidants are incredibly important in quelling this oxidative stress (3), but a good mechanic doesn’t just have one tool in his toolbox. Autophagy is another powerful tool with which we can use to maintain proper cellular function. It is a regulated, orderly process to recycling its own damaged components so that new and healthy versions can be built.
Some of the benefits of autophagy include:
- Increases longevity (5)
- Helps protect against neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease (8)
- Reduce inflammation (7)
Autophagy is our body’s best method to clean up the junk and replace it with shiny new parts, but here’s the catch: it takes time for the process to ramp up. How long until autophagy kicks in for you specifically depends on the nutrient status of your body and the presence of certain nutrients, mainly amino acids, glucose, and ketones, but the general rule of thumb is that a 48–72 hour fast will allow you to reap the most benefits.
This isn’t so much a physiological benefit, but there’s a reason intermittent fasting is so popular compared to other topics in the health and fitness space. It’s simple.
People aren’t good at making drastic changes in their lives. That’s why fad diets never work. However, intermittent fasting simplifies things so that anybody can do it. It doesn’t require any fancy gadgets or any additional knowledge. You simply skip a meal and voila, you’re intermittent fasting! You also save money by eliminating a meal each day, and that adds up over time. Talk about a win-win situation!
What’s even better, in my opinion, is that you save time by skipping meals, and time is invaluable. I’ve personally experimented with multi-day fasts, and the amount of time I freed up by not having to worry about food is ridiculous. Think about it: if you were to prepare a meal from beginning to end, it’d range somewhere between 30 to 90 minutes depending on the meal. Multiply that by 3 meals a day, and you’re spending a minimum of 90 minutes dealing with food throughout the day. Even if you just skipped one meal, that’s about 30 minutes of the day that you’ve freed up. Imagine if you could read for an extra 30–90 minutes per day…
The short answer is: it depends. If you’ve never practiced any form of intermittent fasting before and want to lose weight, I’d recommend starting with a 12 hour fast and working your way up. It would be as simple as eating 2–3 hours before bedtime, sleeping 8 hours, and fasting for another 1–2 hours upon waking. Generally speaking, the longer you’re able to fast the more benefits you’ll reap.
However, I do recommend a shorter fasting window (typically in the 12–14 hour range) or something like the 5:2 method for females. It turns out the hormones that regulate key functions like ovulation are incredibly sensitive to energy intake. Missing your period several times doesn’t sound so bad if you’re not looking to have children yet, but the key here is that the female reproductive system is highly intertwined with the metabolism. Fasting for too long or eating too little food can lead to issues with your thyroid health and metabolic function. Ancestrally speaking, this makes a lot of sense. If women were going through periods of stress, such as a famine, it probably meant that it wasn’t the ideal time to have a baby.
Now if your goal is to build muscle, things become a little different. In order to gain, you would need to be consuming more calories than you burn. I’d still recommend that you implement some form of a time-restricted feeding window, but the length of that window depends on you. If you struggle to consume enough calories within an 8 hour window, bump it up to 12 hours. If you’re able to eat enough within a 4 hour window, then props to you! Ultimately, how long you fast for should be dictated by your goals.
Intermittent fasting has taken the world by storm in the past couple of years. It seems like everybody I know is practicing some form of it, and why wouldn’t they? It’s a simple approach to weight loss with a host of additional health benefits that go beyond the number on the scale.
I do believe everyone should be practicing intermittent fasting to some extent. Even something as short as a 12 hour fast is enough to elicit some of the benefits that I discussed. For added bonus, a 3–5 day quarterly fast will allow you to do some seasonal cleaning and reap the longevity benefits of fasting. At the end of the day, it’s about doing what sticks in the long run. Experiment around with it and see what works for you!
Intermittent fasting can be separated into two distinct categories: time-restricted feeding (keeping your eating window to a certain portion of the day) and extended fasts (not eating for longer than one day).
Time-restricted feeding is a simple yet effective tool for body composition. It naturally reduces your daily caloric intake and causes changes in your hormones that aid weight loss. It can also protect/reverse metabolic diseases such as type 2 diabetes.
Extended fasting promotes autophagy, which is one of the most powerful tools we have for overall health. It allows your cells to recycle old, dysfunctional components for newer ones. It removes the existing debris and self-regulates back to optimal functionality. This can aid in protection against neurodegenerative diseases such as Alzhemier’s, as well as enhance our overall function and lifespan.