I’ve been writing about intermittent fasting (also called time restricted feeding) since 2005 now. The list of potential benefits is long: weight loss, fat-burning, ketosis, cognition, gut health, longevity, and more.
Humans evolved to benefit from fasting. During periods of nutrient deprivation, a pathway called AMPK activates, ketones are made from body fat to fuel the brain, cells start to recycle themselves, and organs shrink. When food is reintroduced, the tissues regenerate. In total this process of creation and destruction is likely an important feature to health, particularly as we age.
So fasting can be powerful medicine—yes—but there’s no consensus on which protocol is optimal. Should we fast for 12 hours, 24 hours, 3 days? And how often should we do it?
The truth is, we really don’t know. Humans are a highly variable species. A two day fast may work great for one person, but provoke sheer misery in another. Others would be better served to not fast at all.
I know it’s a cliche, but the best approach is to go slowly and listen to your body. Don’t start with the whopper fast. Start with the baby fast. I’ll cover this in more detail later.
I’m sure you have many more questions about fasting, and I’ll do my best today to answer them. Read on.
What Is Fasting?
To fast is to abstain from calories for a set period of time. Traditionally, fasting means total calorie deprivation, but some modified fasting regimens allow up to 25% of normal daily calories on fasting days.
There are two main categories of fasting: intermittent fasting (IF) and extended fasting. There are no hard rules here, but I’ll be calling any fast up to 36 hours an intermittent fast. When you cross into two days without food, that’s extended fasting. Yes, it’s a bit arbitrary, but for our purposes I think it helps to organize our thinking.
Fasting is not a recent innovation. Back in Paleolithic times, human civilization wasn’t yet blessed with a Whole Foods on every corner. When the hunt and gather crew failed, you had no choice but to fast.
Fortunately, your body generally doesn’t shut down in a fasted state, but rather switches energy sources. Instead of deriving energy from food, you derive it from glycogen (stored glucose) and body fat. When glycogen stores become sufficiently diminished, body fat becomes your default fuel. This is obviously desirable if you’re trying to lose fat mass.
Some folks, on the other hand, do NOT transition to fasting easily. Folks who are insulin resistant and/or metabolically inflexible can have horrible time fasting, at least initially. One could make the case that these are the very people who SHOULD try fasting, but some caution should be exercised here. It may be easier for these folks to transition to a low-carb, paleo, or keto diet before trying fasting.
Body fat, by the way, is stored as triglycerides—bundles of three fatty acids bound by glycerol (sugar) molecules. Fasting causes triglycerides to split apart into fatty acids, which are then beta-oxidized (burned) predominantly in the liver to produce energy (ATP) and ketones.
Put another way, fasting is like an internal keto diet. Instead of running on dietary fat, your cells run on body fat. The results are similar: increased fat burning, ketosis, and all the benefits that come with keto.
5 Types of Fasting
Even if you’re not a monk, you’ve almost surely practiced intermittent fasting. If you’ve ever gone 12 hours between breakfast and dinner, congrats: You did it.
Here I’ll run through five different types of intermittent fasting. I recommend you start with shorter fasts and work your way up as comfort and schedule permit. ALSO: if you are a hard charging athlete, fasting may not be a good fit for you AT ALL. Yes, our paleolithic ancestors were active, but they were not competing in the CrossFit games. If you are a high caliber athlete I’d be cautious in rolling out any type of fasting protocol unless you are working with an experienced coach and are using the process for a VERY specific goal.
12/12 is my recommended starter fast because it’s both easy and beneficial. To practice 12/12, take a 12-hour break between dinner and breakfast. Stop eating at 8 PM, start eating again at 8 AM (or honestly, any split that works for you such as 6/6 etc).
When you don’t snack overnight, you make your body happy. First, you’re keeping blood sugar and insulin low—which helps you burn more fat. Second, you’re enhancing your circadian rhythm, the 24-hour sleep-wake cycle that controls a huge chunk of the human genome. (Food sends “wake up” signals to your circadian clocks, so it’s best to align feeding windows with daytime hours). This is actually why I’d strongly recommend your schedule looks more like 6/6 than 10/10… although as with everything, there can be caveats. For example, a shift worker might benefit from the simplicity of this later schedule.
On 16/8, your daily feeding window is compressed to eight hours. I’ve done something similar for about 15 years. I eat a huge breakfast around 8 AM, lunch at 2 or 3 PM, and then I’m finished until the next morning. Other folks find skipping breakfast works better. Often it’s easier to skip breakfast over lunch or dinner for social reasons. Up to you – depending on what makes you feel and perform your best.
OMAD stands for one meal a day, and that single meal—be it at 8 AM or 8 PM—will be a big one. To keep your digestive system from hating you, however, avoid stuffing yourself like a boa constrictor swallowing a baby goat.
To practice 5/2 fasting, you choose two non-consecutive days per week to fast, eating normally the other five days. On “fasting days”, you eat anywhere from 0 to 25% of your normal caloric load, which comes to around 500 calories per day.
#5: Alternate-day fasting
Alternate day fasting (ADF) is easy to understand. Just fast every other day. As with 5/2, fasting days can range from 0 to 25% of normal calories. ADF can be socially crippling, but it’s probably the most effective regimen for weight loss.
Depending on the person, fasting may be a super effective medicine. Based on the scientific literature, here are the most important benefits.
#1: Weight loss
Most people fast to lose weight. Through some combination of metabolic enhancement but mainly calorie restriction, it tends to work pretty well.
Which IF protocol is best for weight loss? Not clear—many different regimens have clinical trials supporting them. My guess is that 5:2 and ADF are most effective because they involve calorie restriction multiple days per week. In one study, for instance, non-obese people on a 12-week modified ADF program lost 11 more pounds than controls.
#2: Insulin function
There’s no better way to lower blood sugar and insulin levels than to fast. It even beats keto.
Fasting has proved especially useful for diabetics. For example, a recent study showed enormous promise in reversing type 2 diabetes using a simple fasting approach.
#3: Cognitive health
The path from fasting to ketosis is simple. Fasting lowers insulin levels, which then tells your liver: Yo liver, time to burn fat and make ketones!
Ketones are clean, efficient fuel for the brain, heart, and liver. (Especially the aging brain, which can’t use glucose so well). In one clinical study, elderly people performed better on a series of tests after taking MCT oil—a type of fat that elevates blood ketones.
#4: Longevity (Autophagy)
Fasting increases a recycling program called autophagy in your cells. When a cell undergoes autophagy, it digests old parts and replaces them with new ones. In other words, autophagy turns garbage into gold.
Autophagy is closely linked to longevity. When autophagy is impaired, degenerative disease risk shoots up. As a general rule, long fasts induce more autophagy than short ones, but we still don’t know the specifics for humans.
Who Should And Shouldn’t Fast
Overweight and obese people stand to benefit most from fasting. Lots of body fat to lose. Yet I believe most people should try time restricted feeding—even just 12/12—to capture other benefits, like circadian rhythm enhancement.
That said, certain populations should be careful with fasting. These include:
- Pregnant and nursing women. A growing baby increases Mom’s nutrient needs. Fasting and other forms of calorie deprivation aren’t recommended.
- Underweight people. Being underweight (BMI under 18.5) increases the risk of heart disease and bone density issues. Any protocol that restricts calories should be avoided.
- Those with eating disorders. People with anorexia or bulimia need to mend their relationship with food, not fast.
- Children. Kids need many nutrients—especially protein, zinc, iron, calcium, and folate–to grow. It’s okay to skip a meal now and then, but growth (nutrient-dense, whole foods) should be the priority.
- Diabetics. If a diabetic wants to fast, great—it can be therapeutic. But medical supervision is essential. Without it, blood sugar can dip dangerously low, and even be fatal. I cannot stress this enough: Those on medications—especially metformin and insulin—need to consult their doctor before fasting.
- Athletes. Some folks can implement fasting into an overall athletic strategy, but I’ve seen a lot of folks break themselves by pushing fasting too hard in the context of hard training. Be smart about how you do this. If folks are interested we can look in a future article about how to do this in the context of athletics specifically.
To round this section out, keep in mind that fasting, like exercise, is a stressor. Some is good, but too much can be harmful. If you’re going through a stressful period (breakup, illness, work troubles, etc), think twice before fasting more than 13 or 14 hours.
One final word of caution. Hunger is normal during a fast, but if you feel weak, shaky, or otherwise awful—don’t power through. Replace electrolytes (particularly sodium) and if you don’t feel better within 15-20 minutes, eat something. Fasting should make you feel great. If it doesn’t, it may not be the best strategy for you.
Tips for Fasting Success
Fasting isn’t super complicated. Just stop eating for a set number of hours.
Still, there are strategies to help you succeed. Here are the big ones:
- Start slow. Start with 12 hour fasting and work your way up one hour at a time. This will prevent you from flaming out early.
- Consider going keto. Transitioning to a keto diet helps you fat-adapt. This means you can better access body fat during fasting periods. Plus keto reduces hunger hormones and can curb your raging appetite. I HIGHLY recommend this approach vs going from junk food to no food.
- Drink non-caloric beverages. Coffee, tea, and water will fill your stomach, cut hunger, and keep you hydrated. As a bonus, coffee has been shown to enhance autophagy.
- Break your fast wisely. When the clock hits mealtime, avoid gorging yourself like a starving jackal. It’s not your last meal on Earth. Eat a normal, nutritious plate of food and eat it slowly to let your satiety hormones kick in.
- Increase nutrient density. When you consume fewer meals, you have fewer chances to consume vitamins, minerals, protein, and other essential nutrients. So make your meals count. Fill your plate with vegetables, healthy fats, animal protein… you know, all that stuff that doesn’t come in a package.
- Take electrolytes. Low electrolytes—especially low sodium—are a huge cause of fasting side effects like muscle cramps, irritability, and fatigue. Why do electrolytes drop during a fast? Well for one, you’re not eating any electrolyte-rich foods. But also, fasting keeps insulin low, signaling the kidneys to excrete extra sodium and potassium through urine. To maintain electrolyte levels during a fast, eat plenty of salt and consider supplementing with LMNT. It’s formulated with the optimal ratio of sodium, potassium, and magnesium to support your fast. And it has zero sugar, so it won’t break your fast. Just pour a stick in your water bottle and sip throughout the day.
Okay, that’s all for today. Hope this post answered many of your fasting-related questions. Share your thoughts and questions in the comment section below!