Dr. Anthony Gustin is a board-certified sports chiropractor, functional medicine practitioner, entrepreneur, podcast host, and founder of Perfect Keto.
Intermittent fasting is one of the simplest ways to improve your health.
It involves switching between periods of eating and periods of fasting. For example, eating all your meals in a 6-hour window each day, and eating nothing for the other 18 hours.
Fasting benefits your brain, metabolism, and cellular repair pathways. It increases insulin sensitivity and stabilizes your blood sugar. Fasting can also be a great tool for weight loss, especially if you combine fasting with a keto diet.
But how strict do you have to be to get the health benefits of fasting?
Is black coffee or green tea okay when fasting? What about MCT oil and exogenous ketones?
This article covers the benefits of fasting, what will break your fast and what won’t, and a couple of variations on intermittent fasting that have their own set of benefits.
Fasting is pretty simple: it’s intentionally not eating for a set period of time.
If you want to try fasting, it can be as easy as not eating for any length of time you choose. That said, there are a few popular subtypes of fasting that work well for people:
- Intermittent fasting. Not eating food for fewer than 24 hours. For example, a 20:4 intermittent fast is a 20 hour fast, with a 4-hour eating window.
- One meal a day (OMAD) fasting. Eat a single meal each day and fast the rest of the time.
- Alternate day fasting. Not eating food for a full day, then eating freely the next day. For example, you’d not eat anything on Monday, eat all day Tuesday, not eat Wednesday, etc.
- Extended fasting. Not eating food for multiple days. Popular lengths include 2-day fasts, 3-day fasts, and 7-day fasts. Some people fast for a month. But if you’re going to attempt a 30-day fast, you’ll want to talk to a doctor.
- Fat fasting. Eating a small amount of fat, but no protein or carbs. Fat fasting is easier than skipping food entirely and is a great way to ease into more intense fasts. You may not get as hungry with fat fasting, which lets you fast for longer. That said, you’ll also miss out on some of the benefits of a true fast.
If you want to start fasting, try a few different lengths and styles and find a rhythm that works for you. There’s no right or wrong way to fast. It’s about what feels sustainable for your unique physiology, your goals, and your lifestyle.
Fasting is a powerful way to improve your performance. It’s good for your body and brain in a variety of ways:
Intermittent fasting is a great way to cut your caloric intake without adopting a low-calorie diet.
Ultimately, you have to be in a calorie deficit to burn body fat, but traditional low-calorie diets don’t work well for weight loss long-term. In fact, more than 80% of people who lose weight on a low-calorie diet gain back the weight they lose[*][*].
Many people find that fasting is psychologically easier than a traditional low-calorie diet. Intermittent fasting provides clear, black-and-white structure — you do not eat during the fast — which discourages snacking outside your eating window.
Fasting also lets you eat until you’re full once you finish your fast. And because it’s difficult to pack a full day’s worth of calories into a small eating window, you get the best of both worlds: you’re in a calorie deficit at the end of the day, but you go to bed satisfied.
Short eating windows are even more powerful when you combine them with a keto diet.
Keto suppresses your appetite by regulating the hormones that make you hungry, which makes it even easier to make it through your fast and lose weight without feeling hungry[*].
Autophagy for Anti-Aging, Mental Clarity, and Inflammation
Autophagy is like spring cleaning for your cells. During autophagy, your cells go through all their components and replace any old or damaged parts with shiny new ones.
Your cells end up biologically younger, which is great for anti-aging[*][*]. Autophagy also makes your cells work more efficiently, which translates to practical benefits like a faster metabolism and less inflammation[*][*].
Consistent, shorter fasts (fewer than 24 hours) seem to be especially good for your brain. To quote one study, “short-term fasting induces profound neuronal autophagy”[*].
In other words, fasting clears waste from the brain and rejuvenates brain cells.
A lot of people report mental clarity as one of the main benefits of fasting, and brain autophagy could partly explain why. It makes your whole body run better[*].
Stable Blood Sugar and Insulin Levels
Fasting keeps your blood sugar and insulin levels stable and low, which is ideal for lowering your risk of heart disease, diabetes, insulin resistance, and other common metabolic diseases[*].
Fasting also eliminates blood sugar peaks and crashes, which leaves you with steady energy throughout the day.
Fasting will also help put you into ketosis, especially if you’re already following a low-carb diet.
When you’re in ketosis, your body switches from burning sugar to burning fat for fuel. Ketosis has a wide variety of benefits, from mental clarity to hunger suppression.
You get all these benefits from a full fast (no calories, only water).
If you add a few calories or try fat fasting, you may forego a couple of these benefits — although breaking a fast can be good for you, too.
There are mixed opinions on what breaks a fast. So, it helps to break fasting into two categories: true fasting and fasting-mimicking diets.
A true fast (also called a water fast) is simple. You consume no calories or foodstuffs of any kind. Just water.
There’s debate about whether black coffee interferes with a true fast.
Purists say that you should avoid coffee because it contains caffeine, which affects your liver function (your liver uses enzymes to break down caffeine) and speeds up your metabolism.
However, unless you want to be as strict as possible, black coffee is fine and will not interfere with fasting because it has no calories.
In fact, coffee makes fasting a lot easier. Caffeine makes you more alert and suppresses appetite, which can make your fasts more comfortable and help you extend them.
Just don’t add cream to your coffee — it’ll break your fast. Avoid artificial sweeteners and sugar alcohols (like xylitol and erythritol) as well. If you want to sweeten your coffee, use either stevia or monk fruit extract.
Amino acid supplements (like BCAAs) will also technically break your fast. If you want to do a true water fast, save your supplements for your eating window.
Protocol: Consume no calories. Only water. Black coffee is acceptable, although some people debate whether caffeine interferes with a fast.
Mimicked fasting offers several — but not all — of the benefits of true fasting, with the added benefit of being easier than a true fast.
Also known as fat fasting, mimicked fasting allows you to consume small amounts of anything that doesn’t spike your blood glucose or insulin levels — usually fat.
This is a great place to start for beginners who want to ease into full-on fasts. It’s also a good option if you want to lose weight but struggle with hunger on a true fast.
Carbohydrates and protein both affect your blood sugar, so you want to stick with fat on a mimicked fast.
Medium-chain triglycerides (MCTs), exogenous ketones, and coffee will all suppress your hunger, but won’t spike your blood sugar.
Heavy cream in coffee is acceptable on a mimicked fast (note that half-and-half is not, because it has carbs).
You can also make Keto Coffee with MCTs and grass-fed butter, which will give you energy and keep you full for several hours.
Some people also do bone broth fasts where they only have bone broth for a day. Bone broth likely won’t spike your blood sugar either, which makes it fine as a variation on traditional fasting.
Mimicked fasting is great if you’re fasting for weight loss, or to stabilize your insulin and blood sugar levels, but it may not trigger the autophagy that you get from a true fast.
Protocol: Stick to low calories. MCTs and/or exogenous ketones, healthy fats like coconut oil and butter, and black coffee are acceptable
When choosing the right fasting approach for your body, there are a couple of things to keep in mind.
First, a few of the benefits of fasting — like autophagy — may only apply if you complete a true fast.
However, adding in a small amount of fat or ketones won’t affect your blood sugar or insulin levels and offers many of the benefits of complete calorie-restricted fasting. This also may be more sustainable for your lifestyle.
Your choice of fasting type should depend on your goals:
True Fasting Benefits
- Blood sugar/insulin regulation
- Mental clarity
- Weight loss (possibly greater weight loss than a mimicked fast)
Mimicked Fasting or Fat Fasting Benefits
- Blood sugar/insulin regulation
- Weight loss
- Improved cognitive function (because of MCTs and Exogenous ketones)
- Easing into fasting
- Help you stick to longer fasts
- More sustainable, if you find true fasting is too difficult
The Takeaway: Choose the Fast That Works For You
As you decide on a fasting method, keep in mind that there’s a difference between what is most optimal versus what’s most sustainable.
For example, if your main goal is fat loss, a true fast is your best bet — in theory.
But if you’re miserable and can’t function after 12 hours with no calories, then a fat fast might be a more sustainable option.
You can even shorten your eating window over time from 12 hours to 8 or 6.
Whatever you choose, it’s worth your time to give fasting a try. This free, simple tool is a powerful way to improve your health and performance.
And if you want to make fasting easier and get even more benefits from it, try pairing it with a ketogenic diet. This beginner’s guide to keto has everything you need to get started today.