The idea of fasting in the modern age may sound unnecessary. If you eat your veggies, avoid sugary beverages, and exercise often, do you really need to track timing in between meals?
No, you don’t need to fast. But fasting does have some pretty cool benefits.
Evolutionarily speaking, fasting makes sense. For tens of thousands of years, humans experienced long stretches between meals before the tribe could catch a massive amount of meat or gather enough roots and berries for a party. More recently, families depended on seasonal harvests to supply food throughout brutal winters. Humans managed to survive, and it turns out that our bodies may have adapted to that kind of rhythmic calorie restriction.
There are several health benefits of intermittent fasting. It's great for fat burning, weight loss, stabilizing your blood pressure and blood glucose, managing type 2 diabetes, and increasing your mental clarity, all without any side effects.
Fasting is more popular than ever. Biology nerds, bodybuilders, and everyone in between -- they're all using restricted eating windows and fasting periods with no calorie intake to boost brain function and get ripped.
This article is a beginner's guide to fasting. It will cover how fasting works, the health benefits of fasting, different fasting regimens you can try, and how you can use fasting with a keto or low-carb diet.
What is fasting?
The avoidance or restriction of calories go by many different names. From the more clinical “caloric restriction” or “dietary restriction” to the less appealing “starvation diet.”
The clinical definition of fasting also varies from total caloric restriction – basically water fasting – to allowing up to 200 calories per day, to restricting calories by 60-80% .
Your approach to fasting depends on your goals. If you’re simply looking to lose some weight and balance your blood sugar, intermittent fasting could be for you.
Or you could get hardcore and go for the total immune system overhaul with a longer fast. In that case, benefits seem to kick in after you run through all energy reserves in the form of stored glucose. This takes most people anywhere from 24-48 hours, depending on activity level, caloric intake, and body mass.
For our purposes, we’ll define “fasting” as not eating for over 24 hours, while intermittent fasting (IF) will mean fasting for fewer than 24 hours – for example, 16 hours of no eating to 8 hours of eating (16:8).
Benefits of fasting
Researchers started calorie-restricting worms and rodents about hundred years ago, with pretty promising results – from longer lifespans to lower glucose and insulin levels. Keep in mind, however, that a lot of this hasn’t been reproduced in humans. Here are just a few benefits of calorie restriction in the form of IF:
Fasting – even shorter-term fasting – triggers profound autophagy in rodents and in human cells .
Think of autophagy as spring cleaning for your cells. When you put your body under mild stress, your cells step up to the challenge by getting rid of waste products and old damaged parts. Autophagy translates to “self-eating”; your cells gobble up old, damaged cells and replace them with shiny new versions. Autophagy can help maintain muscle mass and decrease inflammation and aging (again, in rats  and in human cells . It keeps your system running efficiently.
Intermittent fasting can also help you lose weight, largely because it puts you into a convenient pattern of fasting and feasting. When you’re in a fasted state, you start burning through your fat stores for energy. When you have food in your system, you’ll burn that for energy, and fat loss will temporarily turn off.
With intermittent fasting, you cycle between the two states. What makes it especially nice, though, is that fasting makes you especially sensitive to how full you are, so you’re unlikely to overeat once you break your fast.
- Blood sugar stability/insulin sensitivity [5,6]
- Leptin sensitivity (your body’s “satiety” hormone) 
The other nice thing about intermittent fasting is that it can feel less restrictive than many other ways to lose weight. If you can make it through the fast, you get to feast to your heart’s content at the end. It’s a satisfying rhythm (provided you’re eating good foods).
Fasting for three days (we’re talking no food for 72 hours) can kickstart your production of infection-fighting white blood cells, regenerating your entire immune system . This same study found that fasting long enough to deplete glycogen stores (usually 24-48 hours) also decreases an enzyme called PKA and IGF-1 hormone, two markers of aging and cancer risk.
It’s questionable if a life without eating every day is worth living. Still, fasting in worms and rats extends lifespan by anywhere from 15-50% . It makes sense, though, that with the above benefits in action, you’re much more likely to avoid age-related diseases like diabetes, heart disease, and cancer.
You may feel a little lightheaded the first couple times you skip a meal, but if you stick with it, fasting can give you a real mental boost.
When your body runs out of glycogen stores to snack on, it switches to burning fat, aka ketones. This is when the magic happens. Your brain prefers running off of ketones and, in fact, ketones are known as being neuroprotective. So, even though there’s only some preliminary association between fasting and improved synaptic function in fruit flies, it’s likely that this is why so many people report increased mental clarity while fasting .
Pro-tip: Limit your carbs and increase your fat intake for 2-3 weeks prior to experimenting with IF. This could make even short periods of fasting a lot easier.
Remember autophagy? Fasting seems to put your brain cells in repair mode as well. Caloric restriction helps maintain and repair neuronal pathways in rats. There’s also evidence in rats that IF stimulates the production of new brain cells and increases brain plasticity . This is great news when it comes to battling age-related brain degenerative diseases like Alzheimer’s or restoring brain function after injury . Again, though, we need more studies in humans.
It may sound counterintuitive, but fasting can actually help you maintain or even build muscle [13,14]. Fasting can increase something called human growth hormone (HGH), which is one of your body’s most important hormones related to building muscle tissue. HGH production is highest during your teenage years and production wanes as your age. Super low levels of HGH in adults mean less lean muscle mass and more body fat. Even your bone density can suffer without it.
Taking exogenous HGH can get tricky – too much of this stuff is no good and can cause the proliferation of things like cancer cells. Best to boost your body’s natural HGH production. Enter: fasting.
Fasting for just 24 hours can double HGH secretion; fasting for 48 hours offers up to a 5-fold increase in HGH .
Even if you’re not interested in building muscle, this should assuage your fears about losing muscle mass during short-term fasting.
Downsides to fasting
Fasting isn’t for everyone. Check out this list before you get too excited about skipping meals:
- On the discomfort spectrum, restricting food falls anywhere from agonizing, to kind of a bummer, to somewhat enjoyable. If fasting makes you miserable, skip it – although it does get easier after you do it a few times.
- Fasting for more than 24-hours can lead to micronutrient deficiencies. For hardcore fasting experiments, check with your doctor about how to support micronutrient intake.
- Fasting may raise cortisol and other stress hormones, which raises blood sugar and triggers the release of insulin . In most healthy adults, this doesn’t seem to be a problem and regulates over time. But if you have blood sugar regulation or cortisol issues, fasting may not be for you.
Who shouldn’t fast?
- Women who are pregnant or breastfeeding.
- Children and adolescents.
- People with certain medical conditions or taking certain drugs that affect blood sugar levels, insulin response, or hormone regulation. Check with your doc!
- People with a history of eating disorders or with an already low body mass index might want to avoid dramatically restricting calories, even for the anti-aging benefits.
How to fast
Simply “not eating” may sound ridiculously simple, but you need only google “intermittent fasting” to find the mountains of information on just how and when you shouldn’t eat, and how to refuel after your fast. We cover some basic options here and encourage you to leave any questions in the comments.
Option #1: Skip a meal now and then.
Fasting can be as simple as skipping the occasional meal.
It might feel a little unpleasant or strange at first, but you won’t die if you ditch dinner or replace breakfast with some warm lemon water and a pinch of sea salt. This counts as fasting and, over time, can result in some of the same benefits as a longer fast, such as fat loss and a decrease in inflammation and insulin resistance. It’s also a good way to get familiar with how fasting feels and ease yourself into more intense fasting.
Option #2: Intermittent fasting.
IF could mean anything from a 13-16 hour fasting window to a 24-hour fast every 1-3 days (usually referred to as alternate day fasting (ADF)).
You could, for example, only eat between noon and 8PM – that’s 16 hours of fasting and 8 hours of eating (16:8). Or, if you’re a little more hardcore, you could only eat between 5PM and 9PM – a 20-hour fast with a 4-hour eating window (20:4).
There are a number of variations on IF, so you can play around with different iterations and stick with what works for you. IF (as opposed to longer fasts) is super popular because it’s been pretty well studied in humans and you get to eat more often. The promise of eating every day also increases compliance, so you’re more likely to stick with the program.
For more on intermittent fasting, we recommend checking out the following excellent resources:
Option #3: Meal timing (time-restricted eating)
There’s some evidence that your metabolism changes throughout the day, suggesting you can time your IF for maximum effectiveness. Many IFers fast for the first half of the day. It’s simply easier to prolong your fast after sleeping, rather than putting the kibosh on eating at 5-6pm – especially if you’re a night owl.
But humans are diurnal creatures, meaning our metabolic genes fire more during the day and slow down toward dark. So, it makes sense that eating the majority of your calories during the first half of your day can lower post-meal glucose levels, improve insulin sensitivity, and help you lose weight. Try both and see which one feels better for you .
Find the fasting option that’s right for you
Like all things nutrition-related, we don’t know nearly enough about any of this to say definitively what you should do. In fact, the more we learn about the human body, the clearer it is that nutritional needs are rooted in each individual’s health history and goals.
If one or a couple of these fasting techniques sound good to you and your doctor says it’s cool, we recommend testing a couple of fasting windows to see what feels good.
Some markers to track over the span of 1-3 weeks:
- Body mass index
- Mental clarity
- Energy level
Have you tried fasting? Are you currently doing it? Have you noticed the benefits? We’d love to hear your stories in the comments. Thanks for reading.