Intermittent fasting for beginners: A science-based guide

Intermittent fasting should be simple, but it’s complicated by endless noise. Try our 30-day fasting reset diet for effortless weight loss and eternal life. Look good at the turn of the 22nd century!

I wrote this article to cut through the nonsense. So you can decide how and why to pursue intermittent fasting — or if fasting even makes sense for you. Whatever your fasting experience, you’ll find something useful here. 

Why fast to begin with? Most people practice intermittent fasting for weight loss, but that’s not the only benefit. You might also see improved metabolic health, energy levels, mental sharpness, and sleep. You can start seeing these benefits with 12-hour overnight fasts, but there’s a lot of different ways to fast that we’ll walk through later in this article.

Short on time and just here for the quick tips? Here are five ways to improve your intermittent fasts:

1. Take electrolytes to prevent deficiency symptoms
2. Drink to thirst to maintain sodium levels and energy
3. Eat a nutrient-dense, protein-rich diet during your feeding windows
4. Try a low-carb diet to fat-adapt before transitioning to fasting
5. Don’t be a hero (fasting isn’t a competition)

If you have a minute, though, stick around. We’ll cover intermittent fasting benefits, types of fasts, how to choose a regimen, and practical strategies to make fasting easier. But first, let’s define what fasting actually is. 

What is Intermittent Fasting?

Fasting, broadly, is intentionally abstaining from food for any length of time. Intermittent fasting is a type of fasting that involves regularly abstaining from food for 12–36 hours.

It’s worth noting that not all fasts require zero calories. Some regimens allow for reduced calories on “fasting days,” and the science suggests these “modified fasts” have similar benefits to zero-calorie fasts. When over-eaters eat fewer calories, good things tend to happen. Most commonly, though, you’ll see fasters avoiding all calories.

Why is fasting beneficial? Because it signals your body to access its backup energy stores (body fat) when calories are scarce.  

This is an adaptation that’s millions of years old. Our ancestors often failed at hunting and gathering. They needed their body fat (worth tens of thousands of calories, even on a lean human) to last through famines. They stayed lean because they moved frequently and burned through fat stores when mammoth meat was hard to come by. 

Modern society promotes the opposite dynamic. With virtually limitless access to food, we can be eating constantly. This promotes fat storage rather than fat burning, because overeating (especially fat/carb combos) raises blood sugar, which raises the blood sugar-regulating hormone insulin.

Insulin keeps our blood sugar in check by shuttling glucose (sugar) out of our blood and into our cells — tucking excess calories away as fat for safekeeping in case of a famine. Persistently high insulin eventually leads cells to become less responsive to insulin’s signals. In other words, overeating + high insulin leads to insulin resistance, or the root of type 2 diabetes. Insulin-resistant folks put on fat especially easily. 

Intermittent fasting can help reverse this dynamic by reducing your calorie intake, therefore lowering insulin levels and reversing insulin resistance (check out this article for a deeper dive on blood sugar and insulin). In other words, fasting helps shift your metabolism back from insulin-resistant (storing fat) to insulin-sensitive (burning fat).

It’s also worth noting that depriving yourself of calories activates a cellular recycling program called autophagy, which is getting more buzz recently. Essentially, autophagy breaks down the old, damaged parts of your cells, allowing the body to recycle those proteins and lipids into something new. I won’t spend much time on autophagy here, but if you’re curious to learn more, I dig into it in this article. 

Back to it — let’s talk about the benefits brought on by fasting and the shift from insulin-resistant to insulin-sensitive. 

Benefits of Intermittent Fasting

Intermittent fasting can help with weight loss, metabolic health, energy levels, cognition, and your wake/sleep cycle. Let’s explore some science on these benefits. 

#1: Weight loss

All 27 trials in a 2020 systematic review found intermittent fasting (various protocols) led to weight loss in overweight or obese adults — 0.8–13% weight loss, depending on the trial. 

The why makes sense. When people compress their feeding window — the amount of hours in a day that they allow themselves to eat — they tend to eat less.

#2: Metabolic health

Fasting can help folks get off the insulin resistance train. How? Food breaks keep blood sugar and insulin levels down, leading to improved insulin function. Because of this, intermittent fasting is a promising therapy for type 2 diabetes and other metabolic issues. Low-carb and ketogenic diets can also help improve insulin resistance (fewer carbs = lower blood sugar = less insulin production). 

#3: More stable energy

Intermittent fasting improves your fat-burning capacity, decreasing your reliance on carbs for energy. Our bodies burn carbs for energy by default, causing our energy levels to rise and fall with our blood sugar levels (if you’ve ever experienced an afternoon crash where you crave candy, you’ll know what I’m talking about). Fasting removes carbs from the equation, prompting the body to run on fat instead. Fat is a very stable source of energy; the more you run on fat, the more stable your energy becomes.

#4: Mental sharpness

While fasting, your liver burns fat for energy AND produces molecules called ketones. Ketones then supply a portion of the brain’s ravenous energy requirements. A 2016 study found that higher ketone levels (from a high-fat meal) improved performance on various mental tasks in older adults. 

Some people report feeling sharper while fasting, but others don’t. Listen to your body (and brain). Rather than fasting, you can also consider reducing carb intake to boost your ketones. 

#5: Circadian rhythm

Your circadian rhythm (24-hour wake-sleep cycle) governs the quality of your sleep and the function of countless genes that impact your health. Two main factors regulate the circadian rhythm:

1. Light
2. Food

Bright light and food (especially protein) early in the day help wake you up. Avoiding blue light and food at night helps you wind down and produce your sleep hormone, melatonin. Fasting overnight — AKA avoiding those late-night snacks — is a simple way to tune this rhythm. 

Types of Intermittent Fasting

There are myriad ways to intermittently fast. I’ll outline a few of the popular ones here, but don’t be afraid to create a fasting schedule that works best for you. 

Popular Intermittent Fasting Protocols:

• 12/12. Also known as overnight fasting, 12/12 entails 12 hours of eating and 12 hours of calorie deprivation. Typically folks fast from dinner to breakfast without food, such as 7 PM to 7 AM. Most folks can handle a 12-hour fast. 
• 16/8. Popular among athletes and biohackers, 16/8 entails eating all your meals within an 8-hour feeding window (thus fasting 16 hours).
• OMAD. One meal a day (OMAD) involves consuming all your daily calories in one sitting. 
• 5:2. 5:2 entails five days of normal eating with two non-consecutive fasting days per week. Some folks will still consume up to 25% of their usual calories on fasting days.
• ADF. Alternate-day fasting (ADF) entails fasting every other day. Again, some folks will consume up to 25% of their usual calories on fasting days.

How to Choose a Fasting Protocol

If you’re new to fasting, consider starting with a 12-hour overnight fast. Most folks can do an overnight fast comfortably. Plus, it’s great for your circadian rhythm and helps you start adapting to burning fat for energy. 

If you want to extend your fast from there, try adding one hour of fasting to your protocol per week (13, 14, 15, etc.) and see how your body reacts. Jumping straight to longer fasts like OMAD or 5:2 can make folks quit early. For instance, a 2022 study found that compliance was poor among participants doing 5:2 fasting.

Longer fasts have other risks, too. The extra hunger can negatively impact sleep, for one. Also, the longer your fasts, the harder it is to get enough nutrients (like protein) from food. Keep this in mind when choosing how you want to fast. If you’re losing weight and feeling better with a 14-hour fast, that may be your optimal protocol. When it comes to fasting, more isn’t always better.

Along these lines, get clear on why you’re fasting. To lose 20 pounds? To stay lean and mentally sharp? To have more time for work?

If you want to lose weight, the longer fasts may get you there faster. But only if your body and schedule can tolerate them. If it’s no fun, there’s no shame in backing off.

If you want to gain or maintain muscle, shorter fasts are better. They give you more chances to consume sufficient protein and calories for muscle growth (assuming you’re strength training). Interestingly, a 2021 review found that various intermittent fasting protocols (16/8, OMAD, 5:2) combined with strength training did NOT lead to significant muscle loss in participants. To be clear, they still lost some muscle, just not a statistically significant amount, and the included studies were fairly short at 4–8 weeks — perhaps not long enough to show effects of long-term fasting. 

Bottom line? Start small, work up to longer fasts slowly (if at all), and listen to your body.

Groups That Should be Careful With Intermittent Fasting

Certain folks should generally avoid any fast longer than 12 or 13 hours. These groups include:

• Children
• Athletes (check out our article on fasted training)
• Underweight people
• Those with eating disorders
• Pregnant and nursing women
• People with diabetes

Though early evidence suggests fasting is therapeutic for type 2 diabetes, folks with diabetes shouldn’t fast without medical supervision (especially if they’re on blood sugar-lowering medications like insulin or metformin). Don’t wing it. Work with a clinician. 

Tips for Intermittent Fasting Success

The world of fasting is filled with bad advice and pseudo-wisdom. These five tips will help you cut through the clutter. 

#1: Take electrolytes

Fasting increases the risk of electrolyte deficiencies for two reasons:

1. Lower intake of electrolyte-rich foods
2. Higher urinary excretion of sodium and potassium

Using an electrolyte drink like LMNT can help keep electrolyte levels stable and prevent the headaches, muscle cramps, and tiredness that often accompany low electrolyte levels (and, therefore, fasting). Some flavors of LMNT have 1–2 grams of carbs, but that isn’t enough to meaningfully raise most folks’ blood sugar — therefore not breaking the fast.

Check out our sodium intake calculator for some estimations of how many electrolytes to consume while fasting.

#2: Drink to thirst

A typical nugget of pseudo-wisdom is to drink more water while fasting. Unfortunately, if you drink too much plain water, you’ll dilute blood sodium levels and circle back to those same low sodium symptoms (headaches, tiredness, etc). The solution is to add electrolytes to your water and drink to thirst. Thirst is a well-calibrated system to guide fluid intake. Lean on it. 

#3: When you eat, eat well

When you’re eating fewer meals, those meals should be as nutrient-dense as possible. Get plenty of protein, vitamins, minerals, fatty acids, and other essential compounds by eating meat, fish, eggs, and vegetables during feeding windows. Something like paleo or keto tends to work well.

If you’re struggling to digest your massive meals, lengthen your feeding window. Don’t force a fasting protocol if your body doesn’t like it. 

#4: Go keto first

Before progressing beyond an overnight fast, consider shifting to a low-carb diet. A low-carb diet keeps insulin low, helping you access body fat for energy. This “fat adaptation” makes fasting easier — or you may find that keto is working just fine for your goals and you don’t need to fast after all.

#5: Don’t get competitive

People are pretty gung ho about intermittent fasting these days. They get competitive with others and themselves.

But if a 16, 20, or 24-hour fast doesn’t feel right, don’t pressure yourself to power through. Maybe 12 or 13 hours is your sweet spot.

In the end, it’s your body, and how you feel moment to moment is the most critical data point. How’s your energy? How’s your mood? How are you sleeping? Are you feeling strong?

Intermittent fasting should support — not derail — these areas. If it’s not, consider retreating to a lighter regimen as you inch toward your health goals. That’s how you pursue intermittent fasting sustainably.