When someone asks if intermittent fasting works, they’re usually asking if it works for weight loss. It’s the primary reason people fast.
Fasting for weight loss makes intuitive sense. If you cut out the nightly bowl of cashews, you consume fewer calories overall.
Is the formula for weight loss really that simple? For many, yes, it is.
But that doesn’t mean there’s nothing left to learn on the topic. There’s still nuance to losing weight successfully while intermittent fasting.
For instance, did you know that a certain electrolyte mitigates the weight rebound after a fast? We’ll cover that later. Right now, let’s do a quick overview of intermittent fasting.
Intermittent Fasting 101
Intermittent fasting is popular now, but it was also popular back in prehistoric times. Back then, nobody had a fridge, pantry, or a local Whole Foods.
You ate what you could find or kill. And when you couldn’t find or kill anything, you fasted.
A little physiology now. During a fast, our bodies are fueled by stored fat. This body fat is an abundant energy source that we all possess. These reserves provide enough energy to fuel a primitive hunt-and-gather session or, perhaps more realistically, a day at the office.
But modern society doesn’t help us access this fat. Just look at how many aisles in the supermarket are devoted to snacks.
Constant feeding prevents fat burning. Combine this perpetual need to feed with mass inactivity and high sugar consumption, and it's no wonder the Western world is facing an obesity crisis.
Intermittent fasting—the regular practice of caloric reduction—represents a break in this habitual and unhealthy pattern.[*]
Depending on the protocol, intermittent fasting can be anything from:
- Going from dinner to breakfast without snacking (12/12)
- Consuming all your daily calories in an 8-hour window (16/8)
- Eating one meal a day (OMAD)
- Eating 500 calories two days per week (5:2)
- Fasting on alternate days (ADF)
We will discuss picking your perfect protocol later, but now, let’s move on to weight loss.
How Intermittent Fasting Works for Weight Loss
When you take regular breaks from food, you will most likely set yourself up for weight loss. Here’s how:
Fewer calories in
It’s no big mystery why fasting can result in weight loss. People consume less energy than they use.
The fancy term for this concept is negative energy balance. When someone is in negative energy balance, the energy coming in through food is less than the energy expended through resting metabolic rate, exercise, and other activities.[*]
And when less energy comes in than goes out, weight loss often follows.
But wait, doesn’t calorie restriction slow your metabolism? Doesn’t your body adjust to the lower caloric load?
It may depend on the length of the calorie restriction protocol. Longer bouts of calorie restriction (like this six month bout[*]) appear to slow the metabolism, but shorter protocols (up to 28 days) of both IF and calorie restriction don’t appear to have this effect.[*][*][*]
Fat-burning, ketosis, and appetite
Recall that fasting gets you burning fat for energy. This happens because fasting keeps the hormone insulin low, stimulating the breakdown and release of stored body fat—a process called lipolysis.[*]
To be clear, keeping insulin low doesn’t guarantee you’ll lose body fat. You can keep insulin low on a Keto diet but still overeat and gain weight.
Yet, it’s harder to overeat when you’re running on fat for energy. Why? Because burning fat helps reduce hunger and cravings.
Specifically, being in ketosis (the result of burning fat) has been shown to reduce hunger hormones like ghrelin and neuropeptide Y.[*] And a reduced appetite is useful if you’re trying to lose weight.
Intermittent Fasting Weight Loss Studies
The theory is great, but does clinical data support intermittent fasting as effective for weight loss?
It does. Consider the following examples from the published literature:
- A 2018 review published in the journal Cureus found intermittent fasting to be effective for weight loss, even in normal-weight people. The primary intervention reviewed was alternate-day fasting.[*]
- Overweight women lost over 14 pounds after six months of 5:2 intermittent fasting. They also showed improvements in heart disease risk factors.[*]
- A recent review summarized multiple studies that showed time-restricted feeding—eating in a compressed daily window—promotes weight loss in animals and humans.[*]
- In one study from the above review, people with metabolic syndrome lost an average of 7.3 pounds after adopting a 10-hour feeding window for 12 weeks.[*]
Tips for Weight Loss While Fasting
If you’re thinking about fasting to lose weight, mind these tips:
#1: Consider a Keto diet
Keto and intermittent fasting work by similar mechanisms. Both restrict carbs. Both keep insulin low. And both get you running on fat for energy.
In other words, going Keto helps you fat-adapt, allowing you to access body fat during a fast more easily. Being in ketosis may also help to reduce hunger making fasting periods more tolerable.
#2: Consume salt
Much of the initial weight loss during a fast is water weight. This water loss is the result of your body chewing through stored glucose (glycogen) as you transition to ketosis.
When you refeed, some of this water weight will come back. That’s inevitable.
But to minimize this weight regain, take sodium during your fast. Sodium is a major regulator of fluid balance, helping control where water goes in your body.[*]
In one study from the 1960s, sodium supplementation dampened the weight rebound to 39% of what was lost after a 10-day fast.[*]
The recommended daily intake for sodium is no more than 2,300mg.
Be sure to opt for healthy and natural sources of sodium such as sea salt or pink Himalayan salt. Bone broth can also be an excellent source of sodium and a great way to replenish electrolytes when fasting.
#3: Eat during the day
Practicing time-restricted feeding means eating all of your calories in a compressed daily window. To optimize for weight loss and general health, it pays to consume these calories while the sun is shining.
Why? Because along with light, food helps regulate your circadian rhythm, and this 24-hour wake-sleep cycle influences almost every aspect of human health, including weight regulation and fat-burning capacity.[*]
Plus, fasting overnight can help to promote restful sleep, which in turn may reduce hunger.[*]
#4: Pick your protocol
It’s likely true that longer fasts will promote more rapid weight loss than shorter fasts. But this doesn’t mean you should skip the overnight fast and go straight to OMAD or alternate-day fasting (ADF).
Start with smaller fasts of 12, 13, or 14 hours. Develop the habit, then work your way up according to your comfort, desire, and schedule.
Use our intermittent fasting feature to help determine your ideal protocol.
#5: Plan your meals
Planning your meals effectively around your IF schedule can not only help you to stay on track but can also boost the efforts of your fast.
Plan nutrient-dense meals to break your fast, rich in high-quality proteins, healthy fats and low carb vegetables.
Sustainable weight loss starts with sustainable habits. Maintaining these practices once you have shed the pounds will go a long way in helping you manage your weight successfully in the long term. The Carb Manager app can be an invaluable tool in helping you stay accountable and reinforce these healthy habits. By tracking your fasting routine and weight loss all in one place, you can quickly and easily review your progress and celebrate your successes.
Please note that as with any significant diet or lifestyle change, we recommend working alongside a registered health professional, especially if you are currently on any prescribed medications. Fasting is not appropriate for everyone and should be avoided by those with a history of eating disorders, pregnant and breastfeeding women, and anyone under 18.