Intermittent fasting — which alternates fasting periods with an eating window — is a wonderfully flexible principle that you can customize for your needs, wants, and lifestyle. But if you’re interested in intermittent fasting, you’ve probably felt like there are so many options that it’s hard to decide which schedule to choose. 

Considering your starting body mass index, or BMI, can be one way to help you decide. In this article, we’ll explore how intermittent fasting based on BMI can play a role in your planning and how to pick the best schedule for your unique body. 

What is BMI?

BMI is a number based on your weight and height.[1] It’s calculated by dividing a person’s weight in kilograms (or pounds) by the square of their height in meters (or feet). 

• A higher BMI means that someone is heavier relative to their height.
• A lower BMI means that someone is lighter relative to their height.

BMI was originally developed between 1830 and 1850, based on the idea that body weight and how much body fat a person had could influence their health and mortality (i.e., how early they die). 

In 1959, the Metropolitan Life Insurance Company published tables of average body weights for different heights and ages. These tables were used to look at whole populations of hundreds of thousands of people. 

They categorized people as “underweight” or “overweight” if their weight-height ratio was below or above the average.

Current BMI categories

Now, BMI categories for all adults 20 and older include:

BMI	Weight status
Below 18.5	Underweight
18.5 – 24.9	Healthy weight
25.0 – 29.9	Overweight
30.0 and above	Obesity

(BMI is only used for individuals over the age of 18 years old. Other tools are used to assess growth for children and adolescents.)

For instance, for an adult who is 5’9″:

Height	Weight range	BMI	Weight status
5′9″	124 lb or less	Below 18.5	Underweight
	125 lb to 168 lb	18.5 to 24.9	Healthy weight
	169 lb to 202 lb	25.0 to 29.9	Overweight
	203 lb or more	30 or higher	Obesity

Why use BMI?

BMI is now used as an easy screening tool because it only uses height and weight. And, at the extremes — if a person is very light or very heavy — it can tell us about particular health risks.

For example, being extremely light, especially as one gets older, is linked to frailty and a higher risk of falling. Being extremely heavy is linked to joint problems, especially in the lower body.

However, BMI doesn’t measure many other important aspects of health that are often more predictive of outcomes, such as:

• body fat
• muscle mass 
• metabolism and metabolic health factors like blood pressure, blood sugar, insulin sensitivity, and diabetes
• lifestyle habits such as good nutrition, physical activity, sleep, etc.
• emotional well-being and how a person feels, such as their energy levels

BMI and body composition

Because BMI only looks at height and weight, it can’t tell us about body composition — what that body weight is made of.

A high BMI can indicate high body fat, especially at much higher ends of the BMI scale.[2] 

But it can also indicate a person has a lot of muscle mass and/or bone density. For instance, many larger athletes in sports like football, rugby, or weightlifting tend to have higher BMIs.[3]

And women will generally have more body fat than men at all BMIs.[4]

To get a more accurate idea of body composition, we need to use other methods of measurement, like:[5]

• skinfold measurements with calipers
• bioelectrical impedance
• underwater weighing
• dual-energy x-ray absorptiometry (DEXA)

We can also use simple methods for guessing, like combining height, weight, waist circumference, and neck circumference. For instance, waist circumference (WC) and BMI are both correlated with body fat percentage.[6] 

How does BMI impact weight loss?

BMI can affect how quickly a person loses weight and how much weight they can safely lose.[7]

Since a person with a very high BMI likely has more body fat, they may lose weight quickly when they first start changing their habits. A person with a low BMI who simply wants to lose a little weight or a little body fat will likely notice a much slower change.

Best intermittent fasting schedules according to BMI

BMI can affect your intermittent fasting schedule. But should you do intermittent fasting based on body mass index?

While there are many types of intermittent fasting, the one we recommend at Simple is time-restricted eating, or TRE. This alternates periods of fasting that last several hours with eating windows. 

• Higher BMI: In general, if you have more weight and/or body fat to lose, a longer fasting window combined with a shorter eating window works better. Although longer fasts (such as 16:8) are correlated with better weight loss,[8] it’s more important to pick a schedule that suits your lifestyle. Avoid fasting for periods beyond 18 hours, though, as there is limited research to show any additional benefits. 
• Lower BMI: Conversely, if you have less weight and/or body fat to lose, you might find it better to use a shorter fasting window. This is also true of intermittent fasting for beginners. Importantly, we don’t recommend fasting for anyone with a BMI below 18.

We’ve suggested an intermittent fasting schedule BMI chart below. Plus, check out our article on what to eat during intermittent fasting to learn more about supporting yourself with good nutrition, whatever you choose. 

Again, remember that the most important factor is finding a way of eating (and fasting) that suits your lifestyle. So please note: these are only recommendations.

Intermittent fasting by BMI chart

Starting BMI	Our recommendation
18.5 or under	We don’t recommend intermittent fasting.
18.5 to 24.9	You can try intermittent fasting for health benefits or to learn about hunger and mindful eating.	Explore a shorter fasting window, such as 12:12 or 14:10.
25 and over	Intermittent fasting can help with weight loss, especially if combined with calorie restriction.[9]	Try a longer fasting window, such as 16:8 or 18:6.

Not sure which intermittent fasting schedule to choose? We’ve got you covered! Take our Simple quiz, and we can help you figure out the best schedule for your unique body, preferences, and lifestyle. Take the first step to understanding your body type and get personalized insights now! 

How does intermittent fasting affect body mass index?

It can be helpful to consider the pros and cons of intermittent fasting based on body mass index.

Benefits of intermittent fasting for BMI

Intermittent fasting has several health benefits, as well as features that can support weight loss. (See here for more.)

Weight loss: Intermittent fasting can affect BMI by lowering body weight. Fasting usually means eating less overall, which can help people lose weight without feeling deprived or having to “go on a diet.”[10] Longer periods of fasting and shorter eating windows (such as 16:8) seem to help the most for folks with higher BMIs.[8]

Metabolic health: Intermittent fasting improves many indicators of metabolic health for people of all BMIs, but especially people with higher BMIs. These indicators can include:[5–7]

• insulin sensitivity, or how responsive your cells are to insulin — which means better blood sugar management and lowering the risk of type 2 diabetes [11]
• markers of heart health, such as blood pressure or better cholesterol levels [12]
• loss of body fat around your internal organs in your abdominal cavity: intermittent fasting may help you lose this more dangerous belly fat, as well as body fat in other areas

Mindful eating and hunger awareness: Many people find they’re more aware of their hunger cues when they fast,[10] which helps them eat more intuitively (a lovely relief for anyone who’s struggled with their relationship with food). This is important for people of any BMI, as many of us have lost touch with our hunger and fullness cues.

If all of these benefits have you eager to get started, why not head over to our Simple quiz? You’ll be able to set your fasting schedule exactly how you want and get some practical ideas to support you as you start your wellness journey.

Drawbacks of intermittent fasting by BMI

Intermittent fasting is generally safe for beginners and experienced fasters alike. While intermittent fasting does have a few side effects, most healthy adults will do fine with fasting, no matter what their body type. This is especially true if they’re careful to avoid some common intermittent fasting mistakes.

That said, it’s also wise to consider any possible disadvantages of intermittent fasting according to BMI.

High BMI: You may need more than just intermittent fasting to lower your BMI and meet your weight loss goals. 

While people with higher BMIs may be a natural fit for intermittent fasting, it’s also important to consider other factors that may be contributing to high BMIs, such as nutritional choices, physical activity, sleep, and stress. 

(Ask our assistant Avo for ideas about what else you can do to support your journey!)

Low BMI: Intermittent fasting may not be for you.

We don’t recommend intermittent fasting for anyone with a BMI below 18.5. 

And, if you’re on the low end (say, a BMI of 19–20), you might find that more occasional intermittent fasting or fasting with shorter windows (say, 12:12 or 14:10) could be a better option to get the potential health benefits — such as possible autophagy, or cellular cleanup — without any negative impact. (Note that while animal studies show autophagy during fasting, it’s harder to measure in humans.) 

Make sure to speak with a healthcare provider to find out if fasting is right for you.

Does exercise have an impact on your BMI?

Definitely! 

Exercise can help lower BMI by creating an energy demand that helps your body burn fat. Plus, exercise can support weight loss, trim body fat, improve insulin sensitivity, and lower blood pressure.[13]

That said, exercise is important not just for minimizing muscle loss during a weight loss journey,[14] but it may also maintain BMI if you’re working hard to build muscle mass and bone density, especially if you’re starting at a relatively lower BMI.[15]

Of course, your workouts should fit your fitness level and health. People with high BMIs will have more loading on their joints,[16] so talk to a personal trainer and/or physical therapist if you’re not sure what exercises are best for you. To get started, here are our tips on intermittent fasting and working out.

Pro tip: Make sure you stay hydrated while exercising and fasting! Learn more about what to drink while intermittent fasting.

Simple’s tips on practicing intermittent fasting based on BMI 

Set realistic goals. Look for steady and sustainable progress, especially if you have a high BMI. Focus on losing weight at a rate of 1 to 2 pounds per week for a healthy and sustainable journey.[17] (And if your BMI is lower, you might find weight loss is slower … but again, you’re shooting for safe, sane, and sustainable, so that’s OK!)

Focus on changing lifestyle habits. If you’ve had a high BMI for a long time, you might have daily behaviors that could use some adjustment. So, rather than just focusing on losing weight, focus on improving your health and self-care behaviors. 

A small change that you can do consistently almost every day is better than nothing (or a big change you can only do for a few days and then give up). And you’ll start to notice those small changes add up in the long run.

Think quality over quantity. In other words, your body composition — the ratio of body fat to lean mass — is a much greater predictor of health, fitness, and function than just BMI alone.[18] Regardless of the number on the scale, focus on the healthy and fit person you’re becoming every day.

Explore longer fasting windows if your BMI is higher. If weight loss is your goal, play with slightly longer fasts and see how you feel. Remember to keep your fasting window below 18 hours, as there’s limited research available showing any added benefits in going beyond this timeframe.

Don’t fast too much if your BMI is lower. More is not always better. Enjoy fasting for the health benefits, learning about your hunger and fullness cues, and discovering how to eat more mindfully. But keep your fasting moderate. 

Speak with your healthcare provider if you experience any disordered behaviors around eating and fasting.