Intermittent Fasting vs. The Keto Diet

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Written by Brenda Peralta, Registered Dietitian and medically reviewed by Abby Courtenay

Imge of Intermittent Fasting vs. The Keto Diet



Intermittent fasting and the keto diet are two of the most common strategies for achieving weight loss. You can find a lot of information online by looking up each separate diet, but what if you want to compare them side by side?

Let's talk about fasting and keto. What are the similarities and differences between the two? What are the benefits of each one? What are the side effects? What foods can you eat in either diet? Is there a cost difference? Which one is the healthiest?

By the end of the article, you will determine whether to do a fast, a ketogenic diet, or maybe both.


What is intermittent fasting?

Intermittent fasting (IF) is an eating pattern that limits when you can have meals or snacks. There is a specific window of time in which you are allowed to eat, and the rest of the time you are fasting (not eating). 

It doesn't tell you what foods to eat, but when you should be eating. Thus, a fast is not a diet but more of a way of eating.

So how does it work? There are different versions of intermittent fasting. You can do it for 12, 16, 18, or even 24 hours. The number is the amount of time you are going to fast. For example, you don’t eat anything for 12 hours.

The time you are sleeping counts as fasting, so at least 8 of those hours are easy to do!

There are also several ways to approach it. Some people fast for the same time each day. Others alternate their fasting times  or choose to eat very little (less than 500 kcal/day) 2-3 times a week with no restrictions on the other days. Others switch it up. They might do a 12-hour fast three times a week, a 16-hour fast twice a week, and a 24-hour fast twice per week.

How to get started

First, determine how many hours you are going to fast. You can start slowly. Try doing a 12-hour fast, and from there, you can begin adding minutes until you reach your desired goal.

You can drink water or zero-calorie beverages during your fast, like coffee or tea (without milk). Remember to stay hydrated and eat when you are hungry.

Can you do keto and intermittent fasting at the same time?

Yes. In fact, it’s common for people to do keto and intermittent fasting simultaneously.

When you fast, you can eat any type of food you want during non-fasting hours. This means you can combine it with all sorts of diets.

Intermittent fasting caloric intake

Do you need to measure the calories in intermittent fasting?

If your main goal is weight loss, yes, you need to count your daily caloric intake.

Weight loss is achieved when you eat fewer calories than your body needs. Having smaller eating windows can automatically reduce the number of calories you ingest. 

However, if you are not careful and add too many energy-dense foods, you might still gain weight even if you fast.


Similarities between intermittent fasting and keto

Keto or intermittent fasting for weight loss

Both have great results in weight loss. Either strategy relies on having a caloric deficit (eating fewer calories than your body needs). There is a 2-3 pound weight loss per week for both strategies, on average.

Having a weight loss of more than 1% of weight per week is not recommended since it could lead to weight loss in muscle and not fat mass.

Fasting or keto for Alzheimer's

There is compelling evidence on both diets and the prevention of Alzheimer's disease. However, the studies for intermittent fasting have been primarily done in rats. It seems that having periods where you fast can delay the appearance of Alzheimer's.

Also, it seems that having a diet high in carbs can potentially lead to Alzheimer's. Thus, those following a low-carb diet may have a prevention effect.

Keto or intermittent fasting for building muscle

For muscle building, these ways of eating may not be the best options

For you to gain muscle, in addition to resistance exercise, you need a caloric surplus. This means eating more calories than your body needs. Although it could be potentially done with fasting or ketogenic eating, it might be harder.

In a ketogenic diet, although fats are energy-dense, they are very filling. Thus, reaching, for example, 3,000 kcal each day on keto might be challenging to achieve.

Also, having a shorter time window for food means that you need to have more calorie-dense dishes. For example, if you have a 16-hour fast, this means that you might only have three meals a day during your 8-hour window. Consuming 3,000 kcal in 3 meal times means having 1,000 kcal per meal. This can be tough to achieve.

Fasting vs. low-carb for insulin resistance

Both can have significant results in reducing insulin resistance. For people with type 2 diabetes or PCOS, these eating styles can potentially help you achieve better bloodwork.

Differences between fasting and keto

Although there are some similarities between them, there are also some big differences.

Food choices

The most significant difference between both is the food selection. The ketogenic diet restricts the carb intake up to 5-10% of the total daily calories. This means that foods high in sugar, pastries, cakes, and sweet beverages are often off-limits.

On the other hand, you can eat any food you want when doing a fast as long as you stay within your eating time window. Carbs can be kept high or low. It depends on what your preferences are.

Time frame of eating

The other major difference is the eating window. The keto diet allows for eating at any time during the day as long as you stay within your daily carb budget. On the other hand, having a fast restricts when you can eat.

IF vs. keto on ketone levels

In both cases, ketones are produced due to the lack of carbs in the system. As a counter measurement, the body produces ketones to supply energy when there is no glucose available.

Although they both produce high levels of ketones, which results in weight loss, studies have shown that a fast can raises ketone levels more than following a keto diet.

Low-carb or intermittent fasting for diabetes

Both strategies might work with someone with type 2 diabetes. However, the American Diabetes Association doesn't recommend fasting as an option for someone who wants to control their sugar levels since it could increase the chances of having hypoglycemia (low blood sugar), especially if you are taking insulin.


Benefits of intermittent fasting and low-carb diets

Following a low-carb plan has some of the same benefits as fasting. The benefits of IF and keto are:

  • Weight loss. Both are an excellent choice when looking to shed some pounds.
  • Insulin resistance. They can increase your body's sensitivity to insulin, meaning it responds to it more easily.
  • Inflammation. Both strategies are great for potentially reducing inflammation in the body.
  • Brain health. Both may help to reduce the risk of having brain disorders like Alzheimer's.

One of the most significant advantages of following intermittent fasting over keto is the freedom of food choices. Since a fast doesn't limit any food, it might be better for social activities (as long as they take place within your time window). On the other hand, a ketogenic diet might be more limiting when going out.

Side effects of intermittent fasting and keto

Drawbacks of keto

Here is a list of the major side effects of a ketogenic diet.

  • Keto flu. You might get nausea, fatigue, foggy brain, and headaches at the beginning of the diet.
  • Decreased performance.
  • Constipation.
  • More difficulty in social gatherings.
  • Nutrient deficiency.

Negatives of following IF

The biggest side effect of following a fast is that if you are not careful, you might go overboard with energy-dense foods that could still potentially lead to weight loss. Fasting doesn't mean eating unlimited quantities of food. You still need to control your caloric intake.


Compare intermittent fasting with keto foods

Although having a fast and doing keto can have very similar goals, they differ in the food choices they permit. While keto has some major food restrictions, fasting has only a time frame restriction. Thus, the food requirements are very different.

Beans and legumes In moderation Allowed
Condiments and sauces Allowed (low-carb) Allowed
Dairy Allowed (low-carb) Allowed
Sugar-free drinks Allowed Allowed
Sugar-sweetened beverages Not allowed Allowed
Alcoholic drinks Allowed (low-carb) Allowed
Eggs Allowed Allowed
Fish and seafood Allowed Allowed
Fruits Allowed (moderation) Allowed
Grains and starches Allowed (moderation) Allowed
Herbs and spices Allowed Allowed
Meat and poultry Allowed Allowed
Nuts and seeds Allowed Allowed
Oils and fats Allowed Allowed
Processed foods Not allowed Allowed
Sugar-free sweeteners Allowed Allowed
Natural sweeteners

Allowed (low-carb)

White and brown sugars Not allowed Allowed
Starchy vegetables Allowed (moderation) Allowed
Non-starchy vegetables Allowed Allowed


This table summarizes the foods that you can eat, whether you are following keto or intermittent fasting. Although there are major differences, remember that you can adjust which foods you can add when fasting. Thus, you can stick to foods found in a ketogenic diet if you choose.

Keto vs. IF costs

Both depend on the food choices that you make. For example, if you choose proteins that are costly and fats that are difficult to find, following a keto diet might be expensive. On the other hand, there are plenty of cheap low-carb foods. The same applies to intermittent fasting. If you want to have cheaper foods, you’ll need to choose them.

However, when eating out, intermittent fasting is cheaper than keto since you have more options to choose from. Strictly ketogenic options might be more expensive.


Is intermittent fasting better than keto?

Either one is a great option when looking to lose weight. However, since fasting is less limiting, it can be healthier than keto. Having more variety in grains, veggies, and fruits might provide you with healthier nutrition.

But it all depends on what you decide to eat! If you load up on unhealthy, ultra-processed foods when you’re not fasting, clean keto is healthier since it encourages you to eat whole, natural ingredients.


The takeaway

Intermittent fasting might be easier for someone who constantly eats out or has social gatherings. However, if you don't control your portion sizes, you can gain weight by overindulging after your fast and eating more calories than your body needs.

Keto is a great option for those who want to lose weight, and combining it with fasting can help you achieve a ketosis state faster.