Intermittent Fasting and Alternate Day Fasting: Pros, Cons, and Guide

Fasting is a dieting trend with a recent surge in popularity.

There are several ways to go about fasting, each with their own unique variations – mostly related to durations of fasting/feeding periods.

What Is Intermittent Fasting?

What Is Every Other Day Fasting?

  • Alternate day fasting means that you would not eat after every other day of normal eating or after every two days of eating normally.

Each approach operates around the central tenet of reducing total calorie intake to produce weight loss.

Is one approach better than the other? Is fasting better than regular dieting with daily calorie restriction? Should I fast if I also want to workout? Can I eat/drink anything during the fast – like coffee?

Where do I even start? Let’s dive in!

How to do Intermittent Fasting

Also sometimes referred to as, “lean gains.” First things, first, how do you do it?

When to eat when fasting

To get started, think about how each of your days goes and figure out when would be the best times to NOT eat.

In the US in particular, we like to eat all the time, so don’t think about when you want to eat – focus on the not. For most people, they skip breakfast and eat in the early or late afternoon.

However, if you are someone who wakes up starving, there’s no firm rule that says you absolutely cannot eat breakfast. Ipso facto, if you want to pig out before bed, that’s an option as well.

The early to late afternoon is typical because most people aren’t super hungry in the morning and can make it through the first few hours pretty easily, then they don’t want to be hungry when trying to get to sleep, so the feeding period ends a few hours prior to sleep.

To start off, just pick a few days out of the week and “test” it out.

Then go into the second week with a full plan. During at least the first few days, use a wider feeding window, like 8 hours.

This way, it is really only like half of the day that you have to endure the fast since you are sleeping for ~8 hours as well. Gradually, work that down to 7 hours, to 6 hours, 5 hours, then finally 4 hours or even 3 hours.

Some people stick with the 8-hour feeding window, and that can work just fine as well. When most people talk about this, they refer to it as a ratio that adds up to 24, such as 16:8 for 16-hour fasts or 20:4 for 20-hour fasts.

During the feeding period, you can eat “as much as you want.” However, this isn’t a scientific fact.

In reality, if you pack in 10,000 Calories of ice cream in a 4 hour period, you’re probably going to gain weight.

However, also, in reality, eating 8 and a half containers of Ben and Jerrys in 4 hours (that’s 1 container every 30 minutes) is not something you probably want to do.

Well, at least not more than once. Okay, twice. Per week.

How does intermittent fasting work?

One of the ways IF works is by reducing the total amount of food intake.

Because you can only eat so much before getting full and losing the desire to eat, the net result is reduced total weekly calorie intake.

However, instead of traditional diets with moderate calorie reduction over the course of an entire day, and you feeling like you’re hungry all day, intermittent fasting keeps you down right stuffed for a few hours each day.

For many, this improves compliance because they can stick to the feeding windows, whereas “grazing” on a moderate continuous calorie-restricted diet effectively ruins the diet. More on the “how it works” later!

How to do Alternate Day Fasting

Alternate day fasting is similar to intermittent fasting, except that instead of fasting daily, only 3-4 days per week are fasting days.

Hold on – don’t get too excited about this yet! The fasting days are a little more extreme, and you can’t pig out on normal days. However, you may not even want to (see HOW section!).

Alternate day fasting is what it sounds like. You fast on alternating days, so to get started, you again want to consider what days would be best for fasting.

If your office does bagel Fridays, and you love bagels, don’t pick Friday. If you and your family like to go out to eat on Saturdays, don’t pick Saturdays. If your mother-in-law has terrible cooking and is always hosting big family dinners on Wednesday, pick Wednesday!

The fasting days are not always total fasts. They can be just very-low-calorie days with one meal.

On fasting days, calories are between 0 and 800 for the whole day. Early on in your process, go ahead and eat the meal for a little nutrition and satiety.

When you start getting comfortable with fasting, try the whole day! You may find it liberating to not need to worry about eating at any time during the day. Likewise, if you’re not anticipating that one coveted meal, your mind is free to do other, more enjoyable or more productive things.

On your non-fasting days, consume a normal amount of food. Don’t go overboard and pig out like this is a 24-hour feeding window on an intermittent fasting diet.

You can’t do a lot of damage in 4 hours, but a 24-hour all-you-can-eat buffet will definitely eliminate any benefits of the fast if you try to eat it all.

What Types of Food Should I Eat on an Intermittent Fasting Diet or Alternate Day Fasting Diet?

The fasting evangelists like to say, “eat whatever you want!” However, from a scientific and clinical perspective, that strategy is far from optimal.

"Whatever dietary strategy or nutritional approach you subscribed to prior to implementing specific diet patterns, such as fasting (or even the opposite – eating every 3-4 hours or 6 small meals per day), you will probably want to continue eating those sorts of foods during your intermittent fasting or alternate-day fasting diet."

It's difficult to make too many big changes at one time. That’s not saying you can’t – if you think you can, more power to you.

When we’re considering what is optimal, this is going to be something with an emphasis on protein. High-carb or low-carb is still debatable, and perhaps even a moot point if you’re fasting (have I mentioned the HOW section?), but you can use either approach.

Protein and intermittent fasting

Okay, here’s the deal with protein. Unless it becomes impossible – which it pretty much doesn’t unless you’re fasting all day – you should be eating at least 1.2 grams of protein per kilogram of body weight per day.

When it comes to weight change, the number one factor is total calorie intake; the number two factor is protein.

Higher protein diets consistently produce greater fat loss and with better muscle retention than low protein diets, ESPECIALLY when accompanied by calorie restriction.

Conversely, you cannot overeat on protein!

Consuming an extra 800 calories as protein in addition to maintenance level calories does not increase body weight or body fat. If fasting causes you to eat a low-protein diet, I hate to say it, you will probably lose weight, but you still won’t be happy with the way you look.

Why? If you lose muscle tissue at the same rate that you lose fat tissue, you maintain the same fat to muscle ratio. Thus, you’re left with the same proportion of fat sitting on top of your muscles and will still look fat – skinny fat if body weight is “normal.”

Think about the coveted abdominals. If fat is covering the abs, you can’t see them and don’t have a fit appearance.

Likewise, not having any abs to see in the first place gets you the same result. Even if you don’t want more muscle, you need protein to maintain the muscle that is already there!

One popular question about eating during a fast that is related. A little more detail on this in the HOW section, but what can you eat during a fast?

Technically, you’re not supposed to consume anything but water. A few things people get lenient on are BCAA/EAA supplements, black coffee, and ketone supplements or MCT/coconut oil.

Amino acids

Amino acid supplements all contain calories, but we (supplement companies) are not supposed to list them on supplement facts panels because they aren’t intact protein.

In fact, they contain the same amount of calories per gram as protein, which is 4. Technically, they do break a fast, but it’s pretty negligible in terms of weight loss. Discussed in the HOW section (are you sick of this yet?), this may be less negligible for other health or longevity benefits.


Coffee, MCT oil, and ketones all contain calories as well. Black coffee has ~10 calories, so it is quite negligible for all facets (coffee actually tends to improve long term health).

MCTs and ketones

MCTs/ketones “induce fasting” by increasing blood ketones, but this is artificial fasting. All of that being said, it’s not a big deal if you have a serving of BCAA, MCT, coffee, or ketones once during your fast.

Black coffee, I would even say you can drink liberally as your increase in metabolic rate from the caffeine will offset the calories coffee contains.

Diet drinks

Diet drinks are mostly okay as well. Some artificial sweeteners can create an insulin or glucose spike, but it is minor. They are sucralose, saccharin, and aspartame. Some sugar alcohols have the same effect, particularly maltitol and sorbitol.

And then there always seems to be this question that goes hand-in-hand with fasting…

Should I go Keto?

Here’s the fact of the matter. You likely will drift into ketosis just by fasting, so no, you don’t need to consume a ketogenic diet on feeding days/windows.

If your goals are strictly weight loss and maintaining your fasting diet, keto-adaptation can make the fasting periods seem easier because your body is already willing and able to use fat (body fat) as fuel.

This results in less intense and/or less rapid onset of “OMG I’m starving” sensations. Consuming proteins and fats, instead of carbohydrates, also help improve your satiety all by themselves, so it may encourage you to eat less without you realizing it – something called autoregulation. If you get full sooner, you eat less.

Is it a Bad Idea to Fast if I am an Athlete?

The quick answer here is yes, but that’s not always true. Fasting strategically can actually benefit athletic performance.

Of course, if you’re fasting heading into a race or competition, you’re going to suck.

If you’re fasting for a while after or before a training session, you may actually enhance the adaptations to exercise depending on the type of exercise and several other factors.

One simplified concept is that fasting before a training session makes the training session more difficult.

If we know anything about training, it needs to be tough in order to be effective (there are exceptions, but generally speaking). You will undoubtedly feel your training become more difficult if you don’t have the energy (calories) to perform the exercise.

Now, there is a careful balance to be had here because yes, you may enhance some adaptations like mitochondrial biogenesis, fat loss, or efficiency with fasting, but you are also limiting your total exercise volume and intensity.

In broad strokes, you maintain the balance by doing your high-intensity exercise (sprints, some weight training) while fed and your lower-intensity work (general running, cycling, cardio, etc. or dynamic effort weight lifting) while fasting (fed is fine as well here).

This is basically the concept of one of our other articles on carbohydrate periodization, but in that case, it is more of a carbohydrate fast than a total fast. This may be something you can work into a competitive season or reserve for the offseason, but it is a legitimate strategy.

If you’re someone who is active, but not competitive, these types of things are less important. You may still exercise, but there’s no need to fret over timing of exercise relative to when you eat.

Also, like any other diet, fasting diets are MUCH MORE EFFECTIVE when accompanied by an exercise routine. Caps lock warranted – it is a big difference. Don’t forget your resistance training if you’re shooting for optimal!

If you’re somewhere in between competitive athlete and casual exerciser, it’s probably worth it for you to try and time your exercise and fasting appropriately.

In other words, if you consider exercise in the top 3 of your lifestyle priorities, follow the general rules laid out 2 paragraphs ago. You will be bummed if your runs get more difficult or can’t quite do as many reps as you could before.

How Does Intermittent And Alternate Day Fasting Work?

Magic! Just kidding. The HOW section is here at last! We kind of already gave it away, but here is the big reveal!

For weight reduction and fat loss, fasting works by reducing total calorie intake.

Yup, same as regular dieting. Are you disappointed? Don’t be! There are a few other things to talk about here. Calorie reduction is just the main function.

Fasting is basically equal to normal dieting for weight loss when total calorie intake is controlled in a laboratory setting.

Similarly, alternate day fasting and intermittent fasting are equally effective in proportion to the degree of calorie restriction. However, there are a few benefits unique to fasting. One of which is control.

Fasting helps teach the dieter self-control and reduces cravings to overeat (1).

If you’ve ever tried a traditional diet it usually feels like what you’re supposed to eat is never enough, so even when you eat, you want to eat more.

Conversely, on intermittent fasting or alternate-day fasting, you’re hungry for a while, then full, then hungry, then full.

By “teaching your body” that it is fine to be hungry for a little while because food is on its way soon, it becomes easier to maintain your sanity. This isn’t the technical way that it works, but this is what you will likely experience and observe.

As for the technical aspects… Fasting can improve glucose and insulin homeostasis.

The fasting period, and the repetition of those periods over time, “teaches the body” to have better control of blood glucose levels (2). This may be weakly associated with body weight, but it has greater implications for overall health and longevity.

This may be due to mild keto-adaptation and the ability to utilize ketones and fats as a fuel source. In a glucose dominated metabolism – such as with a normal or high carb diet with continuous but low-calorie intake – there is little need to “fat-adapt.”

However, when body fat is virtually the only fuel available, there is an improvement of mitochondrial health, mitochondrial biogenesis, sirtuin activation, circadian rhythm, respiratory chain function, cyclic AMP activity, inflammation status, and other interesting mechanisms that ultimately result in increased longevity (3, 4, 5, 6).

This is one of the reasons some perceive it to be okay to consume ketones or MCTs.

This probably is not a big deal, but if you’re really trying to fast, don’t eat them – just let your natural ketone production do its thing, it will happen even if you eat carbs during your feeding periods (starvation ketosis with fasting vs. nutritional ketosis with MCTs/keto diets).

For athletes, and in particular, endurance athletes, fasting has its own unique set of potential upsides.

This may be near entirely attributed to loss of body fat (which was 11% greater in the fasting group), but one study found fasting to significantly improve exercise economy.

One interesting bit is this improved economy occurred at several intensities ranging from 50% VO2Max to 70% VO2Max and at threshold. Energy expenditure during exercise was reduced after the fasting period by over 10%, and heart rate was reduced by over 7 beats per minute.

This was accompanied by reduced blood lactate and athletes’ perceived exertion (7). Take this with a grain of salt, though, as this study had over 30% calorie restriction but no control group.

For other athletes, we don’t currently know of any benefits other than fat loss and possibly improved endurance. However, this may come at the cost of lost muscle mass – just like “normal” calorie restriction.

To offset some of the loss, BCAA may be supplemented during a fast to increase anabolic signaling to offset the highly upregulated catabolic signals from fasting.

This may help maintain more muscle, which will be beneficial in the long run, without adding many calories.

Should I Incorporate Fasting into My Diet?

Is it a diet or just not eating?

  • In most cases, fasting can’t hurt. If you start slow, it’s not all that difficult. The difficulty of resisting food during the day is one of just a few cons to fasting, and it gets easy fairly quickly.
  • The second and only other drawback is the potential to compromise increasing lean muscle tissue. When energy status is low, the body doesn’t form new muscle tissues and may actually break some existing muscle tissue down. For the majority of endurance athletes, this is not a big deal.

However, for many, the pros far outweigh the cons.

Fasting, despite how hard it may sound if you’ve never tried it, ends up being more enjoyable than traditional dieting.

One of the biggest issues with weight loss diets is compliance – most give up early or cheat too much to be successful. With the defined parameters and simple rules of fasting, it’s easier to find success.

In addition, the extended fasting periods may confer some long-term benefits to health and lifespan.

For some, and under the right conditions, performance can be enhanced as well. If you’ve been thinking about giving it a shot, it’s probably best to just go ahead and give it a go!


  1. Bhutani, S., Klempel, M. C., Kroeger, C. M., Aggour, E., Calvo, Y., Trepanowski, J. F., ... & Varady, K. A. (2013). Effect of exercising while fasting on eating behaviors and food intake. Journal of the International Society of Sports Nutrition, 10(1), 50.
  2. Anson, R. M., Guo, Z., de Cabo, R., Iyun, T., Rios, M., Hagepanos, A., ... & Mattson, M. P. (2003). Intermittent fasting dissociates beneficial effects of dietary restriction on glucose metabolism and neuronal resistance to injury from calorie intake. Proceedings of the National Academy of Sciences, 100(10), 6216-6220.
  3. Chausse, B., Vieira-Lara, M. A., Sanchez, A. B., Medeiros, M. H., & Kowaltowski, A. J. (2015). Intermittent fasting results in tissue-specific changes in bioenergetics and redox state. PLoS One, 10(3), e0120413.
  4. Nisoli, E., Tonello, C., Cardile, A., Cozzi, V., Bracale, R., Tedesco, L., ... & Moncada, S. (2005). Calorie restriction promotes mitochondrial biogenesis by inducing the expression of eNOS. Science, 310(5746), 314-317.
  5. Zhu, Y., Yan, Y., Gius, D. R., & Vassilopoulos, A. (2013). Metabolic regulation of Sirtuins upon fasting and the implication for cancer. Current opinion in oncology, 25(6), 630.
  6. Longo, V. D., & Panda, S. (2016). Fasting, circadian rhythms, and time-restricted feeding in healthy lifespan. Cell metabolism, 23(6), 1048-1059.
  7. Pons, V., Riera, J., Capó, X., Martorell, M., Sureda, A., Tur, J. A., ... & Pons, A. (2018). Calorie restriction regime enhances physical performance of trained athletes. Journal of the International Society of Sports Nutrition, 15(1), 12.