A few months back, I received a question about how fasting affects climbing. I promised to write an article on the topic, but unfortunately it’s taken awhile to get to! Now that I’ve finished an entire series on weight loss, though, it seems like an ideal time.

When we’re talking about diets, “fasting” usually means intermittent fasting, or not eating for certain periods of time daily or sometimes for entire days weekly. People use intermittent fasting for a number of different reasons, but these reasons can generally be summarized to weight loss, health, and performance. There are also people who just happen to follow a dietary pattern very similar to intermittent fasting, but for no reason apart from that’s how they typically eat; we’re not really talking about them, they just do exist.

As with any popular diet, there’s a lot to unpack about intermittent fasting, and that’s where we’ll begin this article: the science and pseudoscience surrounding it.

The Science & Pseudoscience of Intermittent Fasting

The first place to begin is with the biggest claim about intermittent fasting, which is that it’ll improve longevity. To be fair, this claim is more related to caloric restriction than fasting itself, but I see intermittent fasting get rolled up in the same bundle frequently.

In numerous organisms—from less complex animals like worms to mammals like mice and even monkeys—caloric restriction has been demonstrated to improve lifespan. There are a lot of proposed mechanisms for why this may be, including specific gene triggers and broader concepts like hormesis (which is best described as “a little bit of something bad is often good”), but regardless of the reason, we’re now also attempting to demonstrate the same link in humans.

On the headline level, it’s clear why people are excited about the promise of caloric restriction leading to a longer life, because who wouldn’t want to live longer (and presumably healthier; most people aren’t looking to be a modern era Tithonus)? When you dig into what “caloric restriction” actually means, though, it becomes a lot more mundane, and perhaps simply disappointing.

Scientifically, “caloric restriction” or “dietary restriction” tends not to mean a low calorie intake, but rather exactly what the words imply: restricted calories. In animal trials, this means that the “restricted calorie” group is not fed ad libitum, or given free access to food at all times. Instead, it means the scientists performing the study limit their calories, but almost always only to the amount that the animals maintain a healthy weight for their species and sex.

You can see this definition in action in a study on Rhesus monkeys widely cited by proponents of caloric restriction. One group of monkeys was essentially given a never-ending bowl of monkey chow whereas the other group had calories “restricted” to a normal amount for an adult Rhesus monkey.

What this means is that what the studies define as “caloric restriction” doesn’t match up with what you, or I, or any other person would think of caloric restriction, which is the purposeful restriction of calories to below a normal level. Indeed, in the studies that have demonstrated caloric restriction to be healthy, it would be more accurate to say that eating a “weight maintenance diet” is healthy whereas eating too many calories and gaining weight isn’t. This isn’t exactly a revelation!

In the aforementioned Rhesus monkey study, the monkeys who were “calorically restricted” had lived to an average age of about 30 at the time of publication (they can live to about 38 typically when taken good care of in captivity) whereas the ad libitum monkeys lived to only about 25 years. The increased mortality in the ad libitum group was due to an increased incidence of chronic disease, including “cardiovascular disease, neoplasm, diabetes, endometriosis, and kidney failure”. Eleven monkeys in the ad libitum group suffered cardiac events leading to death, while no monkeys in the restricted group did.

In other words, the monkeys who ate as much as they wanted…

Gained excess body weight.

Became more susceptible to “lifestyle diseases” such as cardiovascular disease and diabetes.

Died at a younger age due to those diseases.

On the other hand, monkeys who ate a calorie restricted diet—a diet with only as many calories as a monkey needs to maintain weight—were mostly still alive at the time of publication and very likely lived to or near to their expected lifespan of 38. They had little incidence of the same diseases as found in the ad libitum group, and since they did not suffer from those diseases, they did not die from them.

So, “caloric restriction” does increase longevity, but only when compared to a diet that leads to weight gain. And, it only increases longevity towards a given animal’s “normal”, but not beyond. Thus, the big conclusion we can take from these studies, at least right now, is that you’ll probably live longer if you eat a normal amount of calories—although even that isn’t a given since we humans, unlike Rhesus monkeys, have access to health interventions like surgeries and drugs that can keep a person with one of the aforementioned chronic diseases alive well past what would have been “normal” for such a disease a hundred years ago.

There are still unanswered questions here, so the book isn’t quite closed on caloric restriction, fasting, and longevity. For example, does it matter if you consume more or less calories so long as you’re maintaining weight? Our metabolism isn’t rigid, it’s somewhat flexible and accommodates a range of caloric intakes while maintaining weight. Also, is there any metabolic advantage inherent in consuming maintenance calories, or does 100% of the benefit derive from a lower incidence of chronic disease? What about gaining weight in the form of muscle—does that have the same effect, or a beneficial one?

These questions have not been explicitly answered, but from other studies I think we have enough correlative data to make some good guesses. It seems to me that any benefit of caloric restriction relates directly to avoidance of chronic disease, not from minor metabolic improvements. While I don’t rule out the possibility that these types of improvements do exist or affect our health in some way, I think they are unlikely (compared to avoiding heart disease or diabetes) to contribute significantly to our longevity.

Caloric restriction, then, is something most healthy people are already doing. It’s never not been good advice to consume a calorically balanced diet for your size, sex, and activity level, and we don’t really need more studies to tell us that. At most, what these studies demonstrate is that caloric overconsumption leads to chronic disease across numerous species, and that staying at a healthy weight leads to longer lifespans in most organisms, though not lifespans above what we would consider normal.

As for the effects of fasting itself, the results are even more muddled. Right now, we’re basically trying to understand how fasting affects our cells and biochemically important molecules. As such, there is a lot of preliminary research that sounds very exciting but should be understood in the proper context, which is to say that most things sound very exciting in the preliminary stage. When we move beyond those initial findings and try to link behaviors to objective outcomes, however, they often fall apart or are severely mitigated.

If you’re curious about the proposed mechanisms, you can read this summary paper. Just keep in mind that no matter how many times they propose that fasting can significantly improve health, it still does so most significantly in those who are overweight, and then in conjunction with weight lost. No study has demonstrated an otherwise healthy person to become healthier in any objective way (improving biomarkers that are already within the scope of good health doesn’t count!) because of fasting.

Does Intermittent Fasting Help You Lose Weight?

If you read my recent Weight Loss Series, you should know my answer: yes, if you don’t mind eating that way and it helps you restrict calories, but no if it doesn’t.

Intermittent fasting works because the amount of calories you cut out earlier in the day (or one or two days out of each week) are not fully compensated for in later meals (though it is expected that at least the first meal you consume after a fast will be larger than you would otherwise normally eat). Thus, when intermittent fasting works for weight loss, it works in the exact same way it always does: reducing total calories.

Some people find this method of eating for weight loss much easier as you don’t need to be as careful about meal planning. Instead, you simply skip a few meals and then eat normally (or at least more normally) for your other meals. For some people, this can mean less measuring and worry about getting portions right. It can also mean more fatigue, “hanger”, and loss of focus for others, though.

If you do wish to try intermittent fasting, it has as much support as any other diet—okay, maybe a bit less numbers-wise, but I haven’t seen a study that suggests IF isn’t as effective as any other type of diet so long as calories are matched, and I doubt I will. If not, then don’t worry—it’s not the only effective diet when it comes to weight loss.

Intermittent Fasting and Performance

This doesn’t seem like something we’d have good data on, but we actually do thanks to Ramadan, a month-long Islamic holiday that involves fasting from sunup to sundown. As a result, there’s a plethora of research to try and understand the effect that fasting all day has on Islamic athletes and whether it impairs performance.

Overall, it appears that so long as total macronutrient intake remains unchanged, the negative (or positive) effects of fasting on an athlete are small. This includes changes in body weight or composition as well as anaerobic power, though aerobic endurance may be initially compromised until the body adapts. We might suspect this based on a general knowledge of the role of various energy nutrients in exercise—for shorter duration, higher-power activities, our stores of muscle glycogen are likely sufficient (and will be replenished with later meals); for longer duration aerobic activities without supplemental carbs, muscle glycogen may be too limited, and until our body learns to compensate with fat oxidation, performance decreases.

The key here is that total calorie and macronutrient intake must remain unchanged. If you’re using IF to lose weight, you will suffer some loss of performance (as you would with any weight loss diet) due to the loss of available energy. If you’re only changing the timing of your meals, however, then you should maintain performance while fasting, or at least return to your “normal” within a week or two of your new dietary schedule.

The other thing to consider is that since you have fewer meals in which to get your macronutrients in, you have to eat proportionally more in each meal, which can be challenging for some, especially with carbohydrates. If your carb target is 300 grams a day, for example (not even on the high end of the spectrum), you might need to eat 150 grams of carbs in each of two meals—the equivalent of 4 cups of cooked rice per meal, or 8 cups total, sometimes in the span of only six hours. If you need even more carbs than this, then your “meals” start to look like gigantic piles of food. This means intermittent fasting may not be practical for some people (particularly those who exercise more than 3 hours in a day), or that they may need to alter the plan so they can realistically consume the fuel they need.

Intermittent Fasting and Protein Kinetics

A second thing to consider with intermittent fasting and meal timing is protein kinetics. Protein doesn’t work quite like carbohydrates or fats, which can be consumed in massive quantities and still be fully absorbed and used. Instead, the utility of protein caps at an intake of around 20 grams per meal, and it takes a few hours before our muscles become sensitive to more protein. While I don’t think anybody would fail to get enough protein while fasting, it may prevent optimal intake, and that may be less desirable for athletes who are training hard.

My personal solution to this is to modify the fast so it’s better at sparing protein, essentially by adding in protein-only meals throughout the day. On a “protein-sparing modified intermittent fast”, you would consume one or two protein shakes (or servings of lean meat) during the fasting hours of your day to help the body ward against muscle protein breakdown and also to encourage more muscle protein synthesis in conjunction with training.

Purists might say this nullifies the benefits of fasting, but since we’ve already established that fasting doesn’t really have any benefits (at least not above and beyond any other diet), this isn’t actually a problem! If you accept that fasting intermittently is simply an alternate dietary schedule that works for you, then adding in protein during your fasts doesn’t alter the end result.

Should Climbers Fast Intermittently?

The important question to ask here is, “What do you have to gain through intermittent fasting?” For most climbers, the answer is probably “nothing”. Intermittent fasting is simply another dietary schedule; it comes with its own problems and solutions, but overall doesn’t change anything performance-wise.

That isn’t to say that no climber should use intermittent fasting. For some people, it feels more natural. The point isn’t that intermittent fasting does or doesn’t work, it’s that there’s nothing special about it when it comes to performance, weight loss, or any other physiological metric—whatever you can achieve with intermittent fasting, you can achieve with other diets as well.

If you want to fast, then give it a try—but keep your goals in mind. If you’re training hard looking to improve at climbing, you can’t sacrifice your caloric or macronutrient intake in the pursuit of a new dietary schedule. This is true all diets; calories and macronutrients are more important than schedules.

Whatever you do, though, don’t fall for the hype about the benefits of fasting—they’re far too preliminary, and many of them rely on cultural misunderstanding about what “caloric restriction” means in the various animal studies that have been done. If there are benefits to fasting, it’s unlikely they’ll contribute in any significant way to our long-term health or longevity, at least not when compared to any other healthy method of eating.

And that’s a wrap on intermittent fasting! As always, if you have any questions, leave them in the comments below—I’m happy to answer anything I left out in the article.

While You’re Here…

I’ve got a new project goal up on Patreon: funding a nutrition survey for climbers. This is a baby step in the direction I’d ultimately like to see this site go, which is the funding of nutrition (or sports science) research for climbers—in addition to all the articles, guides, and everything else I already do here, of course.

The goal is $250 a month, with some stretch goals. As of today (2/21/18), we’re still pretty far away: $69 total. You could help change that, though!

Any amount helps, even $1. Especially $1. If half a percent of the people who visit this site this month became a $1 patron, we’d be well past the initial goal.

I understand: it’s hard to commit to these types of things. Certainly I’ve been the guy listening to NPR during a pledge drive, thinking I should pledge and then never doing it, even though I could win that trip to Italy. Sometimes it’s hard psychologically to make monthly pledges when you don’t know what the future holds. But with Patreon, it’s so easy to become a patron, and so easy to cease your patronage, and it’s such a small amount. We just need to get the ball rolling!

Check out Climbing Nutrition’s Patreon page. I’ve got a bit more written about my goal, and you can check out the individual patron rewards. Hope to see you there!