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Fasting & Fitness: What the Science Says


The practice of fasting has been around for centuries. Legend has it that the famous Greek physician Hippocrates recommended fasting as early as the 5th century BCE, for treating various medical issues. Since then, fasting has been used in both religious and health contexts. More recently, the concept of intermittent fasting has been touted, by some, as an effective and simple way to improve health and lose weight.  In fact, a quick google search of intermittent fasting generates nearly 70 million results, with significant conflicting information. In this article, we’ll dig into what the science of intermittent fasting tells us, in an understandable way, ending with a practical implementation plan for those of you that want to give it a try.

Types & Methods of Fasting

To set a level, we have to identify what actually constitutes a fast. As you’ll see, things get complicated very quickly. If you do a little exploring in the scientific and non-academic literature alike, you’ll find many different fasting types and methods of fasts described. Let’s start with the types, and for simplicity’s sake, I’ll keep it to three:

  • Full Fast: consume only water for a defined period of time.
  • Liquid Fast: consume only water, broths, juices for a defined period of time.
  • Partial Fast: eliminate specific food(s) for a defined period of time.

In terms of methods, let’s keep this simple to, by narrowing our list to the following:

  • Full Day(s) Fast: adhering to your fast for one or more days.
  • Time Restricting Eating: not eating outside of a specific time window each day (for example, not eating before 12pm and/or after 8pm). The period where you’re eating is referred to as your “feeding window.”

Now the complexity starts to come in when you start to blend the types and methods. You could do a full day, full fast; a full day liquid fast; or a full day partial fast (like Catholics not eating meat on Friday in Lent). You could pretty much intersect any of the three types of fasts with either of the two methods of fasting to meet your needs.
You could really call any of these methods intermittent fasting if you wanted to, but for the purposes of this article we’ll consider intermittent fasting to be a full day (or more) fast and time restricting eating to be eating in a specific defined time window throughout the day. For other perspectives on intermittent fasting methods check out this article by the Cleveland Clinic and this article from John Hopkins.

Safety of Intermittent Fasting & Time Restricting Eating

To dive deeper into our discussion of this topic, let’s look at the safety element first. Regardless of their effectiveness, if these methods aren’t safe, it’s a non-starter. The good news is, for an apparently healthy population, the risks of fasting are relatively minimal, and we’ll get into those below.
There are some individuals who may want to exercise caution with fasting:

  • Diabetics (Type 1 or Type 2): since fasting results in lower blood sugar levels, this could interact with medications (like insulin) and could result in dangerously low levels of blood sugar.
  • Hypotensive Individuals: people with chronically low blood pressure could see this issue exacerbated by the low sodium and fluid content during a fasted period. This could reduce their blood plasma volume, and therefore their blood pressure.
  • Women who are Pregnant or Breastfeeding: due to the risk of reducing needed calorie intake for fetal growth and breastmilk production.
  • History of Eating Disorders: the necessary preoccupation with eating and non-eating windows may cause psychological distress for these individuals.
  • Certain Medications: some medications may have negative interactions with intermittent fasting.

I think the most important take home message on safety for anyone in the above groups, or with a pre-existing medical condition, is to check with your doctor first before engaging in intermittent fasting or time restricted eating.
As I said above, for apparently healthy individuals the negative side effects of fasting are relatively minimal, but they do exist. Lightheadedness, dizziness, confusion, irritability, diminished cognition, loss of muscle tissue, and impaired physical work capacity are reported side effects of intermittent fasting. However, most of these can be mitigated if you engage in fasting in a progressive and thoughtful manner. We’ll touch on this more in the final section of the article.
Here’s another perspective on the side effects of fasting from the Journal of the American Medical Association (JAMA).

Effectiveness of Intermittent Fasting & Time Restricting Eating

When we talk about effectiveness, we need to first define what effective means in the context of intermittent fasting or time restricted eating. To do this, we’ll break effectiveness into two broad categories: health and weight loss. Since many people research intermittent fasting for weight loss, let’s start there.
In general, the results are quite mixed and inconclusive. Several comprehensive studies of the literature demonstrate these mixed results; such as this meta-analysis and this systematic review. If you dive into these articles, the researchers note a significant amount of variability in how subjects respond to intermittent fasting or time restricted eating protocols. Furthermore, there is very little consensus among researchers as to what the most effective protocol is (see the methods and types section above).
For some, it appears  fasting/restricted eating does result in a calorie deficit and weight loss. For others, it results in no change in calorie intake, and therefore no weight loss. Finally, in other cases it can cause people to experience cycles of restriction followed by excessive calorie consumption (or binging) and cause weight gain.
I realize that summary of fasting and weight loss is quite underwhelming and confusing, so let me sum it for you, based on the totality of the literature. Fasting (intermittent fasting or time restricted eating) does result in weight loss if it causes you to eat less. If it doesn’t result in eating less, your weight doesn’t change. As we’ll discuss below, running an experiment to see how your body responds might be a good idea.
From a health perspective, the results seem to be more promising, but the research is still in its early days. This Healthline article does a good job of explaining the potential health benefits of intermittent fasting and time restricted eating. In general, there are a number of potential benefits in terms of cellular regulation, cardiometabolic health, cognition, and longevity if these fasting protocols can be implemented and adhere to over the long-term. Certainly more research is needed to further verify these claims, however I feel confident saying that based on current evidence, fasting does appear to have some health benefits.

Implementing Intermittent Fasting or Time Restricted Eating

Given everything I said above, it seems like fasting falls into the category of; it won’t hurt you (medical conditions notwithstanding), and it might help you – so it’s worth a try to see if it works for you.
Based on the research, I would consider the benefits more from a health than a weight loss perspective. If you can view it through that lens, play the long game, and see if you notice any of the reported health benefits then there’s a good chance you’ll see some of the  weight management benefits.
If you’re too focused on the scale right off the bat, and it doesn’t change in a way that meets your expectations, you likely won’t stick with fasting long enough to see any benefits whatsoever.
With that in mind, here are some best practices for implementing fasting:

  • Start with time-restricted eating.  Before you jump right into a full day (or days) fast, I’d recommend just tightening your eating window. How to do this will differ for everyone.
    Simple ways to consider are to either wait several hours to eat after waking,  or stop eating several hours before bed. Any time restricted eating is better than none, so even starting with a 12 hour eating window can be a good starting point. A good goal for most people is to get their time restricted eating window down to 8 hours. Progressively try to reduce your eating window to 8 hours and see how you feel.
  • Progress to a very small eating window before a full day fast. If you’re tolerating the 8 hour eating window well, and want to see if a full day fast could work for you, try to cut your eating window down to 4 hours. This will be a small enough window to see if you can tolerate a full day fast.
  • Take your time to progress. In order to give your body time to adapt to the progressively shorter time restricted eating windows,spend  several days at each time increment. I’d recommend spending at least 5-7 days at each restricted eating window, this will make the progression more tolerable. You might start with a week at a 12 hour eating window, then a week at 8-10 hours, a week at 6-8 hours, and a week at 4-6 hours before going to a full day fast.
  • Maintain your protein intake. In order to ensure you maintain your muscle mass try to maintain a protein intake of a minimum of 1 gram of protein per pound of body weight during your eating window. Ideally, you spread that protein intake out into 3 separate feedings with equal thirds of the protein you consume at each feeding.
  • Exercise during a feeding window. Since the fuel source we use for exercise is mostly carbohydrates, planning your exercise when you’re consuming carbs will help you get the most out of your workout. Keep in mind, if you’re exercising during a fasted window, you’ll need to decrease your exercise intensity pretty significantly. If outside of your feeding window is the only time you can exercise, plan to adjust your workout accordingly.

Take Home Message on Intermittent Fasting & Time Restricted Eating

There you have it, a relatively complex topic boiled down into five actionable items to try if you’re looking to implement this. As this article has highlighted, fasting appears safe for healthy individuals and may confer some health benefits. Additionally, there could be some modest weight loss benefits, but those results are mixed.
Approaching fasting as an “experiment” to see how your body responds is probably the right approach. If you feel better and like eating in a manner that’s consistent with fasting, great – keep it up!  If you don’t like it, and it doesn’t work for you, I wouldn’t stress about it in the least. As this article from the World Health Organization suggests, there are many different eating patterns that support health.  Find the one that works for you and run with it!

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