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  • Writer's pictureAllison Koch, MS, RD, CSSD, LDN

Intermittent Fasting: What Is It And Does It Benefit Runners?

One of the most popular health trends of this year so far is hands down intermittent fasting (IF). I’m often asked in my practice whether it’s a good idea and could benefit runners. While the research is mixed, I think there are pros and cons to it, like any eating pattern. Today, I’m going to walk you through the ins and outs of IF and answer the question you are all wondering: is it good for runners?

What is Intermittent Fasting (IF)?

IF is an eating pattern that cycles between periods of fasting and eating. Individuals doing IF cycle through periods of voluntary abstinence of food (or significant calorie reduction), interspersed with intervals of normal food intake. There are no guidelines for ‘on days’ when eating is unrestricted. The goal? To maintain overall caloric intake—simply eat during less hours each day in order to create conditions of fasting. Why? People use it to lose weight, improve their health, improve insulin sensitivity and other metabolic benefits. While not technically a diet, weight loss may be a result, as limiting the hours when you eat may lead to a reduction in the total calories taken in. People aren’t successful with IF if they overeat or consume large quantities of unhealthy foods during the non-fasting windows.

Types of IF

  1. Time-restricted eating (16/8 method): daily 16-hour fasts, skip breakfast and restrict daily eating period to 8 hours; most popular as easiest to stick to

  2. Eat-Stop-Eat: fasting x 24 hours once or twice/week

  3. 5:2 diet: consume only 500-600 calories or about 25% of daily caloric intake on two nonconsecutive days of the week, eat normally the other 5 days

  4. Complete fast on certain days of the week

How does it work?

Whenever we eat, the body releases insulin to help cells convert sugars (glucose) from food into energy. If the glucose isn’t used immediately (like when we exercise), insulin helps make sure the excess is stored in the fat cells. When we go without food for extended periods, insulin is not released, and the body turns to breaking down fat cells for energy potentially resulting in improvements in metabolism and weight loss.

History, Research & Potential Health Benefits

Fasting has been a practice for ages. It is often done for religious or spiritual reasons (e.g., Ramadan). An early experiment almost 100 years ago showed if you fed rats less food, they lived longer. A summary of research to date links calorie restriction with the prevention of age-related disease including tumors, cardiovascular disease, diabetes and dementia; research also suggests it increases lifespan.

In addition to disease prevention and longevity, research shows that restricting eating to daytime hours, an approach that aligns eating patterns with circadian rhythms, has also been shown to have metabolic and weight loss benefits. A 2015 systematic review of 40 studies published in Molecular and Cellular Endocrinology showed that IF was effective for weight loss – with a typical loss of 7-11 pounds over 10 weeks. But IF isn’t necessarily more effective than any other energy restriction regimen (e.g., counting calories or cutting carbs). A year-long study of obese adults published in the JAMA Intern Med in 2017 showed that alternate day fasting did not produce superior adherence, weight loss, weight maintenance or cardio protection over daily calorie restriction.

And while the body of evidence continues to grow, the research is still limited. Most research studies to date have been done with animals or an obese or overweight human population. More randomized controlled studies on a wider range of adult populations – including active, fit adults (like most runners) - would help confirm some of the potential health benefits.

Is IF for everyone?

I caution anyone who is currently underweight or with a history of eating disorders, pregnant or breastfeeding, growing adolescents, diabetics, those with existing medical conditions, people using medication that requires food intake, and/or people over the age of 65 against trying IF without talking to a healthcare provider first. In addition, there is science to suggest that when women restrict their eating for too long, the body increases cortisol – a stress hormone that may lead to holding on to weight around our midsections as well as hindering our training progress.

What about Runners?

My first question for a runner who wants to try intermittent fasting is to understand their end goal. Are they looking for weight loss or improved performance? If the latter, I hesitate to encourage anything that would potentially restrict energy availability. I’ll get into that in a second.

But first, from a research standpoint, it is hard to tell if IF would be beneficial since most of the research has not been done with runners or even athletes. One study looked at runners during Ramadan and showed it negatively impacted their performance. Another study showed that it also was accompanied by significant metabolic, hormonal and inflammatory changes.

Considering food and in particular carbohydrates are a runner’s preferred energy source, it seems less likely that it would be of benefit for performance. Fasting can compromise energy levels and recovery times and long term could actually contribute to decreased performance. Without carbohydrates to fuel your runs and replenish your energy stores and protein to repair the damage done to your lean muscle mass – it’s likely you will bonk in runs and not recover as well if you are doing them in a fasted state.

If a runner is trying to lose weight, a better approach may be to create a small caloric deficit each day versus fasting to allow you to still meet your goals without compromising your runs. This all ties back to energy availability (having enough calories available for the work or exercise you are about to do) – and when you work out consistently in a low energy state – you don’t have enough calories to support your resting metabolic rate (the calories you need to eat, live, breath and sleep), your immune system (to fight off colds), your endocrine system (to convert the calories into fuel) – all the things that allow your body to respond to stress. When taken to an extreme, IF can cause relative energy deficiency in sport (RED-S), often expressed initially as exhaustion, adrenal fatigue and hormonal problems and that can come with longer-term complications like stress fractures, injury and amenorrhea in women.

Bottom Line

Do I think we could benefit from not eating at all hours of the day? Yes. We have 24-hour access to food, 7 days a week. We’ve become accustomed to eating every couple of hours. Some of us graze all day long – never eating an actual meal. It makes sense that we should give our bodies – and our digestive system a rest periodically.

Instead of the current approaches to IF which may be too limiting for runners, I suggest modifying it to meet your needs. Every night you should be aiming for at least 8 hours of sleep. Add to that stopping eating at least an hour or two before bed to allow for adequate digestion – and you’ve got at least a 10-12 hour fast. But by all means – if you are hungry before bed, eat something!

Consider periodizing your nutrition to support your training. In other words, eat more calories around the most active part of your day – for example – if most of your stress – including your workout and your life stress – happens in the earlier part of the day, aim for a bigger breakfast and lunch and lighter dinner. Time your runs for when you’re eating – NOT when you are fasting. Super important for both pre-workout and post-workout to maximize recovery.

I see no problem working out in a ‘fasted state’ if it’s an easy day of cardio equal to or less than 1 hour of low-intensity exercise like a slow jog (just make sure you drink some water!). Following the workout, I encourage refueling with high-quality protein and carbohydrates (i.e., eat breakfast) within 30-60 minutes.

If you are planning a workout longer than an hour or at a moderate to high intensity, you will need some carbohydrate for quick energy. Go without and you may end up feeling tired, weak, lightheaded, nauseous and of course, ‘hangry.’ Wake up early and feel like you don’t have time? Munch on some peanut butter toast or a banana while you get dressed/ready. Trust me - your body will thank you at the 1-hour mark.

Also DO NOT consider a strength training workout in a fasted state. The stress on our muscles is lower during cardio workouts. But during strength training, you are stimulating greater muscle protein breakdown. You want to maximize your muscle gains by making sure you are adequately fueled pre- and post-workout when it comes to strength training.

For more on fasted cardio and whether or not it can improve body composition or metabolic efficiency, check out my blog post on the topic which you can find here.


Do you have nutrition questions? Contact Allison today!

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