Maybe someone in your running group has mentioned they’re doing intermittent fasting, or you’ve seen some news articles pop up about this latest diet trend. Now you’re wondering – should runners do intermittent fasting?
As a dietitian and run coach, I wanted to bring you some thoughts on this topic. Generally, I don’t think intermittent fasting is the best approach for performance-focused athletes. There are pros and cons to this eating pattern, though, so you can make educated decisions for yourself.
Before diving into this topic, it’s important to recognize that this article is not geared towards individuals who are pregnant, who have an eating disorder, who have diabetes, or who have other medical conditions that may affect their nutrition recommendations. This is solely focusing on otherwise healthy runners.
Credit for some of the information below goes to fellow RDs Leslie Bonci, Roberta Anding, and Hope Barkoukis, who presented on this topic at FNCE 2022.
Disclaimer: This post was written and reviewed by Chrissy Carroll, MPH, RD, RRCA Running Coach. It is for informational purposes only and should not be construed as individual nutrition advice. Consult your dietitian or doctor for personal nutrition advice.
Note: This content was first written in 2022 and has been updated for this post in 2023.
What is intermittent fasting?
Intermittent fasting is a style of eating where time – rather than calories or macros – is the limiting factor. You have periods of time where you eat what you please, and other periods where you do not eat anything (or significantly restrict food).
Some people believe this style of eating more accurately represents the way our ancestors ate, where food was scarcer.
Types of intermittent fasting
While there are several types of intermittent fasting that are utilized in research, practically these three seem to be the most common types utilized:
1. Time-restricted feeding (TRF)
This approach restricts daily food intake to specific number of hours, and the individual fasts the rest of the time. The eating window usually ranges from 4 to 12 hours in research, though most people practicing this method use a 6 to 10 hour eating window, leaving a 14 to 18 hour fasting window.
The most common window that seems to be used is the 16/8 method, with 16 hours fasting and 8 hours of an eating window.
Ramadan fasting could also be considered a type of time restricted feeding, since it involves fasting from sun up to sun down.
2. 5:2 Intermittent Fasting
The second most common option utilized, this approach involves eating a regular diet 5 days per week, and eat a significant reduction in calories (around 500-600 calories/day) the other two days of the week.
3. Alternate day fasting
This approach is more rarely used, but worth mentioning. In this case, the person alternates fasting days with feasting days.
Research on Intermittent Fasting and Runners
There were only a few studies that I could locate at the time of publish that were specific to runners:
- A 2011 study looked at the impacts of Ramadan fasting on runners. The study was small – 9 in the control group, and 9 in the fasting group. They examined 5K performance before Ramadan and during the last week. They found a 5% decrease in performance.
- On the flip side, a 2021 study that looked at Ramadan fasting among 15 male distance runners actually found improved time to exhaustion and speed. (However, they noted that the athletes reduced their training volume by 48% during this time, while maintaining the number of weekly training sessions. Could the improved speed and time to exhaustion simply have been the result of what was essentially a few weeks of tapering?)
- A crossover study in 2021 looked at 4 weeks of time-restricted feeding in males using a 16/8 window, compared to a normal 12/12 window. They found the men lost fat mass during time-restricted feeding and maintained running performance.
- Another 8-week study on males doing time-restricted feeding using a 16/8 window found decreased body mass and similar running performance compared to controls.
If we go a bit more broadly towards exercise performance in general, there are some other interesting studies. One of the most impactful is a 2020 article titled Effects of Intermittent Fasting on Specific Exercise Performance Outcomes: A Systematic Review Including Meta-Analysis. This looked at 28 studies that met eligibility criteria and found:
- Small decreases in fat mass with intermittent fasting and no consistent differences in muscle strength with intermittent fasting (Consistent with another 2021 systematic review looking at body composition that found intermittent fasting was linked to less fat mass and no differences in lean mass).
- 36% of studies included noted decreased aerobic performance with intermittent fasting
- Ramadan fasting led to decreases in VO2Max, while time-restricted feeding led to increases in VO2Max
- Only 11% of studies included found decreased anaerobic performance with intermittent fasting
It does seem that the negative performance effects were more commonly linked to Ramadan fasting compared to time-restricted feeding. The exact reasons are unknown, but it could be perhaps due to working against circadian rhythm with timing of meals during Ramadan, or due to a longer feeding window with time-restricted feeding.
Outside of athletic performance, other studies have noted favorable effects on gut health and improved metabolic health markers in those who are overweight/obese (like lower blood sugar and insulin levels). Animal studies have also shown an upregulation of BDNF – brain derived neurotropic factor – an important compound for brain health.
Based on all this, you might be ready to dive into intermittent fasting. But wait a second – because this research doesn’t tell the whole story…
The problems with intermittent fasting research
There are two key problems with many of the studies on intermittent fasting:
- The studies looking at metabolic or cardiovascular improvements are often conducted on overweight/obese sedentary individuals. In fact, the limited research on lean individuals has found that there may not be the same benefits as far as body composition, metabolic, or cardiovascular health.
- The studies on athletic performance are often conducted on men.
This leaves a major group missing – active females. And that’s a big issue, because women’s bodies and hormones and quite different than that of men.
Some experts are concerned that intermittent fasting may affect hormones negatively in females, leading to poorer exercise performance and other potential health risks.
Intermittent fasting may cause a decrease in the production of kisspeptin (a neuropeptide). This compound plays a role in normal reproductive function, and women are particularly sensitive to this effect. This could be one of the reasons why this animal study found fasting adversely affected reproductive function of female rats.
Note that other research, on the contrary, found improvements on certain reproductive hormones with intermittent fasting – but the studies mentioned seemed to focus on women with PCOS.
Additional animal research has indicated that kisspeptin signaling may play a role in energy balance and blood sugar control – mice lacking kisspeptin signaling had higher weights and glucose intolerance.
This mechanism could explain why this study found alternate day fasting impaired glucose responses in women.
Yes, much of this speculation is based on hypotheticals and animal research – we certainly need more actual human research! But there have also been a few interesting studies looking at fasted and fed workouts that are worth noting:
- When research has looked at the impact of gender on fasted versus fed workouts, they found that women did not have the some of the same beneficial adaptations that men had. And even among male athletes, exercising in the fasted state increased muscular oxidative capacity (increasing the ability to use fat for fuel for exercise) but didn’t result in any additional improvements for VO2max and time trial performance.
- This 2020 study on post-menopausal women found no additional benefits with fasted training compared to fed training.
So the limited research on women suggests that fasted workouts may not benefit performance – and could cause some health consequences.
Another concern is the impact on cortisol level. This study notes: “Additionally, a negative correlation between the serum concentration and pulse frequency of kisspeptin and serum levels of cortisol were observed.”
Research shows fasting may increase the level of cortisol production (as well as shift the natural curves of production over the course of the day). High cortisol levels could increase appetite for palatable foods and lead to shifting weight distribution towards more abdominal fat over time. Chronic elevations in cortisol may impair certain aspects of immune system function as well.
Risks of Intermittent Fasting
In addition to the concerns mentioned above, there are some other potential risks of intermittent fasting – whether you are a male or female runner. Intermittent fasting could lead to…
- Not meeting energy intake to support training, also known as RED-S. This is more of a risk for athletes with a high training volume and athletes who are not focused on ensuring they’re eating enough. This can cause numerous health concerns including hormonal and immune dysfunction.
- Reduced exercise performance if a meal is not ingested prior to longer workouts. Research suggests pre-exercise carbohydrate intake enhances performance in exercise lasting more than 60 minutes. (And while some research suggests you may be able to maintain your performance on intermittent fasting, it’s not clear if you’d be able to improve your performance.)
- Feeling fatigued or tired during workouts due to less carbohydrate availability, forcing you to exercise at a lower intensity level.
- Poorer sleep. Several studies have linked this to intermittent fasting, though this may vary based on the type of fasting.
- Muscle mass breakdown, as the body is forced to rely on other fuel sources when fasted. While ideally the body would shift towards burning more fat, there can also be an increase in protein breakdown in the muscles with fasting to support energy needs of exercise. This may be more pronounced among female athletes.
- Disordered eating patterns, which can occur if intermittent fasting becomes an obsession or leads you to ignore your body’s cues for hunger and fullness.
- Side effects like nausea, vomiting, decreased bone density, irregular periods, and other health concerns during prolonged fasting periods.
Should Runners Do Intermittent Fasting?
If running performance is your goal, as a coach and RD I do not believe intermittent fasting is the best choice, especially for female athletes.
I believe a balanced diet that follows your hunger and fullness cues, and that allows for timing considerations with workouts, is likely a better choice.
Even with weight loss or fat loss goals (which I don’t recommend runners focus on, but that’s a post for another day) – some studies have suggested fasting is no better than a slight calorie restriction for this goal. For example, this study found similar results between both approaches in active males, this research found calorie restriction had more sustainable results after 12 months, and this study found fasting participants were more likely to drop out of the study compared to traditional calorie restriction.
But if you have other goals or other reasons for pursuing intermittent fasting as a runner, here are a few helpful tips to try to keep you safe and healthy:
- Avoid jumping into long fasting windows (if you’re not doing this for cultural reasons). Research has not determined a specific “best” fasting approach or “optimal” amount of time to fast. For example, even though many people use a 16:8 approach, this study found that a 14 hour fasting window (with a 10 hour self-selected eating window) led to improvements in cardiometabolic health among those with metabolic syndrome. As such, consider just slightly extending your normal overnight fasting window. For example, if you eat dinner at 6pm, waiting to eat breakfast until 8am would be a 14 hour fast. That’s not unreasonable if you aren’t doing an intense workout early in the morning.
- Be sure you’re eating enough calories and carbohydrates to support your running. By restricting the eating window, intermittent fasting often leads to a lower calorie and carb intake. Dropping too low can leave you without enough energy availability to support your runs, causing health problems.
- Be sure you’re getting enough protein during your eating window to ensure you’re supporting muscle repair and recovery. In general, aim for at least 20-30 grams of protein per meal, and making sure you’re getting enough leucine each day (a key amino acid). It does appear that as long as you maintain enough calories and protein during your eating window, you can maintain lean muscle mass. (There’s not enough evidence to support if you are able to gain muscle mass during this time, though.)
- Fuel before your runs, particularly before long or intense workouts. If you do not do this, you must keep the duration and intensity of the workouts low. Not fueling before workouts can increase the risk of protein breakdown for energy. (There is a big difference between going out for an easy 30-minute fasted run and a more-stress-inducing 2-hour fasted run).
- During long runs (>75-90 minutes), fuel your body during the run. If you are doing these outside of your eating window, try moving them to the middle of your eating window.
- If you find yourself fatigued, lightheaded, dizzy, losing your period, or struggling on your runs – intermittent fasting is likely not a good choice for you. Also, see a doctor to rule out any medical conditions.
The Bottom Line
While intermittent fasting may offer some body composition and metabolic health benefits among certain population groups, it also comes with some risks for runners. Runners would likely benefit more from an overall balanced diet with timing flexibility, particularly among female athletes.
If you choose to embrace intermittent fasting, be sure you working with a dietitian that can help tailor your eating plan to your training goals and ensure you’re meeting your calorie, carbohydrate, and protein needs.
Feel free to pin this post to refer to later!
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