Is Time-Restricted Eating Safe for Athletes?

Does it matter when you eat? Registered Dietitian Matthew Kadey looks at the science behind time-restricted eating.

Photo: Getty Images

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When it comes to feeling and performing better, it’s not just what we eat that matters. When we eat also plays a crucial role.

Over the past decade or so an emerging area of science, called chrononutrition, has begun to unearth valuable information about how the timing of our meals may affect our health and performance during physical pursuits. And born out of this science is a popular feeding pattern called time-restricted eating. Not a diet, per se, as there are no suggestions about what to eat or not to eat, just a focus on when you eat your calories.

But can athletes benefit from shuttering their kitchens at night and delaying their morning oatmeal? Here’s what the science has unearthed about trimming your feeding window to better align with your ticking biological clock.

What exactly is time-restricted eating?

As the term suggests, time-restricted eating consists of limiting daily nutrient consumption to a set amount of time during a 24-hr period as a means of extending the time spent in a fasted state beyond what is now normal in society. Common feeding windows used are 12, 8, and 4 hours.

Various behavioral and physiological pathways follow a predetermined 24-hour cycle known as the circadian rhythm. Molecular clocks are present in almost every cell. And the time of food intake is emerging as a dominant agent that affects circadian clocks in metabolic organs. The belief is that, just like erratic light exposure, erratic eating patterns can throw natural rhythms such as metabolic processes out of whack – known as circadian disruption.Eating erratically and for too many hours daily can provoke chronic circadian disruption or a type of feeding ‘jet-lag’ which is believed by time-restricted eating proponents to be a driver of weight gain, poor sleep, increased risk of developing chronic disease and, in the end, a shorter life. Maybe even less of a chance to reach fitness goals.

In contrast to early humans who may have never had a guaranteed meal, most modern bipedals start cramming in calories upon waking and don’t stop until shortly before calling it a night. The average person now eats across a 15-hour window, and researchers at the University of North Carolina found that the average American consumes 570 more calories a day than they did in 1977. Their findings showed that a modern habit of constantly eating and increased portion sizes were to blame for the rise in calorie intake.

Better nurture your circadian rhythms through a more compatible feeding window, without necessarily greatly restricting calories, and you could be awarded certain health benefits including improved metabolic health. In theory, at least.

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The science of time-restricted eating

A raft of scientific papers suggests that there might be some reasons for athletes to consider reaching into their feedbag less often, especially if the goal is to trim the waistline.

recent study in Medicine and Science in Sports and Exercise randomly assigned male runners to follow a time-restricted eating pattern where all their calories were consumed in an 8-hour period or where they followed their regular diet with no restriction on when they could eat. Participants were permitted to eat what they wanted and as much as they wanted during both trials. Following the 8-week study period, athletes dropped more body mass with time-restricted eating that was attributed to a decrease in overall calorie intake. However, time-restricted eating resulted in no improvements in performance metrics including maximal oxygen consumption or exercise heart rate, nor did blood tests reveal any significant differences in insulin, glucose or triglyceride levels suggesting no benefit to metabolic health. Time-restricted eating may require a longer intervention to bring about any notable metabolic improvements.

In a separate investigation, published in the Journal of the International Society of Sports Nutrition, sixteen elite under-23 cyclists were randomly assigned either to a time-restricted group where they meet their energy requirements during an 8-hour eating window (from 10:00 a.m. to 6:00 p.m.) or a control group where calorie intake was spread between 7:00 a.m. and 9 p.m. during a month-long period of high volume training. Total calories and macronutrients were matched between both groups. Time-restricted eating resulted in a drop in body weight and body fat, but, importantly, no loss in lean body mass. Body mass fell by about 2%, whereas the percentage of body fat loss was about 1%. There were no notable changes in performance between the groups, but the power-to-weight ratio improved for those with time-restricted eating owing to the drop in body weight. The researchers also found some positives in blood markers of immune functioning, most notably less of a decline in leukocytes that can occur during bouts of high-intensity training. Leukocytes are immune system white blood cells involved in helping us fend off infections. It’s worth noting that testosterone levels were lowered in the male athletes as a result of the time-restricted eating pattern, which over time may impact such things as the ability to build and maintain muscle mass.

Other research results suggest that an intermittent fasting program in conjunction with resistance training can decrease fat mass while still maintaining muscle mass in resistance-trained males.  In a meta-analysis of randomized control trials published in the Journal of the Endocrine Society it was found that compared to control diets with no time constraints, time-restricted eating, defined in this report as 4 to 8 hours of unrestricted eating in 24 hours, resulted in improvements in total body weight and body composition (fat mass to lean mass ratio) in people who were overweight or obese. However, the research review found there is not enough data to say that narrowing the eating window can bring about improvements in blood cholesterol (LDL and HDL) or triglyceride numbers. But a different review of previous research published in the journal Nutrients not only confirmed the benefits that time-restricted eating can have on body composition but suggests there indeed are certain cardiometabolic improvements with this style of eating including blood pressure, fasting blood sugar and blood triglycerides. Though the authors concluded that longer-term studies are in great need to confirm these findings.

There could be a few factors at play here as to why time-restricted eating can help the needle on the scale move to the left. Most likely it is simply harder for people to stuff in too many calories when they can’t eat from sun-up to well past sun-down. For most people, there are only so many calories that can be inhaled in a narrow feeding window. Many time-restricted eating protocols discourage feeding after dinner, which helps put the brakes on late-night snacking, a time when many of us eat out of stress or boredom instead of real hunger. And, let’s face it, most often what we reach for at night is nutritional suspect, to say the least. Scientists in Finland determined that compared to people whose calorie intake was proportionally lowest during the evening, those with proportionally highest calorie intake during the evening hours were more likely to be overweight, even if their chronotype suggested they were night types. And not to be overlooked is that eating too much, too close to bedtime could disrupt sleeping patterns, and some research suggests that poor sleep can send people to seek out more calories from junk food.

RELATED: 3 Healthy Ways to Battle the Nighttime Sweet Tooth

There may also be some sort of positive impact on metabolism by noshing in better alignment with circadian rhythms, a concept that can be tricky to prove through research. Some degree of fasting could set the stage for enhanced metabolic fuel switching, thus improving metabolic flexibility. Improvements in insulin sensitivity, blood sugar control and appetite that may result by eating less often could help in the battle of the bulge. There is also the opportunity for people to finally learn what physiological hunger feels like, especially with narrower feeding windows, compared to psychological hunger where we are often eating simply because of factors like daily stress and anxiety.

Longevity experts believe a certain degree of daily fasting allows our bodies to undergo cellular natural selection. A time where old, damaged cells are replaced with newer, healthier ones in a process called autophagy. It’s thought that the weakest, poorly functioning cells can have all sorts of problems such as not dividing properly which disrupts normal tissue functioning and, ultimately, contributes to several life-shortening maladies including cancer. Pathways involved in this cellular replacement are believed to be activated in the fasted state and suppressed in the fed state, the latter of which is what most Americans are in nearly all the time. Feeding marathons robs us of the 12 to 16 hours we need to fully metabolize our last meal and lapse into autophagy mode. (Autophagy has been shown to degrade damaged cellular components to re-use as an energy source during advanced stages of starvation in mice.) So if you’re prone to eating late at night and then again early in the morning, it’s theorized that it is going to be a challenge to take advantage of this cellular turnover that can occur during long hours of overnight fasting. A fascinating concept, but likely one that has so many nuances which need to be sussed out through further research. Exactly how much fasting is required to increase autophagy in humans instead of laboratory animals is completely unknown. In the end, likely not quite the anti-aging miracle we all hope for.

Photo: Getty Images
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What athletes need to know about time-restricted eating

There are certainly some good arguments to be made for taking fewer trips to the kitchen. But it’s important to keep in mind a few things before jumping on board.

In contrast to much of the general population, not all triathletes can benefit from a loss of body weight. Already lean, or too-lean athletes may experience negative health and performance outcomes as a result of shedding more pounds. Nobody should consider a skin-and-bones cyclist post-Tour de France as the ideal physique. So if you’re doing time-restricted eating and are already at an ideal or less than ideal body weight make sure it does not result in an unhealthy amount of weight loss. Time-restricted eating might be most useful during off-season training as a way to help athletes prevent any unwanted weight creep during periods of lesser calorie burn. To date, there is little data to show that the initial weight loss experienced with time-restricted eating is lasting as long-term studies are lacking. So many diets fail to result in long-term adherence and lasting body composition improvements.

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We also desperately need to see studies conducted on female athletes and during various stages of the menstrual cycle. So far it’s mostly been bro science, with not much in the way of data to suggest this style of eating can make you a better athlete. Also, it is still up in the air what health benefits if any people with a healthy metabolic profile such as cholesterol and blood sugar numbers will glean from eating less often. Likely, we may never really know if time-restricted eating can extend lifespan because long-term studies like that are extremely challenging and cost-prohibitive.

Plus, are there any significant differences in 4, 8 or 12-hour feeding windows? So far, most of the weight loss benefits have been associated with 8-hour or less eating times. But there is a dearth of research that has pitted various feeding windows against each other. Also, could it be possible to cycle through periods of time-restricted eating and regular eating? It’s reasonable to assume that a very active athlete has more wiggle room for when they start and halt their eating each day.

Time-Restricted Eating Protocol

If you are interested in giving time-restricted eating a go, here is how to make it work for you.

1. Ease into it

If you have spent years grazing for 16 hours a day making the drastic shift to eating for only 4 hours a day likely won’t go well. Quick, huge diet changes are for most people not sustainable. A better progression might be to start with a 12-hour feeding window, say 7 a.m. to 7 p.m., and then if that goes well for a couple of weeks try narrowing things down from there and see how you feel both during the day and during your workouts. Pushing back your breakfast, or forgoing it completely, and shuttering the kitchen at night can suck at first, but many people find the mind and body do adjust.

2. Choose the window that works best for you

There is some evidence to indicate that greater promotion of health via time-restricted eating may be achieved if a larger proportion of nutrient intake occurs earlier in the day. So a feeding window that mostly finishes before sunset. We are often told to try to allow 2-3 hours after waking before eating and consume your last food 2-3 hours before going to sleep. But not all schedules allow for the same eating window. For instance, if your training occurs later in the day then you will likely be looking to push out your eating. So instead of feasting between 8 a.m. and 6 p.m. you may find it necessary to go with 11 a.m. to 8 p.m. to better jive with your lifestyle. Ultimately, the main goal is to simply decrease how many hours each day you are feasting and snacking.

3. Keep a regular eating pattern

Regardless of how many hours you are allowing yourself nourishment, it’s a good idea to keep regular meal and snack times as much as feasible. This study suggests that a fair amount of variability between feeding times during the week and weekend can contribute to weight gain in the general population. While an investigation published in the American Journal of Clinical Nutrition found that irregular meal patterns can reduce the thermic effect of feeding, that is the amount of calories we burn processing the food we eat.

4. Make sure to eat enough when you can

Remember that the premise of time-restricted eating does not necessarily require eating fewer total daily calories. And if you are training and competing hard it is very important to meet your bodily caloric needs. If you don’t, you risk developing relative energy deficiency in sport (RED-S). This is a fairly new term referring to a situation where there is a mismatch between calorie intake by athletes and the calories they expend during training, leaving inadequate energy available to properly support the bodily functions necessary for health and performance. RED-S has been linked to weakened bones, low testosterone (male athletes), low estrogen (female athletes), higher levels of stress hormones, impaired immunity, menstrual disturbances and altered resting metabolism. So if you aren’t just a weekend warrior don’t be too shy to be generous with your calorie intake during your narrower feeding window. After all, time-restricted eating means you shouldn’t be making up for a light lunch with fridge raid late at night.

5. Don’t forget the big picture

As with so many things in nutrition science context is key. Is the timing of your meals as important as the quality of your diet when it comes to health and performance? No, not likely. Eat a lousy diet full of ultra-processed foods and you’re going to suffer no matter if you just happen to eat these nutrient-poor calories during a limited number of hours every day.

Matthew Kadey, M.S., R.D., is an author and journalist who specializes in sports nutrition and is the recipient of the 2013 James Beard Award for Food Journalism.

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