Physiology Friday #227: Restricting Carbohydrates at Night Improves Aerobic Fitness and Fat Metabolism
"Sleep low" to perform better.
Greetings!
Welcome to the Physiology Friday newsletter.
ICYMI: I’m doing my first subscriber Q&A session soon, where I’ll be answering questions submitted by my audience. If you’d like to submit a question, you can do so here.
Details about the sponsors of this newsletter including Examine.com and my book “VO2 Max Essentials” can be found at the end of the post!
Carbohydrates are key when it comes to high-level athletic performance.
Decades of science have supported the idea that a proper fueling strategy for most types of endurance exercise — and even team sports — involves adequate carbohydrate intake during training and competition to maximize glycogen availability, meet the energetic demands of physical activity, and reduce or prolong fatigue development.
For recreational athletes, the recommended carbohydrate intake is around 3–5 grams per kilogram of body weight per day. If you weigh 70 kg, this means consuming about 210–350 grams of carbohydrate per day. The recommended intake for athletes training at a high volume is around 5–8 g/kg per day. Carbohydrate loading before competition is also advocated. This involves consuming anywhere from 10–12 g/kg of carbohydrate in the 24-48 hours leading up to an event.
The International Society for Sports Nutrition (ISSN) doesn’t shy away from a carb-centric approach to fueling exercise, stating that “...the need for optimal carbohydrates in the diet for those athletes seeking maximal physical performance is unquestioned. Daily consumption of appropriate amounts of carbohydrate is the first and most important step for any competing athlete.”
Though carbs are hailed as king or queen, there has been a recent paradigm shift in how athletes approach fueling their day-to-day training. Many claim that a high-carb approach to sport may have unintended and perhaps deleterious side effects on long-term metabolic health. Carb-loading and mid-race gels might help performance, but at what cost?
There is a growing segment of athletes who wish to optimize not only their athletic performance but also their metabolic health. This means modifying the traditional athletic diet and using strategic, rather than indiscriminate, leveraging of carbohydrate consumption during training. If you’re like me, you’ve probably experimented with or at least taken an interest in the idea of a low-carbohydrate diet for performance.
In theory, there are several benefits to a low-carbohydrate diet for athletes.
At a lower exercise intensity, fat can be used as a fuel and provide necessary substrate for energy production. One characteristic of low-carb athletes is that they are drastically better at burning fat during exercise than athletes on a high-carbohydrate diet. They are able to maximize fat burning at a relatively higher exercise intensity (70% of VO2 max vs. 55% of VO2 max).
An enhanced fat-burning capacity also may have a glycogen-sparing effect and reduce the need for athletes to fuel during exercise with carbohydrate-based gels and sports drinks.
Other potential benefits include better metabolic health (low-carb diets may improve insulin sensitivity and blood glucose control), enhanced recovery, and improved body composition, all of which have been reported (and shown) to occur in athletes following a low-carb diet.
However, when it comes to performance, the data are not in favor of adopting a low-carbohydrate approach. Indeed, several studies have shown evidence that when athletes adopt a low-carb/ketogenic diet during training, they fail to improve their time-trial performance and their exercise economy decreases — despite significant increases in their ability to burn fat during exercise.
Is there a way to reap the benefits of a low-carbohydrate diet and sustain high-level athletic performance? Perhaps.
The concept of periodizing nutrition has become a popular practice among athletes and could similarly be used by anyone undertaking an exercise routine to improve physical performance and health.
Periodized nutrition involves strategically altering one’s fuel availability (by manipulating carbohydrate intake) for each exercise session based on the goals and demands of that session. This concept is also known as “fuel for the work required.”
Periodized nutrition aims to maximize the metabolic and performance adaptations to each workout. This is accomplished by in some cases restricting and in other cases providing adequate carbohydrates to fuel the work being done. Lower-intensity training can be performed in a glycogen-depleted or fasted state, while high-intensity sessions are fueled with carbohydrates to maximize output.
One specific periodized nutrition strategy is known as “sleep low.” When using a “sleep low” strategy, one first performs an exercise bout in the late afternoon which can either be high-intensity, long-duration, or some combination of both. The exercise is designed to deplete muscle glycogen but can be performed in the fed state. Following this exercise session, one consumes a low- to no-carbohydrate meal in the evening. In short, you don’t eat to replenish muscle glycogen. The following morning, you perform another exercise session in the glycogen-depleted state. This session will, necessarily, be lower intensity but perhaps longer duration — zone 2 training, for example.
The benefits of “sleep low” training are similar to those of fasted training. It may improve submaximal exercise economy, time to exhaustion, 10k running performance, and body composition. Fat-burning ability also improves as a result of performing morning exercise in a glycogen-depleted state.
Can the “sleep low” strategy be simplified even further and still have benefits? How well could something as straightforward as cutting carbs at night help our health and performance?
This practical approach to metabolic and performance enhancement was investigated in a recent study published in Nutrients.1
For the study, a total of 22 adults (11 males and 11 females with an average age of 22) were randomly assigned to one of two groups: a nighttime carbohydrate restriction group or a control group who followed their usual diet.
Both of the groups consumed the same number of calories and carbohydrates throughout the entire 1-week study. The control group had no dietary restrictions. The nighttime carbohydrate restriction group had one rule: no carbs after 4 p.m.
(Carbohydrate and calorie intake were the same in each group: ~2,400 calories and ~290 g of carbohydrates in the control group and ~2,200 calories and ~270 g of carbohydrates in the nighttime carb restriction group).
All of the participants performed exercise every day during the study: 60 minutes of outdoor running at an intensity corresponding to 65% of their maximum heart rate.
Study measures were performed at baseline and after 1 week of the training and dietary intervention.
Results
Surprisingly (at least to me), the nighttime carbohydrate restriction group increased their VO2 peak (measured during a cycling test to exhaustion) compared to the control group, who experienced no improvement in aerobic capacity.
The nighttime carbohydrate restriction group also increased their maximal power output (Watts) during a cycling test compared to their power output before the intervention. However, power output at the end of the study was not different between the two groups.
The nighttime carbohydrate restriction group also improved their fat-burning ability during low-intensity exercise after the intervention, as indicated by a lower respiratory quotient or RQ: a ratio that assesses the amount of carbon dioxide produced to the amount of oxygen consumed during exercise. A lower RQ indicates more fat utilization while a higher RQ indicates more carbohydrate utilization for energy.
Some minor changes in body composition were also observed. Specifically, the nighttime carbohydrate restriction group lost 1.4 kg of body mass after the intervention, however, they also lost nearly a kilogram of lean body mass, meaning nearly all of their weight loss was lean body mass.
Importantly, the nighttime carbohydrate restriction had no adverse effects on mood states, including anger, confusion, depression, fatigue, tension, vigor, or friendliness. This is an important outcome as drastic dietary changes can sometimes lead to worsened mood or “hanger” (a scientific term), reducing their sustainability.
What I like about this study is that the strategy implemented was quite straightforward — don’t eat carbohydrates after 4 p.m., continue with your usual training, and observe the improvements.
The participants didn’t need to stop eating after 4 p.m. as in early time-restricted eating, nor did they engage in late-afternoon or evening exercise as practiced in some other “train low-sleep low” periodized nutrition strategies. They didn’t even have to change their diet quality or how much they ate each day.
Even more, this periodized nutrition strategy didn’t even necessitate dietary carbohydrate restriction — the participants consumed nearly 300 grams of carbohydrates per day. “Sleep low” allows you to have your cake and eat it too (just not after 4 p.m.)
With this simple dietary switch, the participants experienced improvements in aerobic fitness, enhanced their fat-burning ability, and even lost a bit of weight. All in just 1 week, during which they only engaged in some moderate-intensity exercise.
I can imagine that we’d see even greater improvements if this practice was implemented for a few months or longer. And that would certainly be a good thing, because the changes in power output and aerobic fitness in this study were, admittedly, quite minor. I should also acknowledge that the participants weren’t trained athletes or even very dedicated exercisers, but rather, recreationally active adults who were likely more responsive to training and dietary manipulation.
If you’re looking to add something new to your exercise routine — something that will possibly yield metabolic and performance benefits — “sleep low” might be worth a shot.
I do not see any apparent negative effects of this strategy because, again, it doesn’t require any type of dietary restriction. In theory, you could maintain or increase your energy and carbohydrate intake while also restricting carbohydrates after 4 p.m. The good thing is that you could try this once, twice, or maybe three times a week rather than every single day.
Having a consistent diet is something that helps me perform well. But there’s always room to test new dietary tools, especially those that have some science to support them.
Thanks for reading. See you next Friday.
~Brady~
The VO2 Max Essentials eBook is your comprehensive guide to aerobic fitness, how to improve it, and its importance for health, performance, and longevity. Get your copy today and use code SUBSTACK20 at checkout for a 20% discount. You can also grab the Kindle eBook, paperback, or hardcover version on Amazon.
Examine.com: Examine is the largest database of nutrition and supplement information on the internet.
I had the same concern. Isn’t loss of lean mass a problem?
> the nighttime carbohydrate restriction group lost 1.4 kg of body mass after the intervention, however, they also lost nearly a kilogram of lean body mass, meaning nearly all of their weight loss was lean body mass.
This strikes me as highly undesirable, and a reason not to "sleep low" regardless of VO2 benefits. Can you chime in on that point?