How short-term nighttime carb restriction boosts exercise and burns fat

By Dr. Liji Thomas, MDReviewed by Benedette Cuffari, M.Sc.Jul 10 2024

Dietary modifications, such as the sleep-low method, may enhance exercise performance. A new paper in the journal Nutrients examines the impact of this technique on exercise performance.

Study: Effects of short-term night-time carbohydrate restriction method on exercise performance and fat metabolism. Image Credit: Izf / Shutterstock.com

Glycogen storage and endurance training

Nutrition is closely related to athletic performance. Carbohydrate availability, for example, is essential to ensure optimal energy utilization before and during high-intensity and prolonged training.

A limited amount of glycogen can be stored within the body to support high-intensity exercise; therefore, inadequate glycogen reserves impair performance during endurance sports. To build and replenish glycogen stores, athletes should ensure sufficient carbohydrate intake before, during, and after exercise and during muscle recovery to reduce fatigue and maintain sufficient endurance levels.

Comparatively, intermittent carbohydrate restriction has been shown to improve muscle adaptation responses to endurance training. Training with low muscle glycogen stimulates fat oxidation by activating oxidative enzymes; however, how this dietary practice affects exercise performance remains unclear.

The sleep-low method

"The sleep-low method has been proposed as a way to sleep in a low-glycogen state, increase the duration of low glycogen availability and sleep and temporarily restrict carbohydrates to improve exercise performance.”

The sleep-low method involves low-intensity exercise following carbohydrate deprivation for a defined period. This activates adenosine monophosphate kinase (AMPK), subsequently increasing the number of mitochondria. 

Prolonged carbohydrate restriction may cause mental stress, which impairs performance and increases the risk of injury. Nevertheless, improved and easier endurance performance has been reported with the sleep-low strategy. These findings were observed during a three-week trial in which night-time carbohydrate restriction four times a week reduced stress associated with more extended periods of carbohydrate deprivation.

The current study explores the potential efficacy of night-time carbohydrate restriction in achieving these effects. Confirming the effectiveness of the sleep-low method has important implications for obese and overweight individuals interested in losing weight and athletes seeking to improve their exercise performance without the accompanying mental stress and within a shorter time frame.

About the study

The current study included 22 university students who were randomized to their usual diet or carbohydrate-restricted diet. During the first week of the study, both groups continued their normal diets without any dietary restrictions.

Total carbohydrate and caloric intake over the first week were measured, and the daily average was calculated. The mean value was fixed as the daily intake over the second week in both groups, during which the intervention group was instructed to refrain from carbohydrate intake after 4:00 PM.  

All study participants, irrespective of group, completed one hour of running every day before breakfast to attain 65% of their maximum heart rate. Various exercise parameters were assessed, including peak oxygen consumption (VO₂peak), work rate max, respiratory quotient (RQ), body weight, and lean body mass.

What did the results show?

Athletes in the intervention group improved in multiple parameters during the second week compared to those in the control group. Fat metabolism was found to respond well and rapidly to even a short period of carbohydrate restriction at night.

VO₂peak was higher after as compared to before the intervention, with this parameter higher in the intervention group as compared to controls. The work rate max also increased after the intervention.

RQ was reduced, thus indicating greater fat oxidation due to increased supply and utilization of fat in the body. Therefore, training in a low-glycogen environment increased the number of mitochondria, thereby stimulating an increase in free fatty acids in the blood.

Low-intensity exercise in a low-glycogen state triggers multiple kinases like AMPK and other transcription-related factors. These molecules promote adaptation to endurance training by increasing the muscle glycogen reserve at rest.

Body weight was significantly lower in the intervention group after carbohydrate restriction began. Body fat percentage did not vary between the groups at any point during the study period.

In contrast, lean body mass was lower after the intervention. This might be due to muscle protein breakdown, which supplies glucose to the muscles through gluconeogenesis.

Conclusions

The study findings indicate that intensive short-term night-time carbohydrate restriction and exercise before breakfast successfully altered energy metabolism to promote endurance.

The timing and type of dietary components are crucial factors to optimize before implementing the sleep-low approach. For example, protein intake after exercise and leucine supplementation with exercise may prevent muscle atrophy and stimulate muscle protein synthesis, muscle repair, and remodeling.

Additional research is needed to elucidate the metabolic changes that occur following carbohydrate restriction. For example, the resulting decrease in insulin secretion could impair muscle glycogen replenishment.

This method can be used to reduce body weight in individuals with obesity and enhance athletes’ performance.”

The current study did not consider certain important parameters despite their significant impact on body weight. These include the effect of menstrual cycles on the body weight fluctuations in female athletes, the type of sport practiced, and the timing of sports events.