Athletes have long obsessed over what to eat, but emerging science points to a different question: when should they eat? A systematic review of 18 studies examining time-restricted eating (TRE) in athletic and active populations reveals early promise that limiting daily food intake to specific windows may trigger beneficial metabolic changes. However, the team of Spanish researchers emphasizes that the evidence remains scarce and that more studies are needed.
Time-restricted eating, also well-known as intermittent fasting, involves condensing all daily meals into a defined period, typically between four and 12 hours, with the remaining hours spent fasting. The review, published in Revista Española de Nutrición Comunitaria, found the most commonly studied approach was the 16:8 protocol, where eating occurs within an eight-hour window and fasting extends for 16 hours. While most TRE research has focused on weight loss, some studies suggest that there may be deeper cellular effects.
One small study of 11 non-athletes following early TRE (eat early, finish by mid-afternoon) for just four to five days found increased expression of molecular markers involved in cellular maintenance and aging regulation. Researchers observed elevated levels of SIRT1, a gene involved in regulating aging processes, and LC3A, a protein that helps form autophagosomes. These microscopic structures capture and remove aged or damaged cellular components in a process called autophagy, the body’s internal recycling system.
The same short-term study found increases in brain-derived neurotrophic factor during the night, a molecule that promotes neuronal growth and survival. These signals are consistent with autophagy pathways but are not direct measures of biological age or lifespan, and were observed after just 4–5 days in non-athletes. Whether similar effects occur in athletes during longer-term eating patterns remains unknown.
Among elite cyclists in a four-week randomized controlled trial, time-restricted eating showed short-term benefits. Athletes lost approximately 2% of body weight and showed an 11% decrease in resting energy expenditure, alongside a roughly 4% improvement in maximum power output relative to body mass. Fat mass dropped while lean muscle remained stable, with a notable reduction in inflammatory markers. During winter preseason training, when intense exercise typically suppresses immune function and leaves athletes vulnerable to illness, this protective effect becomes particularly valuable. Both TRE and normal eating groups showed equal preservation of fat-free mass and athletic performance.
Athletic Performance Stays Intact
Athletes and coaches might reasonably worry that restricting eating times would hamper training or competition results. Across multiple studies lasting three to 98 days and spanning 11 to 271 participants, TRE either maintained or improved athletic performance measures, though most evidence comes from recreational athletes rather than elite competitors.
Research on physically active women combining high-intensity interval training (HIIT) with TRE showed a 1% reduction in body fat alongside improvements in jump performance. Another study of 12 physically active individuals found that after four weeks, participants increased their total work output and fat-free mass while reducing waist circumference. The researchers noted this improvement represented roughly a one-second difference that could influence competitive outcomes.
A study examining eating within just two hours daily in 11 individuals over a short crossover period found no decline in strength, maximum power output, or aerobic capacity. When combined with simultaneous strength and endurance training in people with overweight or obesity, the 16:8 protocol reduced body mass and decreased fat mass by 9% while increasing lean muscle and muscular strength in knee flexion and ankle dorsiflexion.
Metabolic Benefits Beyond the Scale
Beyond performance metrics, intermittent fasting appears to shift metabolic patterns and daily glucose control in ways that could protect long-term health, though most of this evidence comes from non-athletic populations. Studies in people with overweight, obesity, or Type 2 diabetes consistently showed improved glucose control throughout 24-hour periods, with reduced nighttime glucose levels and decreased post-meal blood sugar and insulin spikes.
One experiment in non-athletes compared eating dinner at 6 p.m. versus 9 p.m. Earlier dinner timing reduced blood sugar response following the evening meal and lowered glucose levels from 6 p.m. until 6 a.m. the next morning. Benefits extended into the following day, with increased fat burning after breakfast when dinner had been consumed earlier.
In individuals with Type 2 diabetes, three weeks of eating within a 10-hour window improved glucose balance without changing insulin sensitivity. While 24-hour energy expenditure remained stable, the shift in fuel use was notable: carbohydrate burning decreased while fat burning increased. Participants spent more time in normal glucose ranges and less time with elevated blood sugar, without significant increases in low blood sugar episodes.
Studies in people with non-alcoholic fatty liver disease also documented favorable changes in blood lipids. A 5:2 intermittent fasting protocol, where participants severely restricted calories two non-consecutive days per week, led to reductions in total cholesterol and LDL cholesterol, with particularly pronounced decreases compared to continuous calorie restriction. In one trial, HDL cholesterol decreased only in women. Some time-restricted eating studies showed increases in both LDL and HDL cholesterol in morning measurements, though without corresponding rises in triglycerides or free fatty acids.
Fat Loss in Athletes and Non-Athletes
Weight loss wasn’t universal across studies, but fat mass reduction was a consistent theme. Six studies documented significant decreases in fat mass among both athletes and non-athletes following various time-restricted eating protocols. Lean muscle mass either remained stable or increased in most cases.
Research comparing 16:8 time-restricted eating to continuous calorie restriction in 65 individuals with metabolic syndrome found greater fat mass loss in the time-restricted group without negative impacts on nutritional adequacy or dietary balance. Both groups improved body fat percentage, lean mass, BMI, and waist-to-hip ratio.
Time-restricted eating by itself reduced energy intake by 10-20% in physically active women without explicit calorie counting. This spontaneous reduction in consumption may partially explain the body composition changes, though metabolic shifts in fat burning likely play a role as well.
Hormonal Changes and Hunger Patterns
Time-restricted eating appears to recalibrate hormonal signaling related to hunger and fullness, based largely on research in non-athletic populations. Following Ramadan fasting (approximately 14 hours daily for 30 days), men with obesity showed increased leptin levels, the hormone signaling fullness, with no significant change in ghrelin, the hunger hormone. However, levels of certain gut hormones decreased after the fasting period, including glucagon-like peptide-1, peptide YY, and cholecystokinin.
In elite cyclists, TRE increased adiponectin levels, a hormone that facilitates fat loss and improves insulin sensitivity. The protocol also reduced anabolic hormones without affecting fat-free mass or endurance performance.
Cortisol patterns shifted with eating time restrictions as well. One study in people with overweight showed increased morning cortisol and reduced evening cortisol with time-restricted eating, though meal timing itself didn’t affect melatonin levels or overall circadian phase.
Across studies, participants reported lower hunger ratings, reduced food consumption from lunchtime onward, and decreased nighttime snacking. Participants described better feelings of well-being and positive attitudes toward time-restricted eating approaches.
Real-World Feasibility
Dietary interventions like intermittent fasting only work if people can stick with them. Time-restricted eating showed variable adherence rates across studies. While one 12-week study in people with non-alcoholic fatty liver disease reported 97.5% adherence, another 14-week trial in physically active individuals found adherence rates below 70%. Among the high-adherence group, 90% of participants felt the intervention was worthwhile and 95% would recommend it to others, with only 15% reporting barriers to implementation.
While some studies reported minor undesirable side effects, these were generally infrequent. Most participants tolerated the eating patterns well, though the wide range in adherence rates suggests individual responses vary considerably.
What the Evidence Can and Cannot Tell Us
The review encompassed studies with 11 to 271 participants and intervention periods ranging from just three days to 98 days. Only four of the 18 studies examined athletic populations specifically, with the remainder involving healthy individuals or those with overweight, obesity, Type 2 diabetes, multiple sclerosis, metabolic syndrome, or non-alcoholic fatty liver disease.
The heterogeneity in study populations, exercise protocols, and fasting approaches makes definitive conclusions challenging. Short follow-up periods mean long-term effects remain unknown. Small sample sizes in many trials limit statistical power.
Only one study directly measured markers of autophagy and aging processes, and that study lasted just four to five days in non-athletes. The authors conclude that evidence for anti-aging effects remains scarce and requires substantially more research. Most metabolic and hormonal data comes from non-athletic populations, raising questions about whether the same benefits extend to highly trained individuals with different metabolic baselines.
Despite these limitations, the pattern emerging across diverse study designs points to time-restricted eating as a feasible approach for improving health markers and body composition without compromising physical performance. For strength and power athletes, who research shows may face increased risk of obesity and diabetes in post-competitive life compared to endurance athletes, time-restricted eating might offer preventive value worth exploring.
Source : https://studyfinds.org/can-intermittent-fasting-help-athletes-age-better/