Scientists reveal that shifting mealtimes, without any workouts, can help reshape the body as early eaters lost more weight, but late eaters boost their anerobic power.
Study: Time-Restricted Eating Without Exercise Enhances Anaerobic Power and Reduces Body Weight: A Randomized Crossover Trial in Untrained Adults. Image credit: DenisProduction.com/Shutterstock.com
Time-restricted eating (TRE) and physical exercise could enhance physical performance. A recent study in Nutrients examined the impact of TRE on endurance parameters and body weight in healthy adults without any specific exercise intervention. This is the first time a human study has directly compared delayed and early TRE in an untrained healthy group without formal exercise interventions.
Introduction
TRE refers to a pattern of eating in which the day’s food is consumed within a specific window. TRE, by itself, often reduces caloric intake by around 20%. Animal experiments suggest that TRE improves certain aspects of physical performance even without exercise, but human evidence remains limited and mixed.
Physical performance and metabolic fitness are typically assessed using aerobic endurance and anerobic power. These predict the general health status and the future risk of cardiometabolic disease. Anerobic power is also linked to higher odds of physical function and independence with aging.
Besides exercise strategies like aerobic or resistance exercise, or high-intensity interval training, dietary approaches like TRE also play a key role in building physical capacity. Research from both preclinical and clinical studies points to three main underlying mechanisms.
First, TRE induces regular, prolonged fasting periods, causing a metabolic shift. This forces the body to shift from using glucose for fuel to fatty acids and ketones, promoting metabolic flexibility. In addition, TRE has been associated with increased mitochondrial production in animal studies. Mitochondria supply more energy by breaking down fats into fatty acids and ketones. The aim is to optimize energy efficiency.
Secondly, TRE favors a synchronized biological rhythm across the body systems, since food intake is now confined to the same period across the 24-hour day. This leads to a faster response to insulin secreted after food intake. Insulin stimulates glucose removal from the blood, mainly into muscle and liver cells. Better circadian synchronization also reduces inflammation, improves skeletal muscle efficiency, and promotes cardiometabolic health, especially by avoiding late-night eating.
Autophagy is the third pathway activated by TRE. Cell repair is prioritized instead of cell growth. This leads to removing damaged organelles, including mitochondria, and replacing them with new ones. Animal studies show this occurring in skeletal muscle (which has a high metabolic rate) in response to fasting. The outcome is improved mitochondrial quality and greater antioxidant capacity. However, the authors caution that much of this mechanistic evidence comes from animal studies, with only indirect support in humans.
TRE in overweight or obese people results in decreased body weight, improved blood cholesterol levels, and higher insulin sensitivity. However, the benefits of TRE in highly trained individuals could be modest, as they already have high metabolic fitness. It might be more helpful for untrained individuals without added exercise because they have low fitness levels. The current study used two TRE timings to understand how this affects metabolic outcomes.
About the study
The study included 28 healthy university students, on average, aged 23.4 years. Just over half were women. The participants were randomized to early vs. delayed TRE (eTRE and dTRE, respectively) for four weeks, after which there was a two-week interval. The groups were then switched for another four weeks.
The eTRE timing was between 08:00–09:00 for the first meal and 14:00–15:00 for the last meal, vs 12:00–13:00 to 18:00–19:00 for dTRE. No formal exercise routine was allowed during the study period. After each TRE phase, the body weight, aerobic endurance, and anerobic power were assessed.
Aerobic endurance refers to how long the body can sustain oxygen-fueled physical activity, typically tested by a constant-load cycling test. Anerobic peak power is the maximum force produced without oxygen in a burst of high-intensity effort, sustained for only a few seconds. It is typically tested by sprint cycling, which also allows peak power output to be measured.
Study findings
The mean body weight decreased by approximately 1.6 kg after the eTRE, vs 0.61 kg in the dTRE group. Body weight decreased after both TRE phases, but by 0.95 kg more with eTRE than dTRE.
This corroborates prior research showing that early eating windows are more effective than later ones. This may be due to TRE-associated decreased calorie intake due to an absence of dinner, typically a more calorie-packed meal than breakfast. Additionally, an earlier eating window is better synchronized with the circadian rhythms of digestion and insulin sensitivity. This could reduce fat mass and lower glucose levels. However, the current study did not directly measure changes in body composition, so whether fat or lean mass contributed to the weight loss is unknown.
Aerobic endurance remained unchanged with either form of TRE, despite decreased calorie intake, and without the benefit of prior exercise training. This may reflect a more efficient utilization of energy substrates under restricted eating conditions, but with constant physical demands. This metabolic flexibility favors aerobic fitness, irrespective of the eating window.
Anerobic power increased by 21.25 W vs 35.4 W with eTRE and dTRE, respectively. Power output also increased more after dTRE than eTRE. This may be because the dTRE window correlates better with the body's neuromuscular performance rhythm, where high-intensity muscular work is best done in the afternoon or early evening.
Such gains were not previously visible in trained athletes, perhaps because they are already at the top of their form physiologically. Conversely, other research demonstrates TRE-associated metabolic adaptations that help the body regain energy after short bursts of explosive activity by increasing mitochondrial biogenesis and efficiency.
Conclusions
The study findings demonstrate that eTRE and dTRE are associated with weight loss and increased anerobic power, while aerobic endurance was preserved rather than improved. Weight loss was greater with eTRE vs dTRE, but the opposite was true of anerobic performance gains.
Future studies should examine whether these translate into clinical gains that are sustained over the long term and are suitable for all kinds of groups.
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