Is it better to do aerobic exercise fasted?

Interventions

Appetite

Exercise is inherently linked to less food intake, possible related to less responsiveness to food cues (as behaviours do not change).^[1]

In otherwise healthy normal weight males, running for an hour on a treadmill at 70% VO2 max compared to either fasted or after a standardized test meal (30% daily energy intake and mostly carbohydrate) noted that fed exercise suppressed appetite to a greater degree than fasted (both more effective than control) but there was no significant differences in whole-day food intake.^[2]

Although exercise per se has a significant effect on appetite, there does not appear to be a practical difference between fasted and fed training in the AM when food is consumed afterwards and measured for 24 hours.

Exercise Performance and Fitness

In comparing fasted versus fed training, there is no significant difference in physical output during 70% VO2 running on a treadmill nor the rate of perceived exertion.^[2] A lack of difference in RPE with non-maximal cardiovascular exercise has been noted elsewhere.^[3]

Over time, fasted training may underperform peri-workout carbohydrate for improving VO2 max (training adaptations),^[4] although this is somewhat contested as another study with similar results found no significant difference.^[5] The former study (fasted training underperforming fed training) is thought to be secondary to carbohydrates inherently having a performance enhancing effect for near-maximal exercise;^[6] a benefit that would not be apparent in studies using submaximal exercise.

In submaximal exercise, there may be no differences at all between fasted and fed training. During maximal exercise attempts (which may include intense resistance training or high intensity interval training) being fed with carbohydrates appears to be more beneficial than fasted training for the purpose of enhancing adaptations to training.

Energy Expenditure and Weight

There is no significant difference in energy expenditure between fasted and fed training as assessed by treadmill running, although RER was lower in fasted (indicative of more fatty acid utilization; fed state consumed a large amount of carbohydrates).^[2] This increase in fat utilization may underlie the anti-lipidemic benefits of exercise (normalizing the increase of blood lipids that occurs with excess dietary intake of lipids), which does not occur when carbohydrates are preloaded^[7] nor is it related to the Glycemic Index of the carbohydrate source.^[8] Indirectly, since there are no changes in metabolic rate while a greater percentage of energy comes from fatty acids this reduces glucose oxidation and attenuates the rate of glucose drops induced by endurance exercise (risk of hypoglycemia) when measured over 6 weeks.^[5]

RER and subsequent percentage of lipids being used as fuel seems to be highly correlated to exercise-induced fat loss over time,^[9] and although the intensity that uses most fatty acids as a percent is unchanged fasted training over 6 weeks (relative to carbohydrate preload) enhances how much fatty acids the body adapts to utilize (21% increase over 6 weeks relative to a 9% increase; with exercise per se increasing this capacity).^[5] This increase in fatty acid usage occurs when exercise is performed in a fasted state^[5] and ingestion of a carbohydrate beverage prior to exercise after such a protocol normalizes the differences between groups.^[10] 

The decrease in RER has once failed to be any different between a carbohydrate preload and the fasted state in well trained individuals.^[11]

Fasted exercise results in a greater percentage of calories being derived from fatty acids rather than carbohydrates; this is independent of any actual changes in metabolic rate or work capacity (which, for submaximal exercise, appear unchanged).

One study comparing prolonged cardio exercise in the morning in either a fed or fasted state over 50 days noted that although both groups resulted in a loss of body fat, fed resulted in more weight loss (2.6%) while fasted resulted in less overall weight loss (1.9%) while was able to reach a statistically significant reduction in body fat loss (6.2%) relative to no significant change in fed cardio.^[3] These were independent of any differences in rate of perceived exertion or duration of exercise, but a large confound in this study is that it was conducted during Ramadan (and the fed group, due to eating in the morning, ate 600 more kcal daily for the first half of Ramadan).^[3]

One other study comparing 6 weeks of fasted or carbohydrate preload training failed to note any significant differences in fat mass or body mass in physically active males given a standardized diet.^[5]

Mixed results on whether fasted training can induce more fat loss over a period of time (from selective usage of fatty acids over glucose), with the one controlled study showing promise also having significant differences in food intake.

Fasted training, independent of caloric intake, may not confer any additional fat burning or weight loss effects over a period of time despite increase percentage of fatty acids being used as fuel.

Muscle Mass

Acutely, muscle protein breakdown is enhanced in response to aerobic exercise.^[12] and over time increases the overall rates of protein turnover.^[13]

Lack of differences in skeletal muscle mass may be a result of muscle cells appearing to 'prime' themselves for anabolism in response to fasted training, a conclusion derived from a study noting that carbohydrate-fed endurance training was associated with chronically activated eEF2 and fasted training associated with more responsiveness to eEF2 activation despite not inherently being activated;^[14] eEF2 being an intermediate in protein synthesis.^[15] A reduction of the activity of both anabolic and catabolic genes has been noted in response to caloric pre-loads.^[16]

Fasted training appears to enhance both skeletal muscle breakdown and prime skeletal muscles for apparent anabolism on a genetic level; overall turnover is upregulated in response to fasted training, and to a lesser degree in fed training (although turnover is enhanced per se in response to exercise).

50 days of fasted cardio (relative to the same cardio exercise in a fed state) did not result in any significant differences in lean body mass, despite consuming 600kcal less than the fed state cardio; both groups remained relatively constant.^[3]

Studies that assess lean mass over time and compare fed to fasted aerobic training really do not note any differences in muscle mass chronically.