By Joseph Agu (MSc) | Nutrition Consultant
The topic of fasted vs. fed cardio with the aim of fat loss has been a trending topic recently, both in the fitness industry as well as academia. As such, the aim of this article is to highlight our understanding to date surrounding this issue, dispel some key misconceptions regarding it, and conclude with practical recommendations.
It is worth mentioning that though the potential benefits of manipulating food intake around exercise involve factors other than just fat loss (e.g. training adaptations, markers of health and disease etc.), such topics are beyond the scope of this article.
Rationale for fasted cardio for fat loss
A common strategy among those competing in aesthetic sports (e.g. bodybuilders, fitness competitors etc.) and those competing in weight class sports (e.g. boxing, wrestling, judo etc.) is to perform cardiovascular exercise after an overnight fast, waiting until after the exercise bout to consume breakfast.
The basic premise for this practice is that low levels of glycogen (and/or glycogen depletion during the exercise bout itself) and insulin, shift energy utilisation away from carbohydrate for fuel, thereby allowing greater mobilisation of stored fat that can be used for fuel (fat oxidation).
What does the research say?
Acute effects of fasted vs. fed exercise on fat oxidation
During the exercise bout
Results from several acute studies (Spriet & Watt, 2003; Horowitz et al., 1999) support the idea that exercise in a fed state can result in a decrease in fat oxidation during the exercise bout due to a reduced entry of long chain fatty acids into the mitochondria.
These observations in a fed state have been attributed to several factors, including:
✓ The already mentioned insulin-mediated attenuation of adipose tissue lipolysis (Coyle et al., 1997)
✓ An increased glycolytic flux (Horowitz et al., 1999)
✓ Decreased expression of genes involved in fatty acid transport and oxidation (Civitarese et al., 2005).
That said, things aren’t so straightforward, as several studies have demonstrated that exercising in a fed state has little to no effect on fat oxidation during the exercise bout. Indeed, Coyle’s research group has twice demonstrated that carbohydrate ingestion during moderate-intensity (65-75% VO2max) does not reduce fat oxidation during the first 120 min of exercise in trained men (Coyle et al., 1983; Coyle et al., 1985).
Fasted vs Fed Cardio | Studies
Furthermore, Horowitz and colleagues (1997) studied the fat burning response of six moderately trained individuals in a fed vs. fasted state to different training intensities. The participants each underwent four exercise conditions that involved two hours of cycling.
During two of the trials, they consumed a high-glycemic carbohydrate meal at 30, 60, and 90 minutes of exercise, once at a low intensity (25% peak oxygen consumption) and once at a moderate intensity (68% peak oxygen consumption).
✓ Subjects performed the other two conditions at the same exercise intensities but this time following a 12-14 hour fast.
✓ Results in the low-intensity trials showed that although lipolysis was suppressed by 22% in the fed state compared with the fasted state, fat oxidation remained similar between groups until the 80–90-minute mark (after this point greater fat oxidation was observed when fasted).
Conversely, during moderate-intensity cycling, fat oxidation was not affected by during exercise feeding at any time. This occurred despite a 20–25% reduction in lipolysis and plasma free fatty acid (FFA) concentration.
In another study involving endurance-trained subjects, Febbraio et al (1985) looked at the effect of pre-exercise and during-exercise carbohydrate consumption on fat oxidation. In a crossover design, seven participants cycled for 120 minutes at approximately 63% of peak power output (the intensity at which maximum fat oxidation occurred).
✓ Results showed no evidence of impaired fat oxidation associated with consumption of carbohydrate either before or during exercise, despite elevated insulin levels in the fed conditions.
✓ It seems that such discrepancies in findings are largely due to the training status of the subjects involved in each trial, with the effects of fed status on fat oxidation diminishing in higher trained individuals. It is possible that in endurance-trained individuals, significantly more fat is broken down than that the body can use for fuel.
FFAs that are not oxidised ultimately become re-esterified in adipose tissue (basically meaning that the unused fatty acids are reincorporated into fat tissue), thereby nullifying any lipolytic benefits afforded by pre-exercise fasting.
Though data from the above studies give some insight into answering the question of this article, the effects of feeding or not on subsequent substrate use during the exercise bout are limited to the exercise bout. Rather, looking at substrate use over a 24-h period around the exercise bout would give us a greater insight into the effects of pre-exercise feeding on fat loss.
Fasted vs Fed Cardio | 24-h fat oxidation
A popular study cited by those when this argument comes up is a paper by Paoli et al (2011), titled Exercising Fasting or Fed to Enhance Fat Loss.
In this study, the authors compared the effect on oxygen consumption (VO2) and substrate utilisation (estimated by the respiratory exchange ratio (RER)), in eight men who performed the same moderate-intensity training session (36 min of running on treadmill at 65% maximum heart rate) in the morning in either a fed or fasted state.
✓ Cardio after breakfast increased both VO2 and RER significantly (4.21 vs. 3.74 and 0.96 vs. 0.84, respectively).
Twelve hours after the cardio session, VO2 was still higher in the fed condition, whereas RER was significantly lower, indicating greater fat utilisation.
This shift towards greater lipid use in the fed condition was still significant at the 24-h mark. A notable limitation of this study, however, was that dietary intake wasn’t strictly controlled, though individual subjects food diaries “demonstrated substantial similarity” between trials.
Furthermore, having VO2 and RER measures at more regular intervals may have painted a better picture of substrate use over a 24-h period.
Does training at different times of the day affect fat loss results?
A more recent, strictly controlled study by Iwayama and colleagues (2014) sought to determine whether the time of day that one exercises affects 24-h fat oxidation in a group of nine, young endurance athletes. The subjects performed 100 minutes of treadmill running in a metabolic chamber under three conditions:
1) Before breakfast [AM]
2) After lunch [PM]
3) 50 minutes before breakfast and 50 minutes after lunch [AM/PM].
In addition, meals were provided to meet individuals’ energy balance. Building on the findings of their earlier work (Shimada et al. 2013), the authors observed that although 24-h energy expenditure was similar between trials (signifying no changes in energy balance), 24-h fat oxidation was higher in the AM group compared with the AM/PM and PM groups (1142, 809 and 608 kcal, respectively) .
According to the authors, this higher fat oxidation in the AM group compared with the PM group was due to a transient energy deficit during the AM trial. In other words, in the three hours following exercise, the AM group burned more Calories than the PM group (330 vs. 280 kcal, respectively).
✓ Subsequently, because of the greater glycogen depletion in the AM group, it seems likely that this led to greater levels of fat oxidation. That said, the greater initial energy deficit was nullified by the 24-h mark, demonstrating that there was no residual beneficial effect of fasted training on energy balance.
✓ This data is interesting and in stark contrast to previous research that has either found no difference, or a more beneficial effect, for fed exercise on subsequent energy expenditure, probably due to a greater thermic effect of feeding on subsequent exercise (Lee et al., 1999; Davis et al., 1989; Goben et al., 1992).
Even if we choose to ignore the weight of the evidence regarding an increase in exercise thermogenesis in a fed state, the fact that was a transient energy deficit observed for fasted cardio in the study of Iwayama et al is of little significance given a lack of difference over 24-h.
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Take Home Message
In summary, despite a logical rationale for superior effects of fasted cardio for fat loss when compared to that performed in a fed state, acute oxidation data as well as longer-term trials have failed to support it.
Instead it would appear that there is little to no difference between fasted vs. fed exercise for the goal of fat loss. As such, if fat loss were the aim, my recommendations would simply be to perform the type of cardio that you prefer and can consistently do on a regular basis!