Cardiovascular exercise or simply “cardio” is a common component of many individual’s fitness regimens. Cardio exercise helps a person burn more calories and along with it fat calories, which in turn support the achievement of a leaner and/or lighter body. Performing cardio exercise at a lower intensity (e.g. 55-65% maximal heart rate) is called the “fat- burning zone” because it would allow for a higher percentage of the calories burned during exercise to come from fat.(1,2) Meanwhile, when the body is unfed for several hours (“fasted”), such as during the night the percentage of total calories burned coming from fat is high as well.(3-5) Thus “fasted cardio” would seem like a viable strategy to maximize incremental fat burning. While some acute research suggests that morning, fasted cardio might indeed help increase daily fat burning as a percentage,(6,7) other research suggests that any fat-burning gains (% of calories during exercise) made in fasted cardio are diluted out over a 24-hour (% fat over the entire day) when calories are balanced for weight maintenance.(8) Moreover, other research is not as supportive especially when you look at the impact of this strategy over several weeks when calorie intake would allow for reductions in body weight.(9)
While researchers sort through the finer details of fasted cardio on fat metabolism, an important consideration is the impact of this strategy on muscle protein. That’s because muscle protein balance is in a net negative state.(10) Said differently, more protein is being broken down versus the amount of protein being made. Without a protein source, this continues into the morning, and, consequently, you are losing more and more muscle. Daily muscle protein balance is the algebraic sum of muscle protein synthesis (MPS) (+ factor) and muscle protein breakdown (MPB) (- factor). The former is maximized by protein intake and resistance exercise while the latter is maximized by fasting and inactivity.
Net Daily Muscle Protein Balance = MPS - MPB
Since, the effect of dietary protein to increase MPS is acute, transient and potentially capped, by waiting longer in the morning to nourish muscle with amino acids from diet protein seems counter to most fitness, performance and muscle building goals. So the big question is, can you take in a MPS-maximizing level of protein in the morning a short time prior to cardio exercise and reap the benefits on MPS without disrupting fat burning operations during cardio?
In a new study presented at the International Society for Sports Nutrition (ISSN) 2016 Conference,(11) Dr Chad Kerksick of Lindenwood University had young men perform 30 minutes of fasted cardio (55% max heart rate) a short time after drinking either a:
1) whey protein shake
2) casein protein shake
3) carbohydrate starch (maltodextrin)
4) flavor matched non-caloric fluid (fasted)
In these trials, protein was provided at 25 grams for the whey and casein trials, calorie matched for the protein and carb trials and fluid volume matched for all trials. Further, participants completed all four conditions at the same time each morning and were instructed to follow identical pre-exercise testing conditions. Dr Kerksick and team reported that protein appeared to increase calories expended during exercise and while the percentage of those calories derived from fat decreased a little (not significant), the estimated quantity of fat burned was similar to the fasted (non-caloric) trial. Furthermore, shortly after the exercise was complete, metabolism remained higher in the protein trials and was more favorable to fat burning. Similar measures were reported by researchers at the University of North Carolina, whereby whey protein increased total metabolism during and after three different training modalities and allowed for higher fat utilization after exercise was over.(12)
Based on the information at hand it would seem that during times whereby we haven’t eaten protein for several hours and planning a lower intensity cardio session it would be beneficial to consume a little whey protein (e.g. 25 g). This would allow for a maintenance of a daily protein intake (e.g. >1.75 g/kg target body weight) using a protein-pacing strategy (every few hours) while supporting fat burning during exercise. This would also change the description from fasted cardio to perhaps anabolic cardio whereby muscle enters the cardio session in a net positive (anabolic state).
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