In the summer months I am on my bike almost every day, but in winter it’s the elliptical in the basement for my cardio. I don’t do a lot, just 30min, while watching something or other on netflix or hulu (just finished my Buffy rewatch, in fact). I recently have resolved to try and do my cardio to the morning before the kids wake up, which then raised the question of whether it is better to do it before or after breakfast. A couple of years ago, a NYT article made the case that you burn fat faster when fasting:
Only the group that exercised before breakfast gained almost no weight and showed no signs of insulin resistance. They also burned the fat they were taking in more efficiently. “Our current data,” the study’s authors wrote, “indicate that exercise training in the fasted state is more effective than exercise in the carbohydrate-fed state to stimulate glucose tolerance despite a hypercaloric high-fat diet.”
At the same time, the fasting group showed increased levels of a muscle protein that “is responsible for insulin-stimulated glucose transport in muscle and thus plays a pivotal role in regulation of insulin sensitivity,” Dr Hespel said.
In other words, working out before breakfast directly combated the two most detrimental effects of eating a high-fat, high-calorie diet. It also helped the men avoid gaining weight.
The study quoted by the article has some differences from my situation – it was a deliberately high-intensity workout, and the subjects were eating a high-fat, high-calorie diet. So would the same advantages hold for me, whose cardio workouts are light to moderate, and with a pretty balanced diet? The conventional wisdom now seems to be that working out while fasting in the morning does let you burn fat more efficiently.
However, an article at bodybuilding.com a year later says otherwise. To be blunt, the biochemical explanations sound as hand-wavy as everything else I ever read in these health magazines. But here’s the key argument:
True, the research does show that fasted cardio can increase fat utilization during exercise compared to performing cardio in the fed state. Except this only occurs at very low levels of training intensity.
During moderate-to-high intensity levels, the body continues to break down significantly more fat when fasted compared to after you’ve eaten.
So far, so good. Unfortunately, the rate of breakdown exceeds your body’s ability to use the extra fatty acids for fuel. In other words, you have a lot of extra fatty acids floating around in the blood that can’t be used by working muscles.
Ultimately, these fatty acids are repackaged into triglycerides post-workout, and then shuttled back into fat cells. So you’ve gone to excessive lengths…only to wind up at the same place.
Horowitz and colleagues (2) found that when trained subjects exercised at 50 percent of their max heart rate, an intensity that equates to a slow walk, there was no difference in the amount of fat burned–regardless of whether the subjects had eaten.
These results held true for the first 90 minutes of exercise; only after this period did fasted cardio begin producing a favorable shift in the amount of fat burned.
So unless you’re willing and able to slave away on the treadmill for a couple of hours or more, fasted cardio provides no additional fat-burning benefits, irrespective of training intensity.
Fasted cardio makes even less sense when you take into account the impact of excess post-exercise oxygen consumption. EPOC, commonly referred to as the “afterburn,” represents the number of calories expended after training. Guess what? Eating before exercise promotes substantial increases in EPOC (3).
And guess where the vast majority of calories expended in the post-exercise period come from? You got it, fat!
On top of everything, fasted cardio can have a catabolic effect on muscle. Studies show that training in a glycogen-depleted state substantially increases the amount of tissue proteins burned for energy during exercise (4).
Protein losses can exceed 10 percent of the total calories burned over the course of a one-hour cardio session — more than double that of training in the fed state (5).
The article summarizes all of this as: At best, the effects on body composition won’t be any better than if you trained in a fed state; at worst, you’ll lose muscle and reduce total fat loss.
So, I think I’ll eat first and workout after. But none of this helps me actually get out of bed an hour earlier to do the workout before the kids get up!