The usual
guideline for general health is 30 minutes of moderate exercise about 5 days
per week, even if no afterburn effect is produced. And eating a banana or an
apple after you workout won’t make you gain weight!

The so
called “afterburn effect” postulated to be induced by physical exercise, i.e., whether
the metabolism speeds up for hours after exercise, is an old question that has
been investigated and scrutinized for a century now, with conflicting results.
Some investigators theorized that no such effect exists, while others reported negligible
effects (one found male triathletes burned barely 12 to 30 extra calories after
a workout). Others found as many as 700 additional calories were burned after a
long strenuous workout.

In a
recent paper published in the journal Medicine & Science in Sports
& Exercise, it was demonstrated that after the study subjects (all men) exercised
on a stationary bicycle at a high intensity for 45 minutes, the exercise itself
burned about 420 calories, but over the following 14 hours, the men burned an
extra 190 calories, which is equivalent to an extra 37% of additional burning
of calories (baseline burning of calories while sedentary was 2400 Calories in
these individuals). The authors of this article explained this finding as being
related to the fact that the exercise was so intense. The subjects had had to
cycle at 70% of their VO2 max for 45 minutes, which is the maximum amount of
oxygen a person’s body can take in during exercise.

Another
study using the same measurement technique but which tested the effects of
moderate exercise, documented no afterburn effect. The subjects of this study exercised
at 50% of their VO2 max.

An
evaluation of properly conducted studies concludes that as the duration and
intensity of exercise increases, the afterburn effect becomes even more
substantial.

A recent publication
found that if subjects ran at 70% of their VO2 max or cycled at 75% of it, they
could burn 300 to 700 extra calories after the exercise was over, with 700
calories being rather an exceptional achievement and hence unusual.

Part of
the afterburn effect may be explained by post-exercise energy metabolism: the
body starts using more fat and less carbohydrate after a hard exercise session.
The oxygen system (related to VO2 max) can use
a variety of different energy sources, including protein, although carbohydrate and fat are the primary ones. The carbohydrate is
found as muscle glycogen, liver glycogen, and blood glucose. The fats are
stored primarily as triglycerides in the muscle and adipose cells. The absolute
work rate determines the total quantity of fuel required, while relative
exercise intensity plays a major role in determining the proportions of
carbohydrate and fat burned by the working muscles. As one does mild to
moderate exercise, blood glucose and fat may provide much of the needed energy.
However, the transfer of glucose and fat from the vascular system to the muscles
becomes limited, and you soon begin to rely more on your intramuscular stores
of glycogen and triglycerides. As you continue to increase your speed or
intensity, you begin to rely more and more on carbohydrate as an energy source.
Apparently the biochemical processes for fat metabolism are too slow to meet
the increased need for faster production of ATP, and carbohydrate utilization
increases. The major source of this carbohydrate is muscle glycogen. As such,
there will be transition from use of fat to carbohydrate as the primary fuel
source during increasing intensity of exercise.

Gender, hormones, state of training,
composition of the diet, time of eating prior to competition, nutritional
status, nutrient intake during exercise, environmental temperature, and drugs
are some of the more important considerations. Several
hormones that are released during exercise remain elevated in the blood
afterward, increasing metabolism. And extra calories may be burned when the
body replenishes glycogen, the sugar stored in muscles. But for the most part,
the effect remains a mystery.