(versión en español: pinchar aquí)

A 2010 Study found out that although you burn (oxidize) fat while exercising, if you measure the total fat burnt during 24-h, the results say you are not burning more fat than on an non-exercise day. In most cases you burn less. I present here a few interesting quotes from that study:

«To reduce fat mass, a state of negative fat balance must be achieved«.

Would you mind reading again the sentence in red, please? FAT BALANCE, not energy balance… Isn’t that obvious?

«However, ingesting as little as 60 g (240 kcals) of carbohydrate during the hour before exercise can reduce lipolysis and fat oxidation during subsequent exercise [3], and the increase in fat oxidation during exercise can be blunted for up to six hours following consumption of a meal [21]. This is largely caused by an insulin-induced suppression of lipolysis [7,21], but a decrease in CPT activity has also been suggested [4]. […] Moreover, the endurance training-induced increase in fat oxidation is prevented when a carbohydrate rich diet is consumed [6].»

«Finally, in most of the subjects we have studied, 24 h RQ is slightly higher and fat oxidation is slightly lower on the exercise day compared to the non-exercise day, although on a group level, these differences are not significant. This suggests that the effects of diet in modulating 24 h substrate oxidation are much more potent than the effects of exercise, assuming energy balance.»

Have a look at the bar graph: fat oxidation was basically the same on the non-exercise day (CON) than on the low/high intensity exercise days (LI and HI).

NOTE: be aware that the «solid black» bar (no-exercise day) is not always placed on the left side of the triads.

Let’s have a look now at the «fat balance» of endurance-trained individuals (graph below these lines): seven out of nine had a positive fat balance (they gained fat) on the exercise day (EX). Only three out of nine had a positive fat balance on the sedentary day (LF). Most of them lost more fat on the sedentary day than on the exercise day.

My take on this study: I don’t say physical exercise is bad for you, it isn’t, but exercise won’t help you much reducing your fat mass unless you first unblock fat oxidation. You may try reducing your intake of unnecessary carbs (grains, bread, pasta, etc.) to do that.