By Dr Adam Collins, Form’s Head of Nutrition and Program Director for the MSc and BSc Nutrition courses at the University of Surrey. A qualified nutritionist for nearly 20 years Dr Collins is also a PhD in energy expenditure and body composition. His research interests lie in exercise nutrition, body composition, exercise intensity and energy balance, intermittent fasting and timing of food around exercise.
So far in this series, we’ve explored how fueling with carbohydrates can affect active pursuits of multiple contexts. Besides the differences between professional athlete and everyman, though, we’re yet to touch on the differences that come into play when we consider body composition. Notably, the distinct differences between men and women.
Men Are From Mars
When considering exercise response, it is interesting to note that men and women are not necessarily the same. Given that men and women are physiologically different in a number of (sometimes obvious) ways, it is not beyond rationality to view them differently when it comes to exercise nutrition.
Morphologically, women have more body fat and less muscle per kg of body weight than men. In addition, there are hormonal (endocrine) differences between genders which partly explains body composition changes but also physiological and metabolic responses.
It has been repeatedly observed that at a given intensity of exercise, women tend to burn proportionally more fat and less carbohydrate compared to men. In other words, men preferentially burn carbohydrates, even at a relatively low intensity of exercise.
This is partly explained by the fact that women have slightly higher levels of non-esterified fatty acids (NEFA) in the blood – specifically in the systemic circulation – as a consequence of greater amounts of subcutaneous (peripheral) adipose tissue, which is insulin sensitive. In addition, men tend to release more catecholamines in response to exercise, which prioritises carbohydrate oxidation during exercise as a fight or flight response as it is quicker to break down to supply energy.
The upshot of this is that, during exercise, men rely heavily on carbohydrate, whereas women are better at preserving carbohydrate. Now, when you consider the evidence from the early work on “training low” (training glycogen-depleted or without carbohydrate pre-exercise), this has been shown predominantly in men.
“It has been repeatedly observed that at a given intensity of exercise, women tend to burn proportionally more fat and less carbohydrate compared to men. In other words, men preferentially burn carbohydrates, even at a relatively low intensity of exercise”
So, it could be argued that carbohydrate depletion would be more pronounced in men following training low due to their reliance on carbohydrate during exercise. That’s not to say that women will not get adaptation too, just perhaps not so pronounced.
Indeed, the research that we have undertaken at Surrey University has consistently shown that exercising without carbohydrate before exercise gives the greatest fat oxidation during exercise, and a noticeable improvement in fat oxidation after training in men.
What’s Best For Women?
The impact of feeding in women is more pronounced in the post-exercise recovery period. For men, the exaggerated catecholamine response coupled with the dominant use of carbohydrate during exercise means that – post-exercise – they have a pronounced switch to burning fat and replenishing carbohydrate. Glucose clearance from the blood is very efficient in men post-exercise, as it is rapidly used to replenish liver and muscle glycogen.
In contrast, women are much better at preserving carbohydrate during exercise and have overall better glucose regulation. This, coupled with a lower catecholamine response, means that they have a less pronounced switch to burning fat in the post-exercise period, and also a slower clearance of carbohydrate (i.e. less dramatic glycogen replenishment). Therefore, delivering lots of carbohydrate in the post-exercise period could actually overload women with glucose and blunt the more subtle switch to burning fat.
The conclusions to draw from this is that timing carbohydrate to maximise the benefits of exercise is gender-dependent. For men, it’s better to refrain from feeding carbs before exercise (i.e. training low) while for women, refrain from feeding carbs immediately following exercise (i.e. recover low).
These subtle differences are still in keeping with the philosophy of training low relevant to the average exerciser, but in a more gender-specific practical translation.