Men Are Different, Women, Too - Also, When it Comes to the Metabolic Effects of Exercise | Plus: Differences Make Female 2h Cardio Sessions Look Even More Futile

Men are fat burning machines, women are not!? Review of sex-differences in fatty acid oxidation during and after exercise puts yet another "!" behind the "don't spend 2h on the treadmill if your goal is fat loss"-primacy for women.

We all know: Exercise training is generally a healthful activity and an effective intervention for reducing the risk of numerous chronic diseases including cardiovascular disease and diabetes. And as Gregory Henderson points out, both a result of prevention of weight gain over time as well as direct effects of exercise on metabolism of lipids and the other macronutrient classes during and after exercise.

Unfortunately, the contemporary evidence indicates that resting metabolism does not respond similarly in all individuals to exercise participation. One of the factors that determine the effects of exercise on resting energy expenditure (and even intake) is sex,... no not, sex as in sexual intercourse, but sex as in "men vs. women"!

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In his recent paper in Frontiers in Endocrinology, Gegory C. Henderson reviews the differences in the effects of exercise on resting metabolic rate, fuel selection after exercise, as well as the shuttling of triglyceride and fatty acids between tissues in men and women.

Figure 1. Summary of the main results of Henderson's review of the differential effects of exercise on lipid metabolism in men (M) and women (F) - from an excellent review (Henderson. 2014)

Henderson summarizes his results the awesome figure above. And this is how you read it. Follow the links between "prior exercise" at the heart of the figure and read the "M = F"s, "M > F"s and so on, which tell you that...

• exercise will have a greater lipolytic effect on the fat cells of male vs. female bodies; since the amount of fat that's release into the blood correlates with the increase in fatty acid oxidation (caffeine "burns fat" this way), it's no wonder that...
• exercise will have a more significant effect on resting fuel oxidation (here mostly the oxidation of fat) in men than women, and...
• exercise will have a greater inhibitory effect on the plasma triglyceride response to food in men than in does in women

What we don't know yet, is whether there is a difference between the effects of exercise (in this case mostly aerobic) on the secretion of the heat-unhealthy VLDL cholesterol molecules from the liver.

Is there something you can do about it, ladies? Yes, there is! Hit the weights, or hiit it hard. High intensity interval training, for example, has been shown to increase the capacity for fatty acid oxidation in 8 healthy, recreationally active women by 100% in the first 30 min and still +20% during the last 30 minutes of a 60 min endurance training session within just 2 weeks (Talanian. 2007).

I know ladies, it may really look like we had an unfair advantage, but as long as you've got enough estrogen in your blood you are actually more resistant to body fat gains than we are. 

Figure 2: Relative weight gain during 12 weeks of overfeeding in male vs. female vs. ovariectomized mice (Grove. 2010)

Ok, we are talking about rodent data (see Figure 1), but based on evidence from human data, women have another "unfair" advantage. They accumulate more healthy (subcutaneous) and less unhealthy (visceral) fat than men (Shi. 2009). Fat that they are having a significantly harder time to shed (Ballor. 1991, specifically if they are trying to do it with the "typically female" hours "two hours on the treadmill"-approach to fat loss (Donnelly. 2005).

Never work out only to burn fat.

Bottom line: In view of the "fat exercise disadvantage" of women it appears even more hilarious that they are the ones who usually try to shed the (healthy) fat on their thighs and buttocks during endless cardio sessions. Sessions that are significantly less productive for them, than they'd be for their significant other.

A fact that reminds me to remind you, girls, to take another look at the "The Fallacy of Working Out To "Burn Calories" + Exercise Shuts Down the Carb Cravings: Bench Press, Leg Press HIIT & LISS Are Not Meant to Incinerate the Junk You Eat" before you go ahead and comment on the unfairness of human sex differences in the response to exercise on Facebook.

References:

• Donnelly, Joseph E., and Bryan K. Smith. "Is exercise effective for weight loss with ad libitum diet? Energy balance, compensation, and gender differences." Exercise and sport sciences reviews 33.4 (2005): 169-174.
• Grove, K. L., et al. "A microarray analysis of sexual dimorphism of adipose tissues in high-fat-diet-induced obese mice." International Journal of Obesity 34.6 (2010): 989-1000.
• Henderson, Gregory C., and N. J. New Brunswick. "Sexual dimorphism in the effects of exercise on metabolism of lipids to support resting metabolism." Diabetes 5 (2014): 162.
• Shi, H., and D. J. Clegg. "Sex differences in the regulation of body weight." Physiology & behavior 97.2 (2009): 199-204.
• Talanian, Jason L., et al. "Two weeks of high-intensity aerobic interval training increases the capacity for fat oxidation during exercise in women." Journal of applied physiology 102.4 (2007): 1439-1447.