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It’s that time of year again, the time where I have many athletes contact me and ask if they should try x.y.z diet trend to try to minimize gain weight over the holidays. For my southern-hemisphere athletes, they wonder about long rides on minimal fuel or going out training fasted as a means to mitigate the holiday indulgences (a box or two of scorched almonds, anyone?). Meanwhile, my northern hemisphere peeps look to cut calories or compartmentalize calories around a main event due to indoor training and short days. It’s a pretty complex issue: weight and body composition, and yes, sex does matter.
‘Energy metabolism in humans is tuned to distinct sex-specific functions that potentially reflect the unique requirements in females for gestation and lactation, whereas the male metabolism may represent a default state.’
What does this mean? In general terms, it means that sex chromosomes and sex-specific hormones are associated with fat distribution and how the body uses or stores fat. For example, women tend to store more fat and have a higher whole-body insulin sensitivity (takes less carbohydrate to trigger an insulin response) than men, and men can access and burn stored fat at a much greater rate than women. We see this in fat distribution: women have more overall fat, and we tend to have more subcutaneous fat, both in the abdominal and the gluteofemoral regions. Biologically, we need this extra fat for reproduction, but the mechanisms of action of fat storage are more than macronutrient redistribution and calories in/calories out.
Under resting conditions, after a meal, fatty acid clearance is higher in women (e.g women store the fatty acids), whereas the insulin response in men increases the oxidation and utilization of fatty acids as a resting fuel (eg less fat storage). During exercise, women use more fat and tap into less muscle and liver glycogen; mainly due to estrogen (the higher circulating estrogens, the less carbohydrate availability) but also due to sex differences in sympathetic nerve activity. Overall, this results in a greater whole-body insulin sensitivity in women, with a greater potentiation of subcutaneous fat accumulation.
What about long training rides with minimal fuel? Is this the way to go if women use more fat during exercise? Or should I skip calories and “save up” for the main event?
Carb has become a four-letter word, even among active women, who are increasingly becoming grain and starch phobic. The issue here is the effect low carbohydrate has on the woman’s body. Carbohydrates are indeed essential.
Let’s start with some basic physiology. Your body uses carbohydrates for energy during exercise, to fuel your brain and central nervous system, to help your body burn fat, and to help preserve your precious muscle tissue by preventing your body from using protein as a primary energy source. Remember, too, that glycogen is of limited supply in your body (when fully stocked you have about 10 gram glycogen per kilogram of muscle tissue, and ~40-45 grams liver glycogen. You need at a bare-bones minimum 130 grams of carbohydrate (~520 calories worth) for survival—a.k.a to support the central nervous system, maintain red blood cell production, keep the immune system running, and fuel the brain (which needs ~100 grams daily, ~60% of the body’s resting glucose utilization). This 130 grams does not support physical activity.
What you eat day to day profoundly impacts how much glycogen you have in the tank at any given time. Whether you are training hard for race season in the summer, or putting in some base miles inside or in the cold, it’s important that you have enough on board to get the job done.
While you can likely get through an easy 60-minute spin without draining the tank, as soon as you kick up the intensity to about 75 percent of VO2 max, you’ll burn through your stores in just two hours time. Go harder, like a few 1k sprint efforts and you’ll run through those stores much more quickly. In fact how much glycogen you have available is the biggest limiting factor for going strong and maintaining your effort and intensity for any type of prolonged exercise.
Now here is where I’d be remiss if I didn’t talk about fat adaptation, because I know there are a lot of endurance athletes going high fat and super low carb, but the 100 million dollar question is if they improve performance. And the answer I believe is no. Here’s why:
Yes, low carbohydrate diets do increase fatty acid oxidation during exercise and encourage intramuscular fat storage. The body is smart. If there isn’t enough primary fuel to support the stress it’s under, it’ll go for a secondary source -in this case, fat- then store more of it for the next time it encounters that stress. But this does not translate into improved performance.
From a performance standpoint, the research shows no real performance benefits from low carb over moderate carb diets. In women particularly, high fat, very low carb eating elevates levels of the stress hormone cortisol, which increases catabolism and a harms protein synthesis (i.e. you’re eating your muscles and not making more), which is obviously bad for performance. It also hurts your immune system, which is already being taxed through exercise stress. In the end, your ability to maintain high-intensity and/or prolonged exercise is compromised and your body is put under undue stress. Moreover, the studies investigating fat-adaptation and endurance performance has been done on men, and at low intensities (e.g. 64% Vo2max).
From a body composition change standpoint, going super-low carb and training fasted/low fuel works well in men, but it backlashes against women. I see this alot in my practice, a couple comes in and they have both been following a paleo-eque or intermittent fasting, or other low carb trend. The guy has dropped overall weight, leaned up, and gotten stronger, whereas the woman has gotten more fatigued, put on body fat and lost muscle mass, getting slower and less powerful. I explain to them that it isn’t their motivation or willpower to the diet that is failing them but the actual diet itself. What gives? The basic human protective mechanism of famine.
I’m sure we all know about cortisol- the stress hormone/aka “belly fat hormone” but do you realize that the antagonist to cortisol is DHEA? And every time cortisol is elevated, DHEA drops? Under chronic stress, either through life stress, exercise stress, or dietary stress, cortisol remains elevated. This hits DHEA- why am I so concerned with DHEA? DHEA is extremely important because it makes estrogen, progesterone, and testosterone in both men and women. It also decreases cholesterol levels, decreases the formation of fatty deposits, helps prevent blood clots, increases bone growth, promotes weight loss, increases brain function, increases the sense of well being, helps the body deal with stress, supports the immune system, helps the body repair and maintain itself, and decreases allergic reactions.
So when you are under stress, DHEA is down, you lose the precursor for testosterone production (yes, women do produce testosterone; in women, testosterone increases sexual interest and a sense of emotional well-being. It helps maintain muscle mass and strength. It maintains memory. It helps the skin from sagging. It decreases excess body fat. It helps maintain bone strength. And it elevates levels of norepinephrine in the brain, therefore it has an antidepressant effect).
Cortisol also competes with progesterone receptors; and progesterone and thyroid are directly tied to each other- high cortisol binds thyroid hormone, making it less active… and decreases progesterone, reducing thyroid function as well as the usual effects of progesterone: it works with estrogen for reproductive health, it improves sleep. It helps the body use and eliminate fats. It lowers cholesterol. It offers some protection against breast cancer. It increases scalp hair. It increases metabolic rate. It functions as a natural antidepressive and as a diuretic.
Back to the diet issue…
When women drop too low in carbohydrate (the grams are individual, as each woman is different), it causes a drop in estradiol (the female sex hormone), with a rise in estrone and cortisol (progesterone is converted to cortisol under long periods of high stress). We become more masculinized in our reproductive status and conserve fat. From a survival standpoint, your body is thinking “FAMINE!” and in a widespread famine, the last thing that is needed is new babies. But, in men, the low carbohydrate-famine mechanism is to become “fight ready”: Lean up, increase anabolic activities and increase testosterone. Makes sense right? From an evolutionary standpoint- women need to not reproduce and men need to fight to get food for the tribe.
To add to this, we need to look at post-exercise leucine, if there isn’t enough to trigger muscle adaptation/development, progesterone and cortisol increase the catabolic state of our bodies– which increases cortisol circulation for longer periods of time; and per above- with elevated cortisol, DHEA drops without DHEA we can’t produce testosterone, estrogen, or progesterone.
Are you still with me? Ready for the bullets of what to do to survive the holidaze?
1) Eat low on the food chain (e.g. clean, local, organic when possible).
2) Timing of food is critical: don’t go into an exercise session totally fasted, especially in the morning when cortisol levels are at their highest; just a small hit of protein + carb (~150kcal total) before you head out will counter some of the cortisol. The biggest thing, though, is post exercise recovery! Get that protein or BCAA dose in within 30 min of finishing your session- it doesn’t have to be a special supplement, it can be a split meal- e.g you have the protein component of your breakfast within 30 min, then have the rest within 90 minutes of finishing your session- this will still knock down cortisol.
3) Don’t follow Intermittent Fasting ( IF)- the fasting aspect of IF drives cortisol up, creating an elevated baseline of cortisol- see above for the damage– (also note that long term elevation of cortisol can become adrenal fatigue). Moreover, research shows that calorie deficit from low food intake drives a rebound appetite in women- causing more calorie intake than the deficit, but calorie restrict from high intensity bouts of exercise, suppresses appetite and keeps that calorie deficit in check.
4) Maintain a base of ~130-200 grams of good quality carbohydrate intake a day such as grains, fruit and veggies.
5) Focus on body composition, not weight on the scale- it is less stressful and we aren’t in the “Kate Moss is cool” era any longer!
6) Go with the 80/20 rule: 80 % of the time you are spot on your nutrition and training, the other 20% enjoy life and all that it has to offer!
Want to know more? Here is some recommended reading:
+ Sex-specific differences in lipid and glucose metabolism
+ Appetite and Energy Intake Responses to Acute Energy Deficits in Females Versus Males
+ Metabolic characteristics of keto-adapted ultra-endurance runners
About Dr. Sims:
Dr. Stacy T. Sims, MSc, PhD, is a monthly columnist for Ella CyclingTips. Sims has contributed to the environmental exercise physiology and sports nutrition field for more than 15 years as both an athlete and a scientist. The chief research officer/co-founder of Osmo Hydration, Dr. Sims served an exercise physiologist and nutrition scientist in the human performance lab at Stanford University from 2007-2012 where she specialised in sex differences of environmental and nutritional considerations for recovery and performance. Her personal interest in sex differences and performance has been the precedence of her academic and consulting career, always looking at true physiology to apply innovative solutions in the sport nutrition world.