Different hormones, different fuel mix
According to Mark Tarnopolsky, MD, PhD, at McMaster University Medical Centre in Canada, men and women differ in the type of fuel they burn as energy for muscle contraction during endurance exercise. For everyone, the major energy sources for endurance exercise are carbohydrate and lipid (or fat). However, in an analysis of 16 different studies, Tarnopolsky determined that women derived 41% of their energy from lipid and 56% from carbohydrate, whereas men burned 29% of their energy from lipid and 65% from carbohydrate.
The reason for the difference? The female hormone estrogen. Estrogen increases the activity of enzymes involved in metabolizing lipids. Tarnopolsky believes that higher levels of estrogen in women translate to proportionally more fat burned during exercise compared to men and less reliance on glycogen (carbohydrate) stored in the muscle and liver.
Effective carbo-loading for both sexes
Given this finding Tarnopolsky wondered if men and women differed in their ability to carbo-load as well. Conventional wisdom holds that loading up on carbohydrates 3 to 4 days before an endurance event will improve performance by boosting muscle glycogen stores. However, most carbo-loading studies have been conducted using males. Tarnopolsky decided to test for differences between the sexes.
Not too surprisingly, when men tapered their exercise intensity for four days while simultaneously increasing their carbohydrate intake from 57 to 75% of total calories, they showed a spike in muscle glycogen stores and a hefty increase in the amount of time it took them to exercise to exhaustion. In short, it worked. The shocker came when women exposed to the carbo-loading regimen showed no increase in muscle glycogen storage or exercise performance.
It turns out that to boost glycogen stores, both men and women need to eat in the range of 8-12 grams of carbs for each kilogram (2.2 pounds) of body weight. For male athletes, this amount of carbohydrate isn’t a problem because their caloric intake is so high. But for female athletes, caloric intakes may be too low to achieve the minimum carb intake needed. The following example illustrates the problem:
Putting it into practice
Paul is a 170-pound athlete who consumes about 3500 calories daily, with 55% of those calories (or about 481 g) coming from carbs. This equates to about 6 grams of carbs for every kilogram of body weight, which is appropriate for light-to-moderate intensity training. To carbo-load he increases the proportion of calories coming from carbohydrates to 75% (about 656 g). This equates to about 8.5 grams of carbohydrate for every kilogram of body weight, which is within the 8-12 gram range needed to maximize glycogen resynthesis.
Amy is a 120-pound athlete and more calorie-conscious. She typically consumes 2000 calories daily, with 55% of those calories (or about 275 g) coming from carbohydrate. This equates to about 5 grams of carbs for every kilogram of her body weight. In attempting to carbo-load, she increases the proportion of calories coming from carbs to 75% (about 375 g). Unfortunately, this equates to less than 7 grams of carbohydrate for every kilogram of body weight, which is below the 8-12 gram range needed to maximize glycogen resynthesis.
At 120 pounds, Amy needs a minimum of 436 grams of carbs, or 1745 calories from carbohydrates. Those numbers may look good on paper, but in practice if she’s only consuming 2000 calories daily, getting 1745 calories from carbohydrate (a whopping 87% of total calories) isn’t realistic.
The solution for effective carbo-loading, according to Tarnopolsky, is that women may actually need to consume 30-35% more calories on carbo-loading days. So, instead of 2000 calories, during carbo-loading 2600 calories a day may be needed. Those 600 bonus calories equate to an extra bagel, a cup of oatmeal with raisins sprinkled on top and a PowerBar Performance bar each day. For those concerned about too many calories, no worries: the green light for extra feasting is only for the 3 to 4 days of carbo-loading.
Fueling during and after exercise
Because women use proportionally more lipid during exercise than men, it is reasonable to think that they might use ingested carbohydrate (from energy bars or sports drinks) differently from men. But that’s apparently not the case. Women respond favorably to ingested glucose in a manner similar to, if not even more so, than men.
When it comes to recovery, several studies have shown that the rate of glycogen resynthesis is greater if carbs, or carbs and protein together, are consumed in the early post-exercise period as compared to hours later. Here again, gender differences in carbohydrate metabolism don’t appear to be a factor. Men and women respond similarly to post-exercise glycogen resynthesis regimens.
When it comes to carbo-loading, what works for men won’t necessarily work for women.
· Gradually taper your training 3 to 4 days before your endurance event.
· Simultaneously increase your carbohydrate intake. For optimal glycogen reloading, men and women require 8-12 grams of carbohydrates for each kilogram of body weight.
· Male athletes can usually achieve the higher carb range by simply substituting carbohydrate-rich foods for other foods that tend to be higher in fat.
· For female athletes, effective carbo-loading may require adding foods to the diet. As a rule of thumb, women may need to increase total calorie intake by 30-35% in the 3 to 4 days before the event..