Female athletes fire up fat burning after ovulation, but does it make any difference to weight loss or competitive performance?
Most people are aware that strenuous endurance training can disrupt normal menstruation in female athletes, but few are aware of the effect that the menstrual cycle itself can have on training and performance. Now, researchers at the University of North Carolina have shown that one stage of the menstrual cycle heightens fat burning, while another increases carbohydrate metabolism. The North Carolina research also answers an age-old question: should female athletes attempt to synchronize their menstrual-cycle phases with their workouts and competitions in order to maximize leanness and boost performances?
The luteal phase of the menstrual cycle turned out to be the best time for fat burning. During the luteal phase, fat furnished 70 per cent of the energy necessary to run at 35% V02max, 58 per cent of the energy needed at 60% V02max, and 46 per cent of the fuel at 75% V02max. In contrast, during the follicular phase fat offered only 52 per cent, 43 per cent, and 39 per cent of the required energy, respectively.
Does this mean that women intent on trimming body fat should conduct their long, ‘fat-burning’ workouts during the mid-luteal stages of their menstrual cycles, when fat metabolism is greatest? Can women run their best marathons in the mid-luteal phase, when glycogen conservation is maximised as a result of the augmented fat combustion? Is the mid-follicular stage the optimal time for interval workouts and S-K races, since high-octane carbohydrate metabolism reaches its peak then?
Maximising fat loss
Let’s take a look at the body-fat question first. As you know, the best way to lose body fat is to make a daily habit of metabolising slightly more fat than you eat. If you ‘burn’ 50 grams of fat per day, for example, but your dietary fat intake is only 30 daily grams, you should lose about 20 grams of body fat each day, provided you’re not eating excessive quantities of protein and carbohydrate, which could be stored as fat if consumed in extravagant amounts.. After about 22-23 days of this ’30-in & 50-out’ strategy, you would lose about a pound of body fat.
How does exercise play a role in this fat ‘balance sheet?’ Well, if you’re a normally menstruating female runner and you conduct many of your training sessions at an intensity of 60% V02max (73 per cent of max heart rate), which is a fairly common workout intensity, about 58 per cent of the calories you need to complete this workout will come from fat during the luteal phase of your menstrual cycle, compared to 43 per cent of the calories during the follicular phase. If you were about the same size as the North Carolina women (125 pounds or so), you would burn about 4.3 calories of fat per minute during the luteal phase and just 3.2 calories of fat per minute in the mid-follicular stage.
That makes the luteal phase sound nice, but hold on ! If you run for an hour per workout, that’s 66 calories of extra fat burned per training session in the mid-luteal stage, compared to the mid-follicular phase, or about seven additional grams of lard. Let’s say that you normally conduct 12 such workouts per month, six in the follicular stage and six in the luteal phase, with the rest of your sessions consisting of ‘carbo-burning’ hill efforts, interval workouts, and tempo runs. To maximize the ‘fat burn,’ you decide to carry out all 12 of these one-hour sessions in your luteal phase, with the carbo-burners pushed into the follicular period. The result is a grand total of 42 extra grams of fat broken down per month. After 11 months – provided your eating pattern doesn’t change – you will have dropped one extra pound of body fat as a result of this shift in fat-burning workouts.
A different approach
Another strategy might be to add a two-hour run and a one-hour run to the monthly schedule – on top of your current schedule. If these runs were conducted at an intensity of 60% V02max (73 per cent of max heart rate), they would increase total fat burning by around 570-770 calories per month (The higher figure would apply if both workouts occurred during the luteal phase). After five to six months, total body fat would edge downward by a pound or so. That’ s a bit more than two pounds per year, and close to 10 pounds over five years, as long as food intake doesn’t increase. However, these extra two sessions would have to be added gradually (starting perhaps with IS- and 30-minute workouts, rather than 60- and 1 20-minute efforts) in order to minimise the risk of overtraining. Note that although placing these surplus sessions in the luteal phase would be the optimal strategy, it would only produce a one-pound ‘edge’ in fat loss every 17 months or so, compared to cramming the workouts into the follicular stage. Overall, we can say that adding a reasonable amount of running to your schedule is more effective for fat loss than attempting to shift workouts from one part of the menstrual cycle to another.
What about performance?
Is the luteal phase really the best time to run a marathon? If we look at women who run the marathon at an intensity of 75% V02max (84 per cent of max heart rate) and require four hours to complete the event, such females would burn energy at a rate of 9.7 calories per minute, regardless of whether they were in the luteal or follicular stages of their menstrual cycles. In the luteal stage, fat would cough up 46 per cent of this energy or 4.5 calories per minute. Protein would chip in a minimal amount of energy, so let’s say that carbohydrate accounts for the rest of the fuel requirement – 5.2 calories per minute.
In the follicular stage, fat would offer only 39 per cent of the necessary energy, or 3.8 calories per minute, leaving 5.9 calories/minute for carbos (glycogen) – .7 extra calories of carbs per minute, compared to the luteal phase. Over the course of four hours, that would mean 240 X .7 = 168 extra glycogen calories saved during the luteal phase, or enough energy to run a little over a mile. This may sound like a lot, but bear in mind that it’s an overestimate, since fat burning would likely increase toward the end of a four-hour marathon run. Also, the follicular-woman’s extra loss of glycogen could be more than made up for by the judicious ingestion of sports drink during the event (about three to four sips every 10 minutes, as we always recommend). The bottom line? It’s unlikely that the hiked fat metabolism associated with luteal running will spare enough glycogen to produce a marathon performance bonus.
If you want to lose fat, take it fast
One final point about the North Carolina study: it reminds us once again that the admonition – constantly trumpeted in the popular press – to ‘slow down in order to burn more fat’ is utter nonsense. For the North Carolina women, fat burning advanced from 2.8 to 4.3 to 4.5 calories per minute as the women advanced from 55 per cent of max heart rate to 73 per cent of MHR to 84 percent MHR, respectively, during the luteal phases of their cycles. The same pattern – with slightly lower numbers – applied during the follicular phases. In other words, within the range of pulse rates between 55 and 84 per cent of maximum, the higher the better as far as fat burning is concerned. So, the next time you read someone’s advice to ‘go slow to burn more fat,’ just chuckle.
So what’s the second bottom line? In the North Carolina study, V02max, ventilation rates, and perceived effort didn’t vary according to menstrual-cycle phase, and there were only small differences in fat metabolism. As a result, the differences in performance between the luteal and follicular stages of the menstrual cycle are likely to be slight. If you want to trim away body fat, conducting reasonable amounts of extra training or shifting to a slightly higher workout intensity are the right strategies. Juggling one’s schedule so that more long runs are conducted during the luteal phase of the cycle is unlikely to have a major effect on body fatness.