When Isla Lough was born, the bookies offered odds of 100-1 that the daughter of the marathon world-record holder Paula Radcliffe would one day win the Flora London Marathon. A safer bet would have been to predict that Radcliffe would give birth to a daughter. For this you can "blame" her husband.
Research suggests that male runners who cover more than 30 miles a week – as Radcliffe’s husband and training partner Gary Lough does – are more likely to father female offspring. Researchers at the University of Glasgow divided 139 male runners into three categories: those who were taking a break from running when they and their partner conceived; those who were running less than 30 miles a week when their partner conceived; and those who were running between 30 and 50 miles a week when their partner conceived.
The study revealed that the non-runners and those covering less than 30 miles a week had a 62 per cent chance of fathering male offspring – compared to the average of 51 per cent for the general population. It was a dramatically different story for the runners covering more than 30 miles a week though: only 40 per cent of their babies were boys. The researchers put this trend down to the dip in the male hormone testosterone that occurs as a result of higher running mileage.
Running might affect the sex of your children, but it might also help you conceive in the first place. "Men who run regularly and stay at a healthy weight are more likely to maintain a good sperm count than men who are obese," says Dr Roger Henderson, a GP and marathon runner. Henderson does issue one warning: "Male marathon runners do not appear to have reduced sperm counts, although exercise that consistently heats the testicles, or which requires very tight-fitting shorts, such as cycling, may not help."
There’s little research into how the running mileage of the mother might affect the gender of their children, but research does exist to suggest that, unlike male runners, some female runners are putting their ability to conceive at risk. According to the American College of Sports Medicine, as many as 44 per cent of athletic women experience changes in their menstrual cycle, or have seen their periods stop altogether (amenorrhoea) at some time.
The symptoms may be there but female runners sometimes assume that, even though their periods may have stopped, they’re healthy in other ways thanks to their running. In truth amenorrhoea can lead to infertility and a loss of bone density. In the short term that might mean more stress fractures, but in the long term difficulty conceiving and the early onset of osteoporosis are also possibilties.
In the 1970s experts thought amenorrhoea was linked to weight loss or low body fat but that view has since changed. Now an energy imbalance is thought to be the key: "We think that these women are in a ‘low-energy’ state, meaning they are expending more calories than they are replenishing with dietary intake," says Professor Anne Loucks of Ohio University in the USA. The theory goes that if there isn’t enough energy for the body to function well on every level, it prioritises, using available calories to get you through your 10-mile training run rather than maintain reproductive function.
The good news is that experts believe the infertility associated with amenorrhoea is reversible when menstruation resumes. The key to reproductive and skeletal health is making sure your energy availability is maintained at around 45 calories per kilogram of fat-free mass per day (cal/kgFFM/day) when you’re training. You can determine your fat-free mass with electrical impedance body composition scales. Dip below 30cal/kgFFM/day and reproductive and skeletal health are impaired. A large proportion of the women analysed by Loucks fell below this critical level.
Diets high in carbohydrate and low in fat can also exacerbate the problem. One study found that a daily exercise expenditure of 840 calories (equivalent to an eight- to nine-mile run) on a 62 per cent carbohydrate diet reduced energy availability to a critically low level of 21cal/kgFFM/day.
You can work out your energy availability by adding up the number of calories you eat during the day, as well as the number you use during exercise (most heart rate monitors have reasonably accurate calorie counters, or visit www.caloriecounter.co.uk). Subtract the latter from the former and divide this by your fat-free weight. If you discover you have less than 30cal/kgFFM/day, it’s time to reconsider your diet and training.
Ensuring you consume enough calories to fuel every session should be part of any long-term approach to running – then with any luck you should still be going strong if Isla, or your own little runner, does decide to make her marathon debut.