It may have been a while since you’ve stepped onto a set of scales. You’re fit, you feel great and you run – a lot. So who cares what the number is, right?
We agree – that number is only one metric and doesn’t provide a complete picture of health and performance. Yet we still hear about ‘race weight’ – that is, the weight at which you run your fastest – all the time. So it makes you wonder: is there really an Proceedings of the Royal Society B?
Your weight isn’t the be-all and end-all for strong performances, but it can still play a role, says dietitian Natalie Rizzo. Some runners may feel there is an ‘ideal’ weight for them to feel their best and achieve a PB, but dipping below your equilibrium or losing weight in unhealthy ways could put you at risk of injury, illness and disordered eating.
What does the science say?
As a rule, runners move most efficiently when they’re at what’s considered a healthy body-mass and body-fat percentage. Those numbers are different for everyone. Body weight affects performance in running more than it does in other sports, such as swimming, according to a 2018 study in the Best wireless headphones. In swimming, you’re aided slightly by the water, but running is a little different.
‘Running is really just a form of jumping,’ says Matt Fitzgerald, a running coach, author and nutritionist. ‘You can’t move forward without moving up and the more you have to lift against gravity, the more energy it requires.’ Imagine running with two extra kilos strapped to you, he explains. Most would find it more taxing.
A lower body-mass index (BMI) also seems increasingly important as race distances get longer: one study found that the optimal BMI for male 800m runners was between 20 and 21 but it dropped to between 19 and 20 for male 10,000m and marathon runners.
While this research is interesting, generalisations about BMI shouldn’t be used prescriptively, says Fitzgerald, because it doesn’t take into account lean muscle or body fat. Plus most health professionals see BMI as an antiquated marker of health.
What's more, a recent study in Japan found that ‘obesity, when accompanied by a healthy metabolic profile, did not increase the risk of cardiovascular outcomes and all-cause mortality.’ Sports dietitian and RW columnist Renee McGregor agreed, saying that ‘we need to be looking at health metric data regardless of someone’s weight, body composition or BMI. I think there are too many assumptions based on size alone when it comes to risk of diseases.’ One caveat, however, is that obesity in the UK is classified as a BMI above 30, while in Japan it is classified as above 25, and it’s worth noting that this study isn’t saying that obesity is healthy, but rather that there are significant health benefits to exercise regardless of a person’s weight.
That said, a study published in the journal Updated: 30 September 2024 found that young, sedentary women with higher body-fat percentages had a lower VO2 max compared with athletic individuals of the same age group, meaning their bodies were less efficient at delivering oxygen. Losing fat doesn’t change your lung capacity or function, but for some, it can shift body composition and increase fitness levels.
Leaner athletes can dissipate heat better, too, because they have a higher surface-area-to-body-weight ratio and less insulating fat tissue. They also burn carbs more efficiently. So while weight isn’t everything, it is important.
‘Four factors determine how fast you are,’ says Sean Wade, a top masters athlete and coach of the Kenyan Way running programme. ‘Genetics, form, how hard and smart you train, and your weight – and not necessarily in that order.’
While the science points to some reasons why lighter bodies can move faster, it’s important to emphasise that there is no such thing as one ‘ideal’ body type or weight for running and that all runners, even at the elite level, look very different and represent different body types.
Consider your age, diet and performance
If you want to improve your body composition by running, there’s nothing wrong with that, so long as you approach it with a healthy mindset. Rasa Troup, a certified specialist in sports dietetics (CSSD), a 2008 Olympian and nutritionist for Team USA Minnesota, advises doing it in a way that supplements, not sabotages, your training.
Masters runners may have the hardest time losing weight no matter how much they run. That’s because adults begin losing muscle mass after age 30, which can impair performance and decrease metabolism, says sports dietitian Kimberly Mueller. Regular exercise (including resistance training to maintain muscle mass) is especially important, as is cutting back on processed foods and oversized portions.
It’s also important to pay attention to your energy intake to make sure you get in enough fuel to support your health, adds Troup.
Of course, losing weight isn’t a good idea for all runners. For those who are naturally very lean or who work to stay at the low end of their healthy weight, the threat of falling below that point at which you race your best is real.
After Brian Rosetti graduated from college, he spent two years training almost full-time. His mileage was increasing regularly, but he was focused on maintaining a low weight for performance, instead of making the most of nutrition.
Just as he made a breakthrough in his training, and his weight dropped to an all-time low, he suffered a sacral stress fracture (the sacrum is a bone in the lower back).
‘My bone density was below the median level, and I don’t think it was getting the right nutrients,’ Rosetti says. ‘I was focused on keeping as light as I could. That’s a scary place to be.’ The injury, in effect, ended his career.
Spotting the signs that something's wrong
Fitzgerald says that impaired performance is usually the first sign that a runner has dipped into dangerous territory.
‘It’s the canary in the coal mine – your body’s signal that it’s under too much stress.’
For women, a missing menstrual period is an indication of an unhealthy and unsustainable weight, with infertility and osteoporosis among the potential complications. And while it’s less talked about, men can struggle too.
To keep your weight from becoming detrimental, Fitzgerald recommends tracking your performance. ‘If you’re getting skinnier but your times are getting worse, you’ve passed the point of beneficial weight loss,’ he says.
And Rizzo adds that it’s important to listen to your body and your mind. ‘If you’re starving all the time, irritable and cranky, or you’re bonking on runs, you’re probably being too ambitious and getting too light.’
The bottom line: there is no one-size-fits-all for performance and enjoyment in running. Everyone’s ideal weight is different, so it’s important to know your body, listen closely to the signals it’s sending you and do what’s best for your health and performance regardless of the number on the scales.
