Thymen Arensman of INEOS Grenadiers made headlines this week by candidly discussing his surprising weight gain during the recently concluded Giro d’Italia. The 24-year-old from the Netherlands revealed in a recent interview with Indeleiderstrui.nl that while his power output was at its peak, he faced challenges maintaining a high pace due to an unexpected 10-pound weight gain. So, how does a rider burning thousands of calories each day gain weight?

Spoiler alert: It wasn’t the pre-race pasta or rice causing Arensman to weigh in a bit heavier after several stages. “Weight gain like that would likely be due to inflammation, stress, or fluid retention,” says Kylee Van Horn, a registered dietitian at FlyNutrition who regularly works with ultra-endurance athletes. Considering the energy expended to race the entirety of the Giro d’Italia, it would be virtually impossible for a rider to eat enough to gain weight during those three weeks of racing.

And that’s what Arensman believes as well. “It was probably fluid retention,” Arensman told IDLProCycling. “My skinfold measurements were very low for me, which accounts for about four or five liters of fluid. It’s something useful to figure out for next time exactly what’s causing that.”

Fluid retention—also known as edema—in ultra-endurance events like cycling and running is more common than you might suspect. Research published in the Journal of the International Society of Sports Nutrition found that it’s pretty common for cyclists and runners and often a result of over-drinking, but not always. Research on ultra-runners has confirmed similar results.

Arensman believes he can dial in his drinking strategy to fix this issue in the next hot stage race—though he likely will miss out on racing the Tour de France to test what he learned from the Giro. “It seems like I’m retaining a lot of fluids, which might also be due to my struggle with the heat,” he says. “I perform better in colder conditions, but during the first two weeks of the Giro, it was warmer than expected.” (Well, except for that one snowy stage…)

Funny enough, Arensman isn’t the first INEOS rider to struggle with weight gain during a Grand Tour. In 2019, INEOS rider Gianni Moscon told reporters that he gained nearly 20 pounds during the Tour de France. He cited early season overtraining as one of the reasons, but like Arensman, it’s likely much of the weight was water retention that went away quickly rather than actual weight gain.

Despite the water retention, Arensman did finish sixth overall in the Giro, so it didn’t impact him too severely. Still, he told IDLProCycling that he believes in his podium potential if he can tweak his hydration strategy (and have a slightly better first week of racing).