"You obviously don't have to be an athlete to wear or enjoy Nike wear," she said after stressing that Bella was an accomplished equestrienne before she was forced to quit competitive riding due to chronic Lyme disease.
In addition, many social media users suggested on Instagram that Bella is too thin to represent Nike.
"Nike? She's not an athlete but more importantly it's giving young girls unrealistic expectations yet again," one person wrote.
"Yeh this is why young girls get eating disorders, when being abnormally thin is glorified. Well done @nike," another added.
Foster herself was diagnosed with chronic Lyme disease in 2012, and revealed in October 2015 that Bella and her son Anwar Hadid also have the condition.
Bella, who shared a photo of her treatment in August, previously confirmed her illness ended her competitive riding career.
"It affected my memory so I suddenly wouldn't remember how to drive to Santa Monica from Malibu where I lived," the model told ES magazine. "I couldn't ride. I was just too sick. And I had to sell my horse because I couldn't take care of it."
"If I'd had the chance to succeed in riding, I might not have modeled," she admitted. "I didn't know that I was going to get sick."
Bella, the younger sister of model Gigi Hadid, is known for her modeling work with Balmain and Chanel.
She will walk in the Victoria's Secret Fashion Show for the first time in December.
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