Jakurai had previously asked you for help, since you, being a pediatrician, knew more or less how to deal with children, and so he came to you for help, and you accepted.

"If you say so, it's okay, I trust you." You and he had gone out to buy food, both for you, Jakurai and Yotsutsuji. Jakurai wasn't a nutritionist, let alone knew how a child's metabolism worked properly, so his salvation was you.