Not really sure there’s a patron of weight loss, it’s a very modern issue really.
I suppose for moderation of food you might want to look to St Francis of Padua, who founded the Minims, who take fasting and abstinence from meat so seriously that it’s a 4th vow in their order.
At the other extreme, there is St Thomas Aquinas, who was overweight all his life, and could certainly know where you’re coming from.
Other than that, just eat less and exercise more, it’s really that simple! (If only I could take my own advice :shrug: )