Opposite of a diet

I would like to know if there is a single word to describe the opposite of a diet.

If you are overweight you may say "I will go on a diet".
If you are underweight you could say "I need to go on a ..."

A single word is preferable rather than a phrase

Solution 1:

binge [binj] noun

1) a period or bout, usually brief, of excessive indulgence, as in eating, drinking alcoholic beverages, etc.; spree.

Source: Dictionary.com

*Personal note: Binge eating can be a difficult habit for many people, myself included in the past. If you struggle, please feel welcome to look for support. There are a lot of people with similar struggles working together to help one another. Link

Solution 2:

It’s still a diet, but for clarity you should call it a weight gain diet:

A weight gain diet is very similar to a healthy weight loss diet. In both cases, you will eat foods that are rich in nutrients and not eliminate major food groups. You will avoid "empty calorie" foods (junk foods that contain sugar, salt, and fat, but few other nutrients)

What Is a Weight Gaining Diet?

What's good about "weight gain diet" is that it works no matter who's doing it or why (even if that someone is an animal). It's also neutral and works no matter the level of formality.