Ignore all other answers and let me skip to the truth:
Insulin. Any means of keeping insulin elevated will result in weight gain and eventually symptoms for pre-diabetes.
Skipping breakfast does not slow your metabolism. Calories are irrelevant.
The best way to prevent and fight cancer is fasting. I suggest taking electrolytes.
Your advice to this dangerously underweight individual is to fast?
I was also wondering about insulin. If he ate something like a cookie 30min before each meal it would promote energy storage to fat. Or if he just ate a cookie every hour or something.
For OP, he sounds really active, but not really into any kind of activities that build mass. Based on how you live it's hard to get a lot of mass.
OP also sounds like he's really conditioned to small portion sizes, his stomach isn't keen with stretching too much. When volume isn't an option with food intake, you need to look for density. Fat is the best, so any butter, oil, animal fats,... Is good to try, if it sits well, soak everything you eat in butter or bacon grease, or whatever you like.
Lipids do not create an insulin response. For weight gain, I would suggest fructose. But I think the weight loss is a symptom of the disease and do not see much point in alleviating the symptom.
An insulin spike with a cookie every hour would help keep insulin from falling to a fasted state but I think we are doing more harm than good in this effort.
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