Realistically, if you weigh 400 lbs then you need to eliminate all carbohydrates from your diet and just exist on a steady intake of vegetables, nuts and (if you're not a vegetarian) meat. If you're that large, exercise is going to do nothing but agitate you. Change your diet, stop loading your mouth with extraneous sugar, fruit and sodas, and start to eat healthy. When you're a normal weight, then you can think about exercise -- but at that point, any time your heart rate elevates you're just teething on the border of a severe cardiovascular collapse.
If you ever have a chance to meet Nick Diaz, he will reiterate the same thing -- only with a few more [expletives] woven in.
Therefore if you weigh 400 lbs, don't even pretend like you want to engage in a physical activity with another person. Not only will you gas out in the warm-up, but if you get clocked on the chin, it will take your body a few minutes to fully collapse to the ground. Props to CagePotato for the find.