Let's look at who John is. As a person.

He buys melons in bulk. This can't be all he eats, otherwise he would die of malnutrition, so he's got enough money to buy forty melons at once in addition to regular grocery shopping, most likely delivered in a truck because how else would someone get that much food transferred to their house all at once?

He has a freezer large enough to hold somewhere around forty melons at once, which further suggests he's got quite a bit of money to burn, large freezers aren't cheap.

He's eccentric - who else would buy forty melons at once and store them in a freezer? This means he's probably a wealthy shut-in.

In all likelihood, the melons are just an excuse to not be questioned on why he has such a large freezer. Melons aren't the only thing he keeps in there.

He also stores the bodies of his victims, usually prostitutes and mail-order brides, which he hangs in the back of the freezer on meat hooks - just after the kill, before coagulation or freezing can take, he puts a bucket underneath to catch the blood which he keeps in individual serving-size plastic bags, also kept in the freezer.

Every morning he takes out a bag of blood and a melon and stands there in his kitchen wearing just a robe, watching them thaw, masturbating. Hours pass.

He ejaculates into a blender, then cuts the melon - storing three-quarters and adding the other quarter to the mixture and empties the bag of blood, creating a semen-melon-blood smoothie for breakfast. An entire melon just wouldn't fit in a blender, and the required amount of blood to even the mixture would make a man sick, so the recipe as he sees it calls for 1/4 a melon and 3/4 a pint of blood (a single serving) with the insignificant amount of semen added for texture totalling about a pint of smoothie for breakfast, which is reasonable.

He does this every day for 147 days, feeling less and less healthy every day. He attributes this to the diet, but he's not willing to give it up and sees it as an acceptable trade for what he does.

On the 148th day, with the last quarter of melon blended into the mix, he weakly reaches up and takes a single sip, then collapses on the floor. Dead.

He would later be found by the delivery men, another 37 melons in their van. Coroner's report would discover that he'd had hepatitis A, B, C, D, E, AIDS, every sexual disease known to man and a few new ones that had mutated in his body.

You cannot eat thirteen watermelons and survive.