These dudes aren't a forgotten early sixties doo-wop band; they're a nasty bunch of chemicals that wind up in your bloodstream, menacingly swinging their carbon chains, as an unavoidable consequence of losing weight. Losing weight means burning fat. Burning fat means individual fat cells tapping their reserves, breaking down the complex fat molecules into the simple molecules you burn. Chemical plants produce waste products, and fat cells are no exception. Instead of pumping nasty stuff into the river at midnight that makes aluminium canoes fizz, fat cells dump their waste into the bloodstream in broad daylight, right along with the useful products of breaking down fat.
Most prominent among the waste products of burning fat are a group of chemicals called ketones. As long as you're burning fat, your body will be subjected to a constant dose of extra ketones in the blood, a condition referred to as ketosis. Ketones are, in the contemporary argot, toxic waste, and the prospect of subjecting yourself to a long-term dose of them is off-putting to anybody contemplating a diet.
But the alternative, not losing weight, is much less healthy. With a little knowledge and a simple trick, ketones can be conquered. First, while ketones are toxic, they're only slightly so. Dieting won't make you see huge hairy bats (unless, of course, you saw huge hairy bats before). Second, the waste products of burning fat aren't insidious--they don't accumulate in the body like heavy metals or some organic toxins. Instead, they get swept out of the bloodstream by the kidneys and liver and are excreted in fairly short order. This lets us cope with the problem by a very simple expedient.