Fats are a much more stable energy source than carbohydrates. Sugars and starches, especially in the form of high glycemic index "empty carbs," are absorbed very quickly into the blood stream, causing a sharp spike in the glucose level. This in turn triggers the pancreas to inject a large amount of insulin into the blood, to promote the uptake of the glucose into the body's cells. Carbohydrates ingested without fats are absorbed much more rapidly than carbohydrates buffered by fat, because fat slows down the digestive process. Fats, being digested much more slowly, will become available as an alternative fuel source just as the carbohydrate supplies are becoming exhausted. But this is true only if sufficient fats are consumed with the meal.
Very little excess glucose can be stored in the body for later use, unless it is first converted to fat. The liver can provide a small buffer of glucose stored in the form of glycogen, amounting to no more than 5% of its total mass. Once that capacity is exceeded, any remaining glucose in the blood must be converted to fat to be stored.
A thin child whose diet consists mainly of empty carbs cycles between feast and famine in terms of glucose supply, but suffers chronically from an inadequate supply of fats. This places a lot of stress on the homeostasis system because of the gross imbalance between glucose and fat in the external fuel supply. A solution to this problem can be achieved by piling fat stores on the body, except that the fat stores themselves introduce additional energy needs and the strategy snowballs into obesity.
For the ADHD person, instead of steadily accumulating fat stores and programming the muscles to preferentially consume fats, I argue that their bodies have adopted a strategy of fat conservation. The muscles are programmed to prefer glucose over fat, and the body size is minimized by reducing fat deposits, slowing down the maturation process, and stunting growth. As a consequence of the body's reduced needs for fats, more fat (but still not enough) is available to the brain to support its need to build myelin sheath for the expanding network of nerve fibers.