shadowfoot,
Metabolic dysfunction is complex, but it comes down to inefficiency in both storage and utilization of energy compounds like glucose, fatty acids and amino acids. I like to think of it metaphorically as "plugged pipes". On a biochemical level, this translates to a reduction in the number and sensitivity of receptors, transmembrane transporters and enzymes. Perhaps paradoxically, it often involves an overcompensation by certain hormones like insulin. There is increasing evidence that metabolic dysfunction is the product of (a) a pro-inflammatory diet, lacking in protective micronutrients; and (b) insufficient stress (of the good sort). Your slightly overweight cats sound like they get good exercise from hunting, but otherwise have a low-stress existence. Perhaps introducing a dog to the scene would up their fight-or-flight hormones and help shed a few pounds! (Read Chapter 2 of
The Gabriel Method for just such a story that led Jon Gabriel to lose over 220 pounds). Otherwise, it could be their diet. Your suspicions about vegetable (o-6) oils, grains and artificial flavors may be right. And just having all the recommended vitamins and minerals is often not enough. What about essential fatty acids?
Regarding hyperpalatable vs. homemade food, I agree with you that the latter is much more satisfying. You ask a key question:
If I have homemade food next some something that is "hyperpalatable," the homemade tastes infinitely better. Yet it fills you up instead of giving you that could eat all day feeling. So why does something that tastes worse make you want to eat more? Perhaps someone who has not consumed mostly homemade food their whole life like I have could suggest an explanation. The only thing I can think of is that the "hyperpalatable" food is not satisfying, so you continue eating it in the hopes of being satisfied. This might be because the food is good so that you want to eat it, but not good enough so that it satisfies you.
I think you hit the nail on the head: so-called hyper palatable food generates cravings precisely because it fails to satiate. This then raises the question: exactly why does it fail to satiate?
The answer, I believe, comes back to metabolic dysfunction, and the theory I first proposed on the
Diet page of this blog. The strong flavors of hyper palatable food activate the vagus nerve, which connects the hypothalamus to both afferent (sensing) circuits in the nose and mouth and efferent (acting) circuits in the digestive tract. Hyperpalatable foods typically contain readily assimilated carbohydrates and proteins, which spike insulin. Through experience, the hypothalamus "learns" to secrete insulin and digestive hormones in response to the flavorful stimuli. These causes a more rapid uptake and storage of the glucose, fatty acids and amino acids that are present, depleting the blood stream of these energy molecules -- a fact that the arcuate nucleus in the brain detects. So you are left wanting more, and the hypothalamus turns up the drive to eat more.
In insulin-sensitive individuals, the elevated insulin crosses the blood-brain barrier, where it is detected in the hypothalamus, leading before too long to satiation. In insulin-resistant, leptin-resistant individuals, however, insulin does not readily cross the blood brain barrier, so satiation does not occur. This makes the cravings worse, and the vagus nerve "learns" to secrete even more insulin to compensate, leading to a vicious cycle. Even worse, many of these hyper palatable foods are pro-inflammatory, further reducing the sensitivity of glucose transporters and reducing the permeability of the blood-brain barrier to insulin.
By contrast, home made foods are typically lower in fast-release carbohydrates, blunting the insulin response. And even if they do contain starches and sugars, they generally are non-inflammatory, so the efficiency of insulin and transmembrane transport are improved. The appetite centers in the brain can thus be more readily accessed, leaving you satisfied.
Subjectively, all you know is that the hyper palatable food is generating a stronger craving. But underlying all of this is a biochemical set of events that train the hypothalamus to treat the flavors, aromas (and other sensory stimuli) from these particular foods as more salient by a conditioned release of dopamine -- a neurotransmitter that can lead to addictive behavior when over secreted.
At base, however, this response to flavor is learned. As Seth Roberts discovered in his Shangri-la Diet, breaking the association between the flavor and the calories removes the drive to eat and restores healthy appetite suppression. The flavor by itself -- devoid of calories -- is impotent. There are no "hyper palatable" calorie-free foods.