Summarizing what I’ve discussed so far:
1. Human bodyweight appears to be biologically regulated, that is it makes some attempt (that can be overcome by environment, of course) to maintain body fat within some range or level.
2. The system regulating body fat is assymetrical, for most people it defends against fat loss much more strongly than against weight gain.
3. For proper regulation, the body needs a way of ‘knowing’ two things: how much fat you’re carrying and how much you’re eating; a variety of hormones play a role here.
4. At least in terms of indicating the amount of body fat is present, the hormones leptin and insulin appear to play a major role. Leptin scales with subcutaneous body fat levels (higher in women), insulin scales with visceral fat levels (usually higher in men); there is some indication of a gender difference in response to the different hormones.
Leptin and insulin also both change with changing food intake; leptin levels can drop significantly within a few days of dieting even with no change in body fat levels. Insulin changes meal to meal.
5. When people reduce calories and lose fat, leptin levels drop, and this appears to be a major part of the overall adaptations to dieting in terms of metabolic rate, hunger, etc.
While leptin certainly isn’t the only hormone involved it appears to be one of the major ones not only having direct effects but also impacting how well or how poorly other hormones (such as CCK) work in the brain.
6. While studies have found that raising leptin in overweight individuals typically does little (for reasons related to either leptin resistance or insufficiency in the brain), preventing leptin from dropping during a diet (or raising it) appears to reverse many of adaptations that occur.
Point 6 raises a question that someone actually brought up in the comments: why can’t I find leptin for sale?
And the answer is that it has never (and I suspect will never) been made available outside of research. When I originally wrote my Bromocriptine booklet, an effective dose of leptin came in around $1000 PER DAY. The last time I looked (about a month ago), it’s down to about $500 per day. That’s assuming a chemical company would sell it to you.
That’s not a typo mind you, leptin makes growth hormone look cheap.
For various reasons, it simply hasn’t been developed for human use outside of research applications. Why? I can’t say for sure. I suspect it’s because drug companies primarily want weight loss drugs that cause weight loss and leptin doesn’t do that.
They don’t seem to want drugs that simply make dieting work better. I’d note that the average dieter isn’t looking for that type of compound either. Drugs that generate weight loss without the person having to change their behavior patterns are the real goal.
There is also the issue of leptin being a peptide hormone, meaning it would have to be injected. Injectable drugs are a bitch practically and there’s been a huge push to develop diabetic solutions not involving injectable insulin for that reason; the odds of the typical person injecting leptin twice daily while dieting are slim.
Bodybuilders would, of course, but that small percentage of people trying to get to 5% body fat are not the target market of the drug companies.
End result: nobody is developing leptin for commercial use so far as I can tell and I doubt this will change.
But for dieters and especially the very lean, injectable leptin would be a godsend fixing a majority of the problems that occur with dieting. Unfortunately, it’s a pipe dream at this point.
Where does that leave us?