I guess leaner people become more fixated on their own looks, they like to be attractive. This could increase extraversion.

It also could lead to a different outlook on life, how to enlengthen your attractiveness: lower time preference. Though, low time preference could be only in this area and not spill over to other parts of life, like, say, saving more money.

Body mass correlates with quality of sleep. Weight gain = apnea for many. This would lead possibly lead to different levels of melatonin and serotonin floating around in your system. Just a thought.

I've heard talk of cytokines up with fat, and androgen -> estrogen conversion up in males. I can't vouch for any of it.

Think I read it in Taubes' "Good Calories, Bad Calories", but I understand fatty tissue secretes hormones, such as estrogen, that can have a feminizing effect on personality.

Fat men used to be seen as jolly. If also tall, they were often seen as "gentle giants" and as "surprisingly light on his feet". I'm not sure about old stereotypes for fat women. Anyway, did the stereotypes contain any truth?

Higher body fat means higher estrogen. The leaner the man (the lower his body fat %) the more testoerone he will have with all other factors controlled for such as race.

thanks. i had read the stuff on testosterone but didn't know whether to believe it. hard to navigate the literature on this stuff.

Yes, lower BMI should lead to changing ratios of hormones, such as estrogen:testosterone. Also, as Dex points out, better sleep should lead to hormal changes, such as more growth hormone and different levels of corticosteroids.

As to the personality effect of these different hormonal changes, I don't know. I believe men might become mildly more aggressive/confident if they dropped a lot of body fat and added some muscle, but the age of the man would have a big impact on relative testosterone as well.

As Gary Taubes points out, insulin used to be used as a psychiatric treatment, and when I went on the Atkins diet several years back, my mood did stabilize and improve. Of course metabolic hormones and nutritional content aren't BMI...

Female perspective. Also prolactin. I noticed I gained more muscle and lost more fat when nursing and exercising than before I was pregnant. Combination low estrogen, high prolactin. I ended up about 10 pounds lighter than before I was pregnant and got that body fat estimate at the gym. Anyway at BMI of 18.5, I had far more energy and needed less sleep than at my usual BMI 20. I also found it easier to concentrate. The effect lasted at least a year and a half after I stopped nursing.

I did not have the same experience after my first baby, but I also did not go below my usual BMI of 20.

> As to the personality effect of these different hormonal changes, I don't know.

Well, elevated corticosteroids are found in a large fraction of depressed subjects, but I don't know if it's thought to be causal. It is causal of depression in Cushing's disease, a curable benign tumor that can secrete either cortisol or hormones upstream of cortisol (ACTH, and maybe CRH too but I think not). I'm not sure these subjects, however, don't have rather higher cortisol levels than depressed subjects.

At any rate, the connection was considered plausible enough that pharma developed some inhibitors of the cortisol axis as antidepressants. I think they didn't really pan out though. If you like the cytokine hypothesis of depression (which I do), it's easy to see the cortisol elevation as just being a result of elevated pro-inflammatory cytokines, whose cortisol-elevating effects are well known. If so, it could be a subordinate partial cause of the depression.

If apnea is our only route, though, to higher cortisol in fat people, then those affected will certainly be a smallish minority. Obstructive apnea is extremely common as diseases go, but most people won't have it. One web source estimates a prevalence of 5% in the West. As usual for disease prevalences this seems a little high to me, and I wonder if it hasn't been inflated a bit by grant applicants and other stakeholders, who all alike have reason to err on the high side of the real value... as do genuine altruists who simply want people to be aware of their possibly having the disease.

Free fatty acids (FFA's) are higher in obese people, in addition to the higher levels of estrogens and lower testosterone. It's almost a guarantee that there would be liver damage, too, and that will ultimately end up deranging cerebral blood flow and glucose utilization. I've read some stuff about mitochondrial damage in hypothalamic neurons, partially due to the poor oxidation of FFA's. ATP depletion in some hypothalamic neurons or in the liver induces feeding behavior in rodents, and both of those factors are likely to be present in obesity. The adipocyte-derived cytokines, as some commenters mentioned, would worsen the fructose-induced ATP depletion in the liver. Supposedly, more preadipocytes can differentiate into adipocytes, too, in obesity. It's like a cancer, almost, and obese people have more adipocytes and not just larger adipocytes.