Leptn is often considered the anti-ghrelin. It is produced by fat cells to say “I exist and am full, you do not need to feed me.” Animals with their leptin gene knocked out grow enormously fat. This is a perfectly lovely story that can be conclusively proven by a picture of a fat rat.
If you do not find this story compelling, please consider that I also have a photo of a fat mouse.
Are you convinced yet? Look, I know last week I said all hormones are almost fractally complicated and anyone who says they completely understand one is lying, but that entry forever to write (thanks for publishing that a week early, wordpress), and this entry has pictures of obese rodents. Surely you believe the rodents?
*sigh* I’ve created a monster.
Like ghrelin, leptin is important to fetal lung development because, and I quote, “I don’t know stop asking me.” Leptin is also produced by the ovaries, skeletal muscle, stomach (some cells produce both ghrelin and leptin), mammary epithelial cells, bone marrow, pituitary, liver, and of course adipose tissue.
Leptin stimulates ovulation and sperm production, which makes some evolutionary sense: getting pregnant when you don’t have the resources to carry it to term in a healthy way is extremely costly (men have to be nearly dying before they stop producing sperm entirely, but levels can drop incompletely before then). This doesn’t explain why the ovaries (but not testicles) produce leptin, since they don’t have any independent information about fat stores. This may be an example of an override (in which the ovaries decide they want a baby even though the rest of the body doesn’t believe it has enough fat), but the fact that I can come up with a clever anthropomorphization does not make an explanation legitimate. You can sort of see why leptin facilitates the onset of puberty, since puberty takes a lot of energy.
What you can’t see is why, despite everything we know about pregnancy and eating, the placenta produces leptin. Excess amounts appear to cause hyperemesis gravidarum (extreme morning sickness aka Kate Middleton’s one weakness).
High amounts of leptin appear to be good for your brain. Just so story: brains are extraordinarily expensive, so if you don’t have sufficient savings your body turns on the dimmer switch. They also have a long term protective effect against Alzheimers. On the other hand, high levels of leptin alter the immune system in a way that encourages artery hardening. I am way more afraid of living with Alzheimers than I am of dying of a heart attack, so I will count this as one point for fat.
Leptin’s overall effect on the immune system is complicated. Leptin is an inflammatory agent, possibly to prevent damage from overreating as your body suddenly tries to shove extra calories that won’t fit in the white adipose tissue under the bed and in the coat closet (the organs). Which may explain why ghrelin is an anti-inflammatory. Leptin and ghrelin chose opposite powers and color schemes, like an early 90s superhero cartoon.
Fatness in humans does not appear to be a problem of inadequate leptin production, and more leptin does not make people thinner. Instead, it appears that the brains of obese individuals are less sensitive to leptin. No one knows exactly why, but “crash dieting” is high on the list of suspects. Two people with identical body compositions but different genes or life history may produce very different amounts of leptin, which means they may require very different behavior to stay the same weight, in ways we do not understand at all. Which I could have told you before we went on this magical photographic tour of my childhood. But now we know for sure, plus I learned that fetal lung development is creepily intertwined with food in a way no other organ is. Let us go forth and use this new knowledge