I was reading about a hormone called atrial natriuretic peptide (ANP). It's released from the heart to help regulate the body’s water balance and blood pressure. Well, it turns out that ANP is profoundly fat mobilizing. More interestingly, it works by a completely different mechanism than insulin/adrenoceptors work through.

Basically, ANP appears to side step the whole adrenoceptor pathway to stimulate lipolysis. This might make it feasible to mobilize fat from stubborn fat without having to go through all of the rigamarole involved with inhibiting or overcoming alpha-receptors.

ANP

Increases the release of free fatty acids from adipose tissue. Plasma concentrations of glycerol and nonesterified fatty acids are increased by i.v. infusion of ANP in humans.

Activates adipocyte plasma membrane type A guanylyl cyclase receptors NPR-A

Increases intracellular cGMP levels that induce the phosphorylation of a hormone-sensitive lipase and perilipin A via the activation of a cGMP-dependent protein kinase-I (cGK-I)

Does not modulate cAMP production or PKA activity

And this is a part I found particularly interesting. Among other things, ANP is secreted in response to Atrial distention, stretching of the vessel walls. The atria become distended by high extracellular fluid and blood volume, and atrial fibrillation.Notably, ANP secretion increases in response to immersion of the body in water, which causes atrial stretch due to an altered distribution of intravascular fluid. ANP secretion in response to exercise has also been demonstrated in horses.

So ANP is secreted in response to the body being immersed in water, as well as to exercise. Swimming, basically.

I always noticed as a kid during the summer, that if I spent a lot of time in the pool, I'd get considerably leaner. I more or less thought it was from the resistance of the water, but now I wonder.

Spending time in the pool could be an excellent choice for cardio. I'm thinking especially for pre-contest when the more insulin-loving fat areas are refusing to come off. Really for anyone though.

I realize that the idea of swimming for fat loss is hardly a new one. It's just figuring out something new about a possible mechanism that excites me and I thought I'd share.