Gastric bypass isn’t a highway, but for some obese patients it’s the most effective route to losing weight.

This surgical procedure creates a shortcut in the digestive system, diverting food past some of the stomach and small intestine. So when patients eat they get full faster and absorb fewer nutrients.

The operation also reduces appetite, by changing the body’s hormone balance.

A study published in the January 2006 issue of the journal Annals of Surgery sheds new light on this metabolic downshift.

It showed that after eating, bypass patients had much greater amounts of the hormones G-L-P-one and P-Y-Y in their blood plasma, compared with control groups of thin people and untreated obese people.

These hormones reduce appetite and help regulate blood-sugar levels.

The researchers also found bypass patients had much higher levels of G-L-P-one and P-Y-Y than a group treated with gastric banding. This surgical procedure uses an adjustable collar to divide the stomach, creating a small pouch where food arrives.

Bypasses and banding both reduce the functional size of the stomach. And both deliver the same degree of weight loss. So the difference in hormone response appears to be related to the surgery itself.

The researchers didn’t suggest a detailed explanation of what happened. But they did recommend further studies using drugs to replicate the hormone changes that occur after bypass surgery.

And if we gain a better understanding of these endocrine traffic patterns, maybe we can find new avenues to help people lose weight without surgery.