No one likes feeling down. But down and like a dim bulb? That's just adding insult to injury.

Yet new research suggests this may be exactly what happens to some people. When depression -- even moderate depression -- is combined with aging, it increases the risk of mild cognitive decline in just a few years. And half the cases of mild cognitive decline develop into full-blown dementia. Which underscores why persistently feeling "just a little down" should always be treated, not shrugged off.

While moderate to severe depression is the biggest threat to mental sharpness, even mild depression carries a slightly increased risk.

What's the connection? In some people, depression may actually be an early symptom of neurological disease that will eventually lead to dementia. In others, there might be a genetic link between depression and dementia. It also is possible that depression gradually damages parts of the brain that play a role in memory. Or all of the above may be true; much more research is needed.

Regardless, depression should be treated and monitored closely because there's plenty of evidence that it's bad for physical as well as mental health. There's a wide range of effective treatments. That's one of the miracles of modern medicine. Take advantage.

Depressive symptoms, vascular disease, and mild cognitive impairment: findings from the Cardiovascular Health Study. Barnes, D. E., Alexopoulos, G. S., Lopez, O. L., Williamson, J. D., Yaffe, K., Archives of General Psychiatry 2006 Mar;63(3):273-279.