The earliest warning sign of dementia?

No one wakes up suddenly with heart disease or diabetes. You get warning signs first -- usually plenty of them -- and if you heed those warnings, keep taking your blood pressure, and make some changes, you can avoid the diseases.

It's not as easy with dementia.

By the time the most obvious warning sign -- memory loss -- is noticeable, cognitive decline is already setting in and dementia is often (but not always) on the way.

Some doctors will tell you dementia is impossible to reverse or even slow. That's just not true, as readers of my Health Revelations newsletter will find out next month. (And if you're not a subscriber yet, sign up here and make sure you don't miss it.)

But since dementia is one of the most difficult diseases of all to slow, the best approach isn't to wait until you get it before you take action.

It's to take action now.

The warning signs of dementia may not be as obvious as the ones for heart disease and diabetes, but they can be found -- and one of them can be found by taking your blood pressure.

Not your normal blood pressure, mind you (that would be easy to spot), but central blood pressure, which  is taking blood pressure that measures the pressure of the flow of blood from your heart to the brain through the central arteries such as the aorta and the carotid arteries.

Central blood pressure tends to rise a bit as those arteries stiffen with age. But if they get too stiff, the blood pressure going to the brain can get too high -- and if it gets too high, your brain could actually be damaged by it.

As a result, people with higher central blood pressure tend to have lower scores on cognitive tests, according to one new study.

Specifically, high central blood pressure can slow both your thinking and recognition abilities -- two risk factors for more serious cognitive problems, according to the study of nearly 500 Australians.

Clearly, you want to keep an eye on your central blood pressure -- but it's not as easy as keeping on eye on your regular blood pressure. Until relatively recently, measuring central blood pressure was an invasive procedure.

Now, there are some noninvasive techniques for taking blood pressure  that can get the job done, and your own doctor may even have some of them available in his clinic.

If yours is high, you'll want to take action to reduce it. Along with increasing the risk of cognitive problems, high central blood pressure can also lead to problems in the eyes, kidneys, and (of course) the heart.

Reducing central blood pressure isn't as difficult as it might sound. In fact, most of the same lifestyle changes that can reduce ordinary blood pressure will do the same with central blood pressure.

Start with diet. Eat real, fresh foods and skip processed junk, and your numbers will almost certainly decline.

(I recommend the delicious and healthy Mediterranean Diet. Click here to learn more.)

In addition, be sure to eat plenty of the dark blue, red, and purple fruits and vegetables. They're rich in the pigment anthocyanin, which has been shown to reduce central blood pressure (especially in women).

Finally, don't forget to exercise. Aerobic exercise in particular is great for reducing central blood pressure as well as your normal blood pressure. It's also a great way to lose weight and keep fit -- and that, in turn, will also slash your dementia risk.