Your blood sugar. You can't feel it or taste it, but if your blood sugar levels start to creep up, you're putting yourself at increased risk for everything from heart and kidney disease to dementia, nerve damage and early death. You don't even have to be officially diagnosed with diabetes to find yourself in trouble. A condition called "insulin resistance," in which your cells prevent insulin from "unlocking" the door so energy in the form of glucose can enter, also increases your risk of heart disease, as does full-blown diabetes.

The remedy? Watch your diet, get regular physical activity and try to maintain a healthy weight—nothing new here. And if you're 45 and older, make sure you're getting regular blood sugar tests at least every three years. Your health care professional may want to test you more often depending on your risk factors for developing diabetes. Also measure your waist every few months. A high waist-to-hip ratio increases your risk of insulin resistance and diabetes.