Coffee good for you, but it's OK to hold back

Story highlights

  • Coffee may prevent type II diabetes and Parkinson's disease, evidence shows
  • It may also have anti-cancer and antidepressant effects
  • Some people can't tolerate coffee because of side effects
  • Doctor: If you don't drink coffee and want to start, ease into it

(CNN)If you can't get through your day without a coffee break or two, here's good news for you: What scientists know so far suggests coffee may help you stay healthy.

As usual with medical research, the operative word is "may."
It's hard to know for sure whether coffee is really causing good effects -- lifestyles or behaviors associated with coffee consumption may also influence health. Also, different people have different tolerances for coffee -- it can have short-term side effects that make people steer clear of morning brews.
So, doctors aren't quite convinced enough to prescribe coffee -- but they probably don't need to, because so many people indulge in it anyway.
    The point is: In general, regular coffee drinkers won't be discouraged from continuing the habit, although there are exceptions.
    "For most people, for people who don't experience the side effects, the benefits far outweigh the risks," said Dr. Donald Hensrud of the Mayo Clinic.
    Why would coffee be good?
    More is known about the overall association between coffee and positive health effects than about the mechanism behind it, said Dr. Alberto Ascherio, professor of epidemiology and nutrition at the Harvard School of Public Health.
    Antioxidants are one potential reason that good outcomes are seen from coffee. Our bodies produce oxygen radicals, which are damaging to DNA. Antioxidants prevent them from doing damage, Ascherio said.
    Although antioxidants are found in fruits and vegetables, research has shown that coffee is the top source of antioxidants for Americans.
    Caffeine itself may also contribute to coffee's positive effects on brain health. That may be because caffeine is an antagonist to adenosine receptors. These receptors normally slow down neural activity when the chemical adenosine binds to them, producing a sleepy feeling. But if caffeine binds to the receptors, the activity of neurons speeds up.
    Coffee also appears to lower levels of insulin and estrogen, which is perhaps why a study last year found a lower risk of endometrial cancer in coffee-drinking women. Insulin also plays a role in prostate cancer, another disease coffee may help stave off.
    What good it may bring
    The evidence is fairly strong for coffee preventing type II diabetes and Parkinson's, and reasonably good for antidepressant effects, too, doctors say.
    Just in the last few months, several new studies have been published highlighting other possible benefits of coffee. Again, none of them prove that coffee causes any effects at all; they are just associations.
    People who drink two 8-ounce cups of coffee daily appear to have an 11% lower risk of developing heart failure, compared to noncoffee drinkers. That's according to a meta-analysis