Dear Doctor: I have read about the health benefits of dark chocolate for the brain and the heart, but that it must contain about 80 percent chocolate to be effective. How much sugar or additives (nuts, fruit) can be added before chocolate loses its health benefits?

Dear Reader: Ah, chocolate, a food that so often inspires a passionate response. The fact that it also offers a host of proven health benefits seems almost too good to be true.

A number of studies over the years have linked cocoa, the basic building block of chocolate, to cardiovascular benefits, cancer prevention, a lower risk of stroke, better blood flow to the brain, lower blood pressure and improved insulin metabolism. Cocoa also contains important minerals, including zinc, iron, magnesium and potassium.

With your question about the percentage of cocoa in a chocolate product, you've zeroed in on the vital detail. At what point does chocolate go from being just another kind of candy to becoming a beneficial food?

Let's start with a bit of background.

Chocolate is derived from cocoa beans, which are the seeds of the cacao tree, a small, shade-loving evergreen in the tropical regions of the Americas. These beans are high in a substance known as flavonols -- sometimes also referred to as flavonoids. Flavonols, which fall into the category of antioxidants, have anti-inflammatory properties.

The catch is that cocoa -- or pure chocolate -- is quite bitter. That means sweetening agents and milk solids are added to make it more palatable. Depending on the percentage of cocoa in any particular preparation, the result is a product that contains varying degrees of sugar and calories. And while cocoa has proven health benefits, sugar definitely does not.

When it comes to the health benefits of chocolate, the higher the percentage of cocoa content, the better. Sixty percent cocoa is widely considered the minimum when looking to a chocolate product for health benefits. But don't rely solely on the words "dark chocolate" when looking at the label.

In the U.S., a product with as little as 35 percent cocoa may be designated as dark chocolate. European chocolates set the bar a bit higher, with a minimum of 43 percent cocoa. And if you're one of those who love white chocolate, we're afraid there's some bad news. White chocolate is made from the fat of the cocoa bean and contains no actual cocoa at all.

And now, amid all this amazing chocolate news, a bit of a reality check.

A group of scientists from Harvard who reviewed the data from 24 studies pinpointing various health benefits associated with cocoa found that the average dose of flavonols ingested by the test subjects was 400 milligrams per day. Turns out, that's the equivalent of (are you sitting down?) eight bars of dark chocolate, or 30 bars of milk chocolate per day.

If you want to match that intake, cocoa supplements are available that skip the sweeteners and fillers that make chocolate so delicious. But if you're going to add actual chocolate to your diet, limit it to 2 ounces per day, and go with as high a percentage as you will actually enjoy.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: I'm almost 90 years old and always tired. I took two capsules of 2,000 mcg vitamin B12 the other night, and felt stronger and not as tired. Is it safe?

Dear Reader: My first thought is that you might have B12 deficiency, caused either by inadequate intake or by inadequate absorption of B12. Animal products such as meat, dairy and eggs are the only sources of B12 in humans, and the recommended daily allowance is 2.4 micrograms per day. The average intake of an individual in the United States is approximately 3.4 micrograms per day, but if you follow a vegan diet, you obviously lack sources of B12. Thanks to evolution, one-half of your B12 intake is stored in your liver, and it takes many years for this to be depleted.

If your intake is sufficient, it may be your ability to absorb B12 that is compromised. This can occur if you have inflammation of your stomach lining or disorders of the small intestine, such as Crohn's disease. Antacids and proton pump inhibitors such as Prilosec, Protonix, Prevacid, Aciphex, Zantac and Pepcid also can lead to deficiency, because they reduce the stomach acid necessary to release the B12 from food. Metformin, a drug used to treat Type 2 diabetes, can also decrease absorption of B12 in the small intestine in 10 percent to 30 percent of patients.

Lastly, age can be a risk factor for B12 deficiency. Multiple studies in this and in other countries have found deficiency in 10 percent to 24 percent of people 75 and older. The causes may be both inadequate intake and inadequate absorption. Significant B12 deficiency can lead to anemia, nerve damage and dementia.

But let us say that your B12 levels are normal. Is there any benefit to taking more, and is there any danger in doing so? Many of my patients, who are not B12 deficient, feel greater energy and improvement of their health with B12 injections. I am not certain why this happens, and it doesn't appear to have a lasting effect. It is possible that extra amounts of B12 can improve the metabolic function and allow the vitamin to be utilized better.

Water soluble vitamins, like B12, can be tolerated at higher doses without adverse effects upon the body, and the National Academy of Medicine has not established an upper limit of vitamin B12 intake. The dose of 2,000 micrograms of B12 is more than 800 times the recommended daily amount. So, on the face of it, this dose seems too high, but as you increase the amount in your diet, less of a percentage will be absorbed.

Before continuing the B12, I would recommend getting your levels tested with your doctor. If you show a deficiency, your doctor may want to investigate and gauge how much of the vitamin you might need.

If you are not deficient, I believe it is safe to take a supplement at a lower dose; you don't even have to take it every day because B12 is well-stored by the body. Further, if you want to ensure better absorption of B12, you can try preparations that can be absorbed under the tongue. The important thing is to investigate why this B12 dose had such an impact -- and how you can safely replicate the effects.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: About a year ago, when I turned 44, I started using reading glasses that I purchased at the drugstore. They seemed to work just fine. Now I need a stronger pair, and I wonder whether using them has weakened my eyes.

Dear Reader: We're happy to reassure you that using over-the-counter readers -- or any kind of corrective lenses, for that matter -- will not weaken or damage your eyesight. However, the fact that you find you need to move to a higher level of magnification does indicate that your vision is changing. And considering that many adults begin to experience changes to their ability to see clearly at close range after age 40, it's not all that surprising.

This change in close focus is known as presbyopia, which occurs when the lens within your eye gradually becomes less flexible. This is believed to be the result of age-related changes to the proteins within the natural lens in the eye, and to the tissues of the surrounding muscles that cause the lens to focus. The loss in flexibility to the natural lens results in blurred vision when doing close-focus activities such as reading.

While you could certainly opt to buy a new pair of drugstore reading glasses with a higher level of magnification, we recommend that you use this shift in vision as an opportunity to visit an eye care professional.

One-size-fits-all reading glasses are certainly affordable, and a quick trip to the store is more convenient than a medical appointment. But chances are the generic readers will not correct your eyesight to the highest level of accuracy. That's because, for most of us, the prescription in each eye is at least slightly different. Many individuals also have a small amount of astigmatism correction in their prescriptions as well. Wearing the wrong glasses can lead to headache and fatigue as your eyes strain to achieve optimal focus.

A comprehensive eye exam not only yields a corrective prescription tailored to your specific needs, it also includes several other tests to detect vision problems, assess eye health and screen for eye disease. For example, your eye care professional will use special drops to dilate your pupil and examine the important tissues at the back of the eye, including the retina, the macula and the optic nerve. A test of the pressure within the eye, known as tonometry, screens for glaucoma.

If you already wear glasses to correct farsightedness, you have the option of blending the two prescriptions in a pair of bifocals, trifocals or progressive lenses. A pair of multi-vision glasses for life on the go, and a pair of reading glasses for sustained close work, will give you the best of both worlds. If you prefer contact lenses, multifocal contacts correct near, intermediate and far vision.

Age-related changes to vision, once begun, will continue. However, these changes can occur so gradually that they may be difficult to notice. That's why it's important to schedule regular eye exams and safeguard your vision.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)