DEAR DR. DONOHUE: It has been said that you can't get sufficient vitamins and minerals from a very low caloric intake. When is this low calorie threshold reached for a dieter? Also, I thought carbohydrates were bad for you, the starches, etc. Why do you insist they be a big part of a diet? - Mrs. H.R.

ANSWER: As a general rule, multiple vitamin and mineral supplementation should kick in when diet intake is under 1200 calories a day. It's hard to get those nutrients with that low an intake, not even if you follow the 60-30-10 rule - 60 percent carbohydrate, 30 percent fat, 10 percent protein calorie mix.The word carbohydrate is sometimes misunderstood. We are not speaking of the sugary kind, but the pastas, cereals, vegetables and fruits. They are the so-called complex carbohydrates.

Remember, too, that when you restrict calories you lose muscle tissue along with the fat. So you have to boost protein intake with lean meat, fish, etc. to make up. This is doubly important if you are, as you should be, adding exercise to the weight loss plan.

The underlying idea is to make the body draw on its stored fat for energy as much as you can. And you don't have to sacrifice eating enjoyment to do that.

Finally, to keep off the weight you've lost you have to make such a diet plan a way of life after reaching your goal.

DEAR DR. DONOHUE: I have an underactive thyroid gland. What causes that? Is it because I am so overweight? I find it hard to lose. Incidentally, I am taking thyroid supplements for my gland problem. - A.S.P.

ANSWER: Your overweight did not cause your thyroid to become underactive, but your underactive thyroid could have led to your weight problem by slowing your metabolism.

Thyroids malfunction for a variety of reasons. A common one is inflammation, an immune system attack on the gland tissue. It's called Hashimoto's thyroiditis. Other causes of gland malfunction include pituitary gland tumors and viral infections. Long ago, the chief cause of underactivity was lack of iodine. That is rare in developed society with the availability of iodized salt.

With your thyroid problem diagnosed and with the supplemental thyroid hormones you're taking, things should improve for you soon.

DEAR DR. DONOHUE: What's new in genital herpes treatment? - K.K.

ANSWER: You can't get any newer than the availability of acyclovir in cream and pill forms. And for those who may have had their immunity lowered, acyclovir can be given intravenously.

This is not to say that the herpes experimental front is quiet. Interferon, a body-produced virus fighter, has been made into a gel and is being tested for its effectiveness against genital herpes. Don't look for that yet, because it is experimental.

- Dr. Donohue welcomes reader mail but regrets that, due to the tremendous volume received daily, he is unable to answer individual letters. Readers' questions are incorporated in his column whenever possible.