Q Could you please discuss GNMA unit investment trusts? I had read about GNMA being a very secure investment, but the units I bought last year have decreased in value since then. If I hold the units to maturity, will they pay back the principal I invested?

A GNMA (Government National Mortgage Association) is a very secure investment. Payment of both interest and principal is guaranteed by an agency of the U.S. government, and you will get both your principal and all interest due if you hold the trust units to maturity.

However, that guarantee does not protect you against intermediate fluctuations in value. If interest rates rise above what your units are paying, sale of your units before maturity would give you a loss.

Although I like GNMA unit trusts for certain types of investors, I only recommend their purchase if you expect to hold them to maturity.

There is another element in GNMAs that may be confusing you. The underlying security is a portfolio of insured mortgages. Like any mortgage, the income to the trust -- and in turn to you -- consists of two segments: Interest income, taxable as ordinary income on receipt; and return of principal, very low in the first few years and progressively larger as time goes on.

As a result of this continuous return of principal, the value of the units decreases each month regardless of interest rate changes. In other words, with each check you get, you are receiving repayment of a part of your original investment. The payment advice you get with the check tells you how much of the check is interest and how much principal.

You should put the principal portion away for later investment; if you spend it with the interest portion, you must realize that you are blowing part of your principal. To avoid this, you might ask the sponsors to reinvest the principal portion each month in their GNMA accumulation fund, sending you only the interest.

So the answer to your last question is not a simple yes or no. If you hold the units to maturity, you will not then get back your total investment; but you will have gotten the entire principal back, in small bits and pieces over the years.

In past replies to questions about withdrawal options for Individual Retirement Accounts at age 70 1/2, I have said that the minimum annual withdrawal is based on life expectancy at 70 1/2. A man of that age has a 12-year life expectancy, and so must withdraw one-twelfth of the starting balance in the first year, one-eleventh the second year and so on.

Starting this year, the Tax Reform Act of 1984 changes those rules. The withdrawal schedule starts the same way; but in subsequent years, the payee recalculates his or her life expectancy to determine the fraction that must be withdrawn that year.

At age 75, for example, a man has a life expectancy of nine years; that is, he is then expected to live to 84, instead of the 82 originally postulated. So where a man of 75 would have had to withdraw one-seventh of the remaining balance under the old rules, the minimum payout is now only one-ninth.

You must understand, of course, that these figures are minimum withdrawals only. Once you reach 59 1/2, you may take out of your plan any amount you wish, up to the entire balance. But reducing the minimum withdrawal means that the principal can be stretched to last several years longer than it would have under the old method.

The IRS announced that the annual interest rate charged on tax underpayments went up to 13 percent on Jan. 1. (The same rate will apply to interest paid by the IRS on qualifying overpayments and refunds.) The old rate, for the period July 1-Dec. 31, 1984, was 11 percent.

The interest rate is adjusted semi-annually, and is based on the average prime interest rate for the six-month period ending March 31 or (as in this case) Sept. 30.

The six months from April 1 to Sept. 30, 1984, saw relatively high prime rates, so the new IRS rate is higher than might be expected, considering the drop in rates in recent weeks. The present lower rates will be reflected in the next setting, expected to be announced in late April for the last six months of 1985.