Food service industry sales are expected to increase nationwide in 1981 after two years in which the restaurant and food service industry registered declines, according to a forecast by the National Restaurant Association.

Higher sales in fast-food restaurants are expected to be responsible for the overall rise.

Overall sales are estimated to reach $122.7 billion in 1981, a 10.2 percent increase over 1980. When inflation is taken into account, that translates into real growth of only 0.6 percent -- a modest increase but an improvement, if it occurs, over 1980 when real sales declined 1 1/2 percent and 1979 when they were off by 1.8 percent.

The gain is expected to be made by fast-food restaurants, where sales are projected to increase by 2.3 percent. Sales in other restaurants are expected to decline by 0.3 percent as consumers continue to cut back.

Inflation has produced trends among restaurant customers, including "trading up" -- exchanging two or three visits to lower-priced restaurants for a special occasion -- "trading down" to lower-priced restaurants, or "trading out" -- giving up restaurant meals entirely -- according to the NRA.

"Overall the industry has been subjected to declining customer counts for the past two years. More recently traffic has been up at fast-food and family restaurants while it has been off at other types of eating places," according to the NRA's report.

The report also examined sales regionally and found them increasing fastest in the South and the West, where the movement of manufacturing has produced relative prosperity.

In this area, food service sales are expected to increase 5.6 percent in the District of Columbia, 8.8 percent in Maryland and 10.8 percent in Virginia.