A year after sounding alarms about lowcalorie liquid protein diets, the Food and Drug Administration yesterday revised its order for warning labels on the products.

The agency said that, although use of liquid protein for diets "has greatly decreased since FDA warned the public about them last year, there is still a potential for serious health problems if the products are misused."

The warning labels were first proposed on Dec. 1, 1977. Yesterday's order is tentative until public comment can be reviewed.

The regulation would require warnings on all protein products that provide more than half of a person's calories and are promoted for use in losing weight or as a food supplement.

The label on protein products used solely for weight reduction would say: "Warning -- Very low calorie protein diets may cause serious illness or death. DO NOT USE FOR WEIGHT REDUCTION WITHOUT MEDICAL SUPERVISION. Use with particular care if you are taking medication. Not for use by infants, children or pregnant or nursing women."

Less dire warnings would be used for products in which the person consumes 800 or more calories per day, or simply takes protein as a food supplement in an athetic training program or other circumstances.

Liquid protein was widely promoted last year as a means of losing large amounts of weight. But the FDA said 16 persons, mostly women, died while on the diet for prolonged periods.

The agency is still trying to pinpoint what led to the deaths. Research has indicated that the victims suffered from erratic heartbeats possibly caused by the liquid protein.