First, check out your daily diet. Are you eating three meals a day plus some healthy snacks. Often with slim people, there are not enough calories, it is not just a lack of protein. Try to make the changes that are easy, adding some richer foods for your usual; creamy or thick soups instead of broths, sauces on vegetables, a little meet or cheese with crackers and fruit for a snack, and so forth.

Eggs are a cheap source of complete protein. If you do not have high cholesterol and do get regular exercise, you can eat some eggs in your diet. If you can afford them, dry milk powder or whey protein powder can be added to drinks anbd puddings to make them richer.

Good luck with your plan. Slim people do tend to live longer lives than much heavier folks, you know.