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grams is weight -ounces, pounds. Calories is the amount of food energy in the weight of what you ate. there is not much calories in lettuce since lettuce is mainly water and water has no calories. but the same weight of cheese has tons of calories since most cheese is one third fat and fats are the most energy packed foods. So 15 grams (about half an once) of lettuce (a large leaf)will have next to no calories (2 calories) - while 15 grams of Cheddar cheese will have a lot of calories(60 calories)
Liquids can also have calories . water has none, but milk will- the calories coming from the fat and sugars in milk. Whole milk (milk with no fat removed will have in 1 cup 250 grams, -160 calories, 72 calories from fat and most of the rest ,about 88 calories from sugars ; skim milk -milk with all its fat removed will have in 1 cup, 250grams,- 90calories almost all from sugars in the milk)
how much of what you should eat is individual. Go click on library above and look up questions and answers. If it still is confusing ask questions either in the forums or directly to CC.