Calorie density simply refers to the number of calories by mouthful. For optimum weight management we should aim to eat less high calorie dense foods (except during training and racing) and more low calorie dense foods.
Here's a list of common foods & drinks that are calorie dense:
Fruit juice, full-fat dairy products; nuts; dried fruit; all commerical
sports nutrition - bars, gels, protein drinks; processed meats like bacon, sausages and any meat with a fat content over 10%; skin/fat on meat; soft drinks; commerically made sweets, biscuits, cakes, donuts, pancakes; mayonaise & salad dressings, pizza, lasagna and other cheese-based meals like macaroni cheese; all fast foods; fried foods; alcohol. Find ways to limit, substitute or modify your intake of these high calorie dense foods. Note that the first few foods in this list are "healthy" options but the take-
home message is that these are only good choices when your calorie intake needs are high (such as post-workout recovery).
Here's a list of common foods that are low in calorie density:
Fresh fruits and vegetables, lean chicken (skin removed before cooking), turkey, pork (fat removed before cooking), lean beef and lamb (grass fed preferred), unbuttered popcorn (popped in microwave), puffed wheat cereals and rice cakes (crackers). Note that if your calorie intake needs are high (such as to gain weight or meet higher Basal Metabolic Rates), then you will need to eat large quantities of foods in this list.
Remember, food is fuel so endurance athletes will select high density calories at specific intervals during training, however there is no advantage in consuming empty calories with no nutritional value at any time. Outside of racing conditions, always choose a real food option over a supplement (more on that later).