There may be a difference between a food's listed calorific value, and the amount of calories it actually delivers - but only a small one.
A friend of mine knows exactly how many calories are in Starbucks lattes (265 for a large one with whole milk). He eats nothing without first checking its calories and has been slim for ever. Yet according to the Daily Mail this week, "counting calories is virtually meaningless because we all digest food differently". The story seems to come from a feature in Scientific American by Rob Dunn, a biologist at North Carolina State University that says calorie counts on food labels are unreliable. They don't take into account the amount of processing the food has gone through, the way we cook it and the individual bacterial colonies we have in our guts. These factors can all make our net calorific intake (the amount we absorb) substantially different from what it says on the label. A study from USDA often quoted to support this argument, but which is hardly dramatic, shows that almonds are less completely digested than their levels of fat, protein and carbohydrate suggest. Instead of an average serving providing the 170 calories indicated on the label, it delivered only 128. This is because almonds are fibrous and take more effort to digest, and because bacteria in the intestine take some of the calories for themselves. Proteins require 10 to 20 times as much heat energy to digest as fats, says Dunn, but the loss of calories through generating heat is not accounted for. So in reality you take in fewer calories than it says on the packet. Studies on mice show they get more energy from cooked than raw meat. So should we ignore calorie counts and just go for raw foods that are tougher to eat?
There may be a difference between what calorie counts say and how many calories a food actually delivers - but not by much. A calorie (Kcal) is the amount of energy needed to raise the temperature of 1 kilogram of water by 1 degree celsius. Most calorie counts come from indirect estimates using the Atwater system, developed by Wilbur Atwater at the end of the 19th century. This adds up the calories provided from the constituents of the food - protein (4Kcal/g), carbohydrates (4Kcal/g) and fat (9kcal/g). Foods also now have 2Kcal/g added for fibre.
You'd be wrong to ignore calories, because that way fatness lies. "We have to restrict our calorie intake overall," says Sue Baic, a dietician and spokeswoman for the British Dietetic Association. "Being aware of calories and comparing them is useful." She also wouldn't advise that we should go for raw: harder to digest food. "Some nutrients are much easier to get from cooked food." So calorie counts are not meaningless at all.
In Picture: Do you know how many calories there are in a latte? Photograph: Alamy