How the recommended daily calorie allowance failed us all

Calories give us an idea of how much energy foods and drinks contain, and have been a dietary obsession in recent decades. Most of us can recite that women need 2,000 calories a day, and men need 2,500.

But as NHS guidelines point out, “an ideal daily intake of calories varies depending on age, metabolism and levels of activity, among other things.”

Calories are nice theoretical units, says Dr Duane Mellor, a British Dietetic Association spokesperson and registered dietitian, but are flawed. While a hazelnut contains a certain amount of energy potential, when eaten it requires quite a lot of energy to break it down – and still more energy will escape digestion.

“This is because the energy is inside the fibre, which we can’t break down. Whereas the same number of calories from instant noodles are a lot easier to get hold of,” he says.

Mellor points to the work of Kevin Hall at the US Department of Health and Human Services, who has found that eating ultra-processed foods compared to unprocessed foods led to greater energy intake and weight gain, despite the foods having the same calories, sugar, fat, sodium, fibre and macronutrients.

Then we need to account for variation in unprocessed foods. “If you’ve got a number on a label, because of biology, it can vary by about 20 per cent,” says Mellor.

Read more:

• The five-a-day fallacy: How to actually get enough fruit and veg in your diet
• How are calories in food calculated?

Even apples from the same tree can differ in sugar content by 10 per cent, according to which got the most sunshine.

“If you’re going to look at calories, go for foods with more fibre, protein, vitamins or minerals in them which give you bulk and more satisfaction,” he advises.

But rather than sweat the numbers, he says to focus on more variety in terms of colour, fibre and nutrients.

Berry doesn’t believe that calories are a useful measure for health. “We know that there is much more to food and health than calories. Moving the conversation away from calories and how much you weigh, and towards how you feel and improving our relationship with food and eating healthier foods, is much more helpful. Focus on how healthy a food is, not on how many calories it has.”

As studies from personalised gut-health and health research company ZOE have found, different foods cause harmful blood sugar or fat spikes in different people.

“Understanding what works for you is simple but crucial,” says Dr Sarah Berry, associate professor of nutritional sciences at King’s College London, and ZOE chief scientist.

“A lot of people are out of touch with their hunger and fullness cues, their energy levels and their enjoyment of food. Our members want to feel better, not worry over a number on the scales. Nutritious foods lead to enjoyable meals.”