December 31, 2020
So if you want to lose weight you need to eat fewer calories than you burn.
Sounds easy, right?
We sat down with Christopher Gardner, Professor of Medicine at Stanford University, and Sarah Berry, reader in nutritional sciences at King's College London and lead researcher of our PREDICT studies, to find out why this simple equation doesn’t always hold up.
Calories measure the amount of energy present in food. It can be calculated in a lab using a special piece of equipment known as a bomb calorimeter, but people usually just use standard calorie value tables that have been around for many years.
Generally, we estimate the number of calories from the amount of carbohydrates, fats, proteins and alcohol (macronutrients) in a food. One gram of carbohydrate or protein provides around 4 calories, while one gram of fat notches up 9 calories.
However, these amounts are averages and there are small differences between the calorie content of different carbohydrates (sugars and starches), between animal and vegetable proteins, and between different types of fats.
That imprecision is also reflected on food package labels. The calorie values that appear on labels are useful to compare between similar types of foods, such as breakfast cereals or yogurts, but they’re a terrible way of calculating how many calories you’re actually eating.
The numbers are estimates with a wide margin of error, based on previously calculated calorie values from individual ingredients. It’s also easy to be caught out by portion sizes, as the calorie count on the label may be for a far smaller portion than you actually eat.
Figures on food labels tell us nothing about other important factors, such as the structure of the food (the food matrix), how the food was processed, and any interactions between nutrients.
For example, grinding or chopping almonds affects their food matrix and allows us to absorb more calories from them than from the same amount of whole nuts. Similarly, finely ground oatmeal causes less healthy blood sugar spikes than larger oatmeal pieces.
Calories also don’t tell us anything about the quality of the food we’re eating. A hundred calories of candy is very different in nutritional terms from a hundred calories of nuts or kale.
Avoiding some foods that are high in calories, such as olive oil or avocados, means we miss out on the healthy fats present in those foods. So, while counting calories has a role to play in weight management, many other factors are important too.
“In my opinion, the back of pack labelling or the nutrient composition and the calories, only tells us one piece of a thousand-piece puzzle,” Sarah says.
The simple take on calorie counting is that if we eat fewer calories than we burn, we will lose weight. Conversely, we gain weight when we eat more calories than we use.
But we don’t just eat ‘calories’. We eat foods, and foods are complex.
Our individual calorie needs are affected by our age, sex, height, weight, and levels of physical activity.
We burn around 70% of our calories just carrying out the metabolic processes that are essential for keeping us alive.
On average we burn another 20% on physical activity and the last 10% is used up on digesting our food and on non-exercise activities such as walking, doing tasks, and even fidgeting.
However, these are estimates and the recommended intakes of 2000 calories for women and 2500 calories for men are averages for the population.
But nobody is truly average. Our PREDICT studies have shown that each of us has our own unique responses to food, so our calorie needs are going to be personal too.
It’s difficult to measure how many calories we consume because that figure fluctuates a lot. Studies have shown that it can go up or down by an average of 700 calories per day, and we generally find it hard to keep track of exactly how much we’re eating.
Challenges in figuring out how many calories we need to eat is one reason why some people struggle to manage their weight with calorie counting. But there are many others.
Food is complicated but humans are complicated too, and many factors affect our response to food, from our genetics and body shape to our microbiome and sleep patterns.
Our PREDICT studies show that even identical twins can have very different responses to the same foods. And identical twins can gain different amounts of weight in response to overeating by the same amount.
“A lot of the reason there is so much discrepancy out there in the published research and so much confusion about why that diet worked for that person and not for that person is because we are complicated, and we all respond so differently,” Sarah explains.
“With the PREDICT studies we are trying to untangle and unravel what determines how each individual person responds to food.”
Despite its popularity, calorie counting is a tricky and unreliable tool for managing your weight.
“You do have to eat have fewer calories to lose weight, but they’re hard to count. Don’t neglect calories completely, but don’t count on calorie counting to work for you,” Christopher says.
Restrictive diets are also often unsustainable in the long term. For some people calorie counting can become all-consuming and have a negative psychological impact, leading to a fixation with numbers rather than focusing on eating delicious, nourishing food.