How many calories should you eat to lose weight?

September 1, 2020

Many of us use the calorie counts on food packaging to figure out whether a particular food is healthy and fits within our daily needs.

But the big problem with these basic calorie counts is that they are often oversimplified and misleading. Our own research shows that each of us responds very differently to meals containing exactly the same back-of-the-pack calorie information.

So, should you be focusing on calories to be healthier and control your weight in the long term? We believe the answer lies in better understanding your own body and letting your biology do the work.

Calories are confusing

The concept of the calorie was originally invented by scientists to measure the energy stored in a material and is now used as a measure of how much energy is in our food. What we refer to as a ‘calorie’ today is defined as the amount of energy needed to raise the temperature of 1 kilogram of water by 1 degree Celsius.

(To be pedantic, it’s actually 1,000 calories – technically known as a kilocalorie or kcal – but it’s usually called a calorie to make the numbers easier to handle).

How many calories should you eat to be healthy?

The FDA and government bodies around the world have issued guidelines about how many calories a person should eat to maintain a healthy weight, based on their age, sex, and activity level. For female adults, this ranges from 1600 to 2400 calories, and for males from 2000 to 3200 calories.

The great thing about using units of energy to measure our food is that it makes maintaining our weight simple (in theory). The law of the conservation of energy demands that all energy must go somewhere, so it is easy to understand that we must burn the calories we eat or they are stored in our body.

None of us can break the laws of physics, so to lose weight, we must therefore eat fewer calories than we burn, right? Unfortunately counting calories in and calories out isn’t always as simple as it seems.

The calories in a food depend on who’s eating it (and who made it)

One of the first problems people encounter when trying to count calories is human error. People are notoriously bad at measuring and logging their food. What’s more, food prepared by a person instead of an automated process can vary wildly in terms of calorie content. 

We don’t all burn the same food with the same efficiency. Our genetics, the bacteria that live in our gut, and many other factors can affect how we respond to food and absorb calories.

As a result, even if the calorie counts on food packaging were correct for the ‘average person’ they still wouldn’t be representative of how much energy your body will get from that food.

Same food, different responses

The results from our first nutrition study, PREDICT 1, showed that the basic properties of a food - such as the calories, grams of fat, protein and carbohydrate - only account for up to a third of people’s unique responses to food.

We gave participants exactly the same meal at the same time of day - a strange breakfast of three muffins and a milkshake, containing 50 grams of fat and 85 grams of carbohydrates and adding up to 830 calories. Then we measured how their blood sugar and fat levels changed over the following six hours. 

Here’s some real-life data from a pair of identical twins in the study. Time is running along the bottom of the graphs, blood sugar or fat levels are up the side.

Twin 2’s responses are pretty healthy - their blood fat rises a little then heads back down, and their blood sugar rises aren’t too big. Their sibling is very different. Fat rapidly accumulates in their blood - something that may contribute to health problems like heart disease - and their blood sugar spikes are higher.

The fact that identical twins - who share all the same genes - respond so differently to exactly the same meal tells us that it’s not enough to look at the calorie listing on the back of the pack - you need to know how your body responds to a food to understand how it fits into your nutritional choices.

Let your biology do the work instead

Our advice on counting calories? Don’t. It’s frustrating and, more often than not, ineffective.

Rather than focusing on calories, our approach is to go right to the root cause of poor metabolic health. By analyzing your blood, poop and other biological factors, ZOE provides you with a deeper understanding of your metabolic health.

We start by nourishing your body with the right foods for you, and allowing your biology to do all the work. But if you want to know how to be your healthiest weight and improve your gut health, you need to know how your body works.