What are calories, and how do they impact me?

A calorie is a unit that measures energy. 

Specifically, 1 calorie is the amount of energy it takes to raise the temperature of 1 gram of water by 1 degree Celsius. 

When it comes to calories and health, we’re talking about the amount of energy that our bodies can use from the foods and drinks we consume.

For example, a drink’s nutrition label shows the number of calories to let you know how much energy a serving of that drink provides. 

Despite what you may have heard, calories are not bad for you.

Our bodies need them for growing, thinking, moving, and for functions like breathing and pumping blood. 

The number of calories you eat doesn’t paint a full picture of your overall health. Instead, the quality of the foods and drinks containing those calories is key.

Different nutrients provide different amounts of energy. Carbohydrates and protein each provide 4 calories per gram and fat provides 9 calories per gram. 

How many calories do you need?

How many calories you need depends on many factors, including:

  • sex

  • age

  • body size

  • activity level

  • whether you’re pregnant or breastfeeding

  • medications you’re taking

  • your personal health goals

But some general recommendations can be a good starting point.

The most recent Dietary Guidelines for Americans advises women to consume 1,600–2,200 calories per day and men to aim for 2,200–2,800, depending on a number of factors.

On average, men generally need more calories because they tend to have larger bodies — but of course, this can vary. And women who are pregnant or breastfeeding need more calories than women who aren't.

How active you are also has a big impact on how many calories you need.

Movement and activity require energy. The more active you are, the more energy you need. People with more sedentary lifestyles generally don’t need as much energy as people who get regular exercise. 

Age is also a factor. Our bodies burn less fuel in older age, so people may need fewer calories over time. However, our bodies are also less efficient at absorbing nutrients, and therefore energy, in later life.

While these recommendations are a general guide, determining how many calories you need is much less straightforward. Your personal energy needs are specific to you and your body.

The ZOE at-home test analyzes your body’s blood sugar and blood fat responses, as well as your gut microbiome. From this information, we can provide you with diet recommendations specific to you.

Take our free quiz to get started.

Calories and weight loss

If you’ve ever tried to lose weight, you’ve probably come across the phrase, “calories in, calories out.” How you balance the energy you take in with the energy you burn may be relevant to weight loss, but this isn’t the whole picture.

The idea behind the phrase is: When you consume more energy than you burn, your body stores the extra energy as fat, and your weight goes up. If you burn more energy than you take in, your body breaks down these stores to use as fuel, causing your weight to go down.

If you consume the amount of calories that your body needs, your weight remains the same.

While this sounds pretty straightforward, it’s really not so clear-cut.

In his book Spoon-Fed, ZOE co-founder Prof. Tim Spector expands on the pitfalls of calorie counting, saying, “The general misconception is that they are a direct and precise measurement of how fattening a food is.”

How your body processes calories is unique to you. At ZOE, we run the largest nutrition science study in the world. Our data show that even identical twins can have very different responses to an identical meal. 

So, even if you eat the same number of calories as another person, your bodies will respond in different ways, depending on factors like your metabolism and the microbes living in your gut.

The quality of the foods you eat is also more important than simply how many calories you’re getting.

What do calories really tell us?

The number of calories in a particular food shows how much energy it contains, but it doesn’t show the value of that food to your body. So, a food's calorie count doesn’t necessarily translate to how nutritious it is. 

Rather than focusing on calories, you can look at other things to help guide your food choices.

High quality, nutrient-dense foods are typically minimally processed. They’re also usually rich in important nutrients — like fiber, protein, and healthy fats — that keep you feeling fuller for longer and support your metabolic and gut health. 

Rather than counting calories, focus on eating a variety of foods such as:

  • vegetables

  • fruits

  • whole grains

  • nuts and seeds

  • legumes

  • dairy

  • seafood

  • lean meats

Some foods give your body plenty of energy but few or no nutrients. These are things like sodas, alcohol, candy, and energy drinks.

Because they have little value for your overall health, it's best to have them less often. When looking for quality foods, opt for those that are unprocessed or minimally processed. 

When a food is minimally processed, someone, maybe the manufacturer, has only changed it slightly, so its nutritional value is intact. An example is freezing a bag of vegetables. This type of processing is not necessarily a bad thing.  

As a food undergoes more processing, it loses nutrients. For example, removing the skin from an apple takes away fiber and other nutrients. Applying heat can also destroy certain nutrients. 

Processing also changes the structure of the nutrients in food, also known as the food matrix. Altering the food matrix changes how our bodies break down and use these nutrients.

Ultra-processed foods have gone through industrial processes that have changed them significantly. Also, they can contain large amounts of unhealthy fats, sugars, and additives.

These foods can have high amounts of calories, and they tend to have fewer nutrients. Some examples include cookies, chips, and sodas.

The occasional cookie is not going to sabotage your health. But evidence shows that regularly eating ultra-processed foods as part of a Western-style diet is linked to an increased risk of many chronic health conditions. 


A calorie is a unit that measures how much energy is in a particular food or drink. This energy is crucial for basic bodily functions, moving, thinking, and other important actions.

There are general recommendations about how many calories should be in people’s diets, but your specific needs depend on a number of factors.

Counting calories for weight loss might seem straightforward — but not all calories affect your body in the same way.

Overall, the quality of food is more important to your health than how many calories your meal contains. Try to aim for a varied diet that’s rich in minimally processed, nutrient-dense foods.

At the end of the day, there is no one-size-fits-all approach to nutrition. 

With the ZOE at-home test, you can learn about your body’s unique blood sugar and blood fat responses. We’ll also give you a breakdown of the “good” and “bad” bugs living in your gut. 

With this information, we’ll provide you with diet advice tailored to your unique body.

To get started, take our free quiz.