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What is energy?

Energy is the fuel we get from food that allows us to sustain life and be active. Energy requirements vary depending on your age, gender, body size and level of physical activity. Fats, proteins, and carbohydrates are the macronutrients found in food and beverages we consume which all provide us with energy (known as kilojoules or calories). Fats provide more energy per gram than protein or carbohydrates. Balancing energy intake (calories) with energy expenditure (physical activity)  is essential to maintaining a healthy body composition. 

Does food contain energy?

Food is our body’s only source of energy. Whatever we eat, our body turns into glucose which is either used by our cells for immediate energy or stored for later. We need a minimum amount of food to create the energy necessary to get through the day. When we’re more active, we need more food to sustain us.

Every type of macronutrient – carbohydrates, fat, and protein – provides us with different amounts of energy. Fat has the most at 9 calories per gram, which is why it’s often stored by our body for long-term energy. Carbohydrates and protein both have 4 calories of energy per gram.

Which structures break down food into energy?

When we digest food, it’s sent to our stomach where it’s broken down using our stomach’s acid and enzymes, and turned into glucose. From there, the lining of our stomach and small intestines absorbs the glucose and releases it back into the bloodstream. Our body uses insulin, which is produced by the pancreas, to help absorb the glucose into every cell.

Once the cell receives the glucose, it’s broken down into energy by mitochondria in a process called cellular respiration. The cell’s mitochondria are like a microscopic version of our digestive system. They create energy from glucose for each individual cell.  

What is the body’s main source of energy?

Carbohydrates are the body’s main source of energy. They can be found in a huge variety of food groups, from fruits and vegetables to dairy. The carbohydrates in every piece of food we eat are digested and turned into energy either slowly or quickly, depending on whether it’s a simple or complex carb. 

Simple carbs tend to make us feel more energetic in the short term, but complex carbohydrates are better for our body since they offer a steadier and more reliable source of energy.

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Photo of Kristin Ricklefs-Johnson

Medically reviewed by:

Kristin Ricklefs-Johnson, Ph.D., RD

Kristin is an RDN who also earned her Ph.D. in Nutrition from Arizona State University with an emphasis on insulin resistance, lipid metabolism disorders, and obesity. She completed her post-doctoral fellowship at Mayo Clinic where she focused on nutrition-related proteomic and metabolic research. Her interests include understanding the exact mechanism of action of various genetic variations underlying individual predispositions to nutrition-related health outcomes. Her goal is to help all individuals prevent chronic diseases and achieve long, healthy lives through eating well.

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