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Ever wonder exactly how the food you eat helps to power you throughout your day? Learn everything you need to know about what it takes to turn food into the energy you need to survive and thrive.

No matter what we engage in as humans – big or small – we rely on nutrients to perform our everyday activities. Our ability to extract energy from the food we ingest helps determine how optimally we can function day-to-day. Our metabolism entails the chemical reactions in our cells that convert the energy in our food into the energy our cells need. 

Macronutrients: the suppliers of energy

A full belly keeps us feeling good and satiated, but have you ever stopped to think about exactly how the food we eat supplies us with the fuel we need to live? It all comes down to macronutrients. 

Our diets mainly consist of foods containing combinations of carbohydrates, fats, and proteins. As an example, a slice of pizza provides a mingling of fat and protein from the cheese along with a heavy dose of carbohydrates from the wheat flour-filled crust. These three nutrients are metabolized to provide us with most of our energy and are the building blocks of immune function, cellular growth, and repair. That’s why they’re called essential macronutrients; they are essential because our body needs them in more significant amounts to function properly. Micronutrients, on the other hand, are also found in our food and are necessary for us to survive, although we need a smaller amount of them to get by. These include vitamins and minerals like iron, iodine, and calcium and help the body produce critical hormones and enzymes required for development and growth. They also play a central role in maintaining tissue function and making sure our metabolism is working correctly. 

Carbohydrates, fats, and proteins provide us with energy that is measured in the form of calories, with carbohydrates containing 4 calories per gram, fats containing 9 calories per gram, and proteins containing 4 calories per gram. The balance between the calories we ingest and our ability to burn them helps to determine our body weight and composition.

Each macronutrient has a unique function in the body, so let’s take a deep dive and learn about the specific role each one plays in keeping you healthy.


The carbohydrates found in our food are compounds made of carbon, hydrogen, and oxygen that can be digested and broken down directly into glucose, the primary type of sugar in the blood and the body’s preferred source of energy. It’s preferred because it is easier for the body to convert carbohydrates into immediate energy than transforming fat or protein into fuel, which both require more oxygen to burn. Glucose is used in the brain and the cells of the body, and any unused glucose is stored in the muscles and liver as glycogen. The number of carbohydrates we eat each day will determine how much glycogen is stored. When we exercise, the glycogen in our muscles is converted back into glucose, which the muscles then use as fuel. Glycogen from the liver also gets converted back into the bloodstream as glucose but is mainly used to maintain an even blood sugar level.

Carbohydrates are classified as either simple carbohydrates or complex carbohydrates

Simple carbohydrates are naturally found in processed sweet foods like white sugar, honey, syrup, milk, and yogurt. They are easy for the body to break down into glucose because they are made up of only one or two sugar molecules. Fruit also contains simple carbohydrates but has beneficial vitamins, minerals, antioxidants, and fiber, an undigested type of carbohydrate that can aid in digestive and heart health by helping to rid the body of waste and keep cholesterol levels low.

Complex carbohydrates are also made up of sugar molecules but are linked together in long, tough strands which makes these carbohydrates longer for the body to break down. Foods rich in complex carbohydrates include beans, whole grains, rice, and starchy vegetables like corn, potatoes, and peas. Like simple carbohydrates, complex carbohydrates release glucose quickly, but they also offer vitamins, minerals, and fiber critical for health. Because of this, most of the carbohydrates we eat should come from whole foods rich in complex carbohydrates rather than the refined added sugars that come from simple carbohydrates. The American Heart Association recommends getting no more than 6% of our daily calories from added sugars.


Fat gets a bad rap for being calorie-heavy, but we really couldn’t live without it. The fat in our food enables us to store energy, make certain hormones, cushion our organs, absorb fat-soluble vitamins A, D, E, and K, and help keep our cell membranes intact.  But not all fats are created equal. Some can be health-promoting and some can be damaging, so it’s essential to know which fats to embrace and which to steer clear.

There are three types of fat: saturated fat, unsaturated fat, and trans fat.

Saturated fat is the type of fat found in animal products with a high-fat content like lamb, pork, beef, chicken with skin, cream, lard, butter, and full-fat dairy and cheese. Eating too much saturated fat can raise our total blood cholesterol, including harmful LDL cholesterol, putting us at higher risk for heart disease. 

Unsaturated fats are known in nutritional circles as “healthy fats” because they may actually lower the risk for heart disease. This fat mostly comes from plant sources like nuts and nut butters, avocados, seeds, olives, and oils. In addition, fatty fish like salmon, tuna, sardines, and mackerel have heart-healthy unsaturated fat. Due to its healthful properties, the American Heart Association suggests we replace saturated fats with unsaturated fats whenever we can, as doing so may cut our heart disease risk down as much as a cholesterol-lowering statin medication could. 

Trans fat comes from adding hydrogen molecules to unsaturated fats, making hydrogenated oil commonly found in margarine, shortening, baked goods, and fried foods. Trans fat can be found in small amounts in animal foods like dairy and red meat, but it is mainly found in processed food. Trans fat is considered the worst type of fat for our health because it boosts heart disease risk by raising LDL cholesterol and lowering beneficial HDL cholesterol. It is also associated with weight gain and type 2 diabetes, so it should be avoided whenever possible.

Like carbohydrates, fats can be converted into fuel for the body. The most common type of fat in the body are triglycerides we get from food, which come from sugary, processed foods, butter and oil, alcohol, and foods high in saturated fat or trans fat. Triglycerides are either used as energy or stored in fat cells for energy use later on if we ingest too many calories that our body doesn’t need right away. The glucose in carbohydrates may be the body’s preferred fuel source, but fat is the more energy-dense and efficient source of the two. In other words, fat yields more energy per unit of weight than carbohydrates. At 9 calories per gram, fat packs more than twice as much energy as carbohydrates or protein. Also, fat by nature repels water, so when it’s stored, it does not bring the weight of water. On the other hand, every molecule of glycogen carries two grams of water, which means that if we stored the same amount of energy as glycogen and water compared to fat, our body weight would double.

So now that we know that we get more bang for our energy buck when we use fat for our fuel rather than carbohydrates, how do we get to the place where we can make the switch over from burning glucose to burning fat? We get there by depriving our body of accessing glucose and glycogen – and this happens when we restrict the number of carbohydrates we eat while upping our fat intake. When glucose and glycogen are low, the body turns to fat as the next best alternative. It breaks down triglycerides into fatty acids, which further break down into ketones, energy-rich substances used for fuel. Using ketones is helpful for weight loss because the fatty acids that produce the ketones come from our fat stores. Another benefit to burning fat for energy instead of carbohydrates is that the body has an unlimited ability to store fat, which means you’ll never run out. In contrast, glycogen needs to be replenished once it’s gone – which means we need to eat more just to keep up.

The ability to burn fat as fuel, also known as “fat oxidation,” can be a critical approach to keeping chronic disease at bay by managing weight and reducing insulin resistance. We know that what and how much we eat can heavily influence our ability to oxidize fat, and recent research shows that when we eat can make a big difference as well. A 2018 study reported in The Journal of Nutrition looked at how the composition of breakfast influences fat oxidation across the 24-hour day. In the study, either a high-fat or a high-carbohydrate meal was consumed in the morning for four weeks among 29 sedentary men and women. The high-fat group was given a breakfast consisting of 45% energy from fat and 35% from carbohydrates. The high-carbohydrate group received a breakfast with 20% energy from fat and 60% from carbohydrates. Both groups’ lunch and dinner were metabolically neutral, with 50% carbohydrates, 20% protein, and 30% fat, including less than 10% saturated fat. The results were pretty clear: the high-fat breakfast group showed significantly greater fat oxidation after lunch and dinner and throughout the day than did the high-carbohydrate group. This finding suggests that eating a high-fat, low carbohydrate meal could assist in our ability to burn fat and slash our risk for chronic disease in the process. 


Unlike carbohydrates and fats, protein is not stored in our body as fuel and typically meets only 5% of the body’s energy needs. Protein instead has the essential role of building, maintaining, and repairing tissues and producing critical enzymes, hormones, and neurotransmitters. But in some extreme cases, such as when we aren’t ingesting enough calories and carbohydrates and fats aren’t available, the body will be forced to break down lean muscle mass to convert amino acids into glucose as a last-ditch effort for energy.

ATP: the energy currency of the cell

Now that we’ve dug deep into the three essential macronutrients and why their roles are so critical to our survival, let’s explore exactly how these nutrients transform from the food on our fork to the energy we depend on.

The carbohydrates, fat, and protein in the foods we eat are each digested and metabolized differently in the body, but all eventually deliver carbon dioxide, water, and adenosine triphosphate (ATP) molecules. ATP is the body’s immediate fuel source for energy and consists of high-energy compounds like “batteries” converted from stored energy to energy supplied to our cells. So, anytime we need energy, to jog around the block, to type out an email, or to breathe, our body uses ATP to shuttle the energy to places within the cell where it is needed.

Get to know your metabolism using Biosense

Speak to someone about healthy eating and weight loss, and the topic of metabolism is usually not far behind. Metabolism is a reasonably understood terminology, but few appreciate the complexity of converting the food we eat into the energy we can use. The truth is, as unique as we are as humans, our metabolisms are just as unique. Each of us uses food to burn energy in our own way based on factors like our basal metabolic rate and general ability to digest and absorb food. So, metabolism is not a one-size-fits-all process – which makes it all the more critical to be able to learn exactly how your own body utilizes energy. The Biosense® breath ketone monitor is the first of its kind to precisely show you how your particular physiology metabolically relates to food using a simple measured breath. Biosense achieves this by capturing the time and depth of your ketone levels several times a day, letting you know exactly how effectively you’re metabolizing fat for energy. 

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