We are what we eat. The foods that we consume each day are used by our bodies to build, repair and power the trillions of cells that make up a human being. To do this, our bodies start by digesting the foods we eat. Digestion is the process by which our gastrointestinal tracts break down a food into its basic components, called nutrients. Those nutrients are then absorbed by the cells of the gastrointestinal tract and transported by our circulatory system to all the different parts of our bodies. Once those nutrients are delivered to the cells that need them, those cells begin the process, called metabolism, of using those nutrients to build, repair and power themselves.

To feel healthy, for our bodies to function properly as they grow and sustain themselves, we must regularly feed them a specially balanced combination of nutrients. However, the variety of foods we have to choose from contain different amounts of nutrients in varying combinations. In order to decide what we will eat, it is important to understand more about the nutrients we need to consume.

Nutrients are classified into two main types: macronutrients and micronutrients. Macronutrients are needed in large quantities every day and are typically measured in grams (g) or ounces (oz). Micronutrients are required in much smaller daily doses and are typically measured in milligrams (mg) or micrograms (mcg). Every food can be thought of as a combination of bulky macronutrients with a light sprinkling of micronutrients.

There are six kinds of macronutrients which make up the foods on our plates: proteins, fats, starches, fiber, water and sugars. All of these macronutrients are what gives a food its weight and each plays a distinct role in our metabolism. With the exception of water, all the macronutrients contain calories. In addition to their unique functions, these calorie-bearing macronutrients all act as fuel which our body’s cells transform into energy. The water we drink in our beverages, and eat in our foods, acts as a necessary solvent to allow these metabolic reactions to occur in our cells. Those metabolic reactions are supported by a variety of micronutrients which fall into two categories: vitamins and minerals. These micronutrients, though often only required in trace amounts, are nonetheless essential for the production of energy in our cells.

Proteins are an essential building block of human body tissue. They are required for the growth and repair of our cells, and following water, they are the most abundant kind of molecule in our bodies. Proteins represent the major structural component of our muscles, internal organs, blood, skin, hair and nails. Proteins themselves are made up of chains of amino acids. The human body requires 22 different amino acids, which it transforms into enzymes, antibodies, hormones and other molecules essential for human life. Since nine of these essential amino acids cannot be biosynthesized by our bodies, we must consume them through the proteins in our diets. When not used as building blocks, proteins can be converted to energy by our cells, yielding four calories for every gram.

Fats are necessary for the absorption of several essential vitamins and are used for the storage of metabolic energy. Fats insulate and cushion internal organs, regulate body temperature, maintain healthy skin and hair, and promote cellular function. Fats also act as a buffer, allowing our bodies to absorb diseases or toxins, diverting them away from internal organs until the problematic substances can be expelled. Similarly to the amino acids found in proteins, two of the fatty acids our body requires cannot be biosynthesized, so we must consume them through our diets. When burned by our cells as fuel, fats yield nine calories per gram, more than double the other macronutrients.

Starches are a type of polysaccharide produced by most green plants as their primary energy store. Also known as complex carbohydrates, they are the main source of energy in our diets. Unlike with proteins and fats, there are no starches which we must consume because our bodies cannot synthesize them. Nonetheless, almost half of the calories we eat each day should come from starch. As our main source of fuel, starches are converted to energy by our cells at a rate of four calories per gram.

Fiber is another type of polysaccharide, which acts as the structural component of most green plants. These complex carbohydrates, unlike starches, are not fully absorbable by our bodies. Nonetheless, dietary fiber has the essential function of regulating our gastrointestinal system to promote the digestion and absorption of the other nutrients. Cellulose fiber is not convertible to energy because the human body lacks the enzymes to break it down. Instead, cellulose serves a bulking function which helps pace our digestive tract. Other types of fiber are fermentable by the bacteria in our digestive system. When broken down these types of dietary fiber can regulate the absorption of sugars or be converted to energy themselves. When fiber is metabolized by our cells, it will yield four calories per gram.

Water is essential for life. It is the most abundant molecule in our bodies, accounting for about 60% of our weight. We often say we are “made of water,” but it might be more accurate to say humans are “made out of, through, with, in, on and around water.” All of the body’s major functions involve water, it is the primary ingredient in the blood that moves through our circulatory systems, and all the metabolic reactions that occur in our cells require it. Water is continuously excreted from our bodies and so it is necessary to regularly rehydrate. To replenish those lost liquids, in addition to what we drink in beverages every day, 60% of the food we eat, by weight, should be water. Water is unique among the macronutrients because it does not contain any calories. However, as with all macronutrients, maintaining a balanced water consumption is essential.

Sugars are monosaccharides and disaccharides, which are also known as simple carbohydrates. They are readily absorbed by humans and used as a source of energy. Glucose is metabolized by nearly all known organisms. Known as a ubiquitous fuel, glucose acts as blood sugar in our bodies. Sucrose and fructose are secondary energy stores of many green plants, found in their roots, fruits and nectars. Lactose is found in the milk of mammals, but unlike the other simple carbohydrates, it requires a unique enzyme to be digested. Different sugars are absorbed by the gastrointestinal tract at varying paces, but when they are metabolized by our cells, it occurs at a rate of four calories per gram.