How Food Works

By: Marshall Brain

Food is essential to life. Check out these food proportion pictures.

­It is safe to say that one thing you'll do today is eat some food -- food is pretty important to all animals. If you don't eat, it can cause all sorts of problems: hunger, weakness, starvation... Food is essential to life.

But what is food? What's in food that makes it so important? What happens to the food once you eat it? What is food made of? How does it fuel our bodies? What do words like "carbohydrates" and "fats" really mean (especially on those "Nutrition Facts" labels you find on almost everything these days)? What would happen if you ate nothing but marshmallows for a week? What is a calorie? Why can't we eat grass like a cow does, or wood like a termite?


If you have ever wondered about food and how your body uses it, then read on. In this article, we'll give you all of the information you need to understand what a hamburger or a banana does to keep your body running every day.

The Basics of Food

Think about some of the things you have eaten today -- maybe cereal, bread, milk, juice, ham, cheese, an apple, potatoes... All of these foods (and pretty much any other food that you can think of) contain seven basic components:

  • Carbohydrates (simple and complex)
  • Proteins
  • Fats
  • Vitamins
  • Minerals
  • Fiber
  • Water

Your body's goal is to digest food and use it to keep your body alive. In the following sections, we will look at each of these basic components to understand what they really do and why they are so important to your body.


(Note that there might be a few non-food things mixed in with what you eat, especially if you are eating lots of processed foods. Things like artificial colors and chemical preservatives are the most common. Those are additives, not part of the natural foods.)


You have probably heard of "carbohydrates" and "complex carbohydrates." Carbohydrates provide your body with its basic fuel. Your body thinks about carbohydrates like a car engine thinks about gasoline.

The simplest carbohydrate is glucose. Glucose, also called "blood sugar" and "dextrose," flows in the bloodstream so that it is available to every cell in your body. Your cells absorb glucose and convert it into energy to drive the cell. Specifically, a set of chemical reactions on glucose creates ATP (adenosine triphosphate), and a phosphate bond in ATP powers most of the machinery in any human cell. If you drink a solution of water and glucose, the glucose passes directly from your digestive system into the bloodstream.


The word "carbohydrate" comes from the fact that glucose is made up of carbon and water. The chemical formula for glucose is:

You can see that glucose is made of six carbon atoms (carbo...) and the elements of six water molecules (...hydrate). Glucose is a simple sugar, meaning that to our tongues it tastes sweet. There are other simple sugars that you have probably heard of. Fructose is the main sugar in fruits. Fructose has the same chemical formula as glucose (C6H12O6), but the atoms are arranged slightly differently. The liver converts fructose to glucose. Sucrose, also known as "white sugar" or "table sugar," is made of one glucose and one fructose molecule bonded together. Lactose (the sugar found in milk) is made of one glucose and one galactose molecule bonded together. Galactose, like fructose, has the same chemical components as glucose but the atoms are arranged differently. The liver also converts galactose to glucose. Maltose, the sugar found in malt, is made from two glucose atoms bonded together.

Glucose, fructose and galactose are monosaccharides and are the only carbohydrates that can be absorbed into the bloodstream through the intestinal lining. Lactose, sucrose and maltose are disaccharides (they contain two monosaccharides) and are easily converted to their monosaccharide bases by enzymes in the digestive tract. Monosaccharides and disaccharides are called simple carbohydrates. They are also sugars -- they all taste sweet. They all digest quickly and enter the bloodstream quickly. When you look at a "Nutrition Facts" label on a food package and see "Sugars" under the "Carbohydrates" section of the label, these simple sugars are what the label is talking about.

There are also complex carbohydrates, commonly known as "starches." A complex carbohydrate is made up of chains of glucose molecules. Starches are the way plants store energy -- plants produce glucose and chain the glucose molecules together to form starch. Most grains (wheat, corn, oats, rice) and things like potatoes and plantains are high in starch. Your digestive system breaks a complex carbohydrate (starch) back down into its component glucose molecules so that the glucose can enter your bloodstream. It takes a lot longer to break down a starch, however. If you drink a can of soda full of sugar, glucose will enter the bloodstream at a rate of something like 30 calories per minute. A complex carbohydrate is digested more slowly, so glucose enters the bloodstream at a rate of only 2 calories per minute (reference).

You may have heard that eating complex carbohydrates is a good thing, and that eating sugar is a bad thing. You may even have felt this in your own body. The following quote from The Yale Guide to Children's Nutrition's explains why:

If complex carbohydrates are broken down to monosaccharides in the intestines before they are absorbed into the bloodstream, why are they better than refined sugar or other di- or mono-saccharides? To a great extent it has to do with the processes of digestion and absorption. Simple sugars require little digestion, and when a child eats a sweet food, such as a candy bar or a can of soda, the glucose level of the blood rises rapidly. In response, the pancreas secretes a large amount of insulin to keep blood glucose levels from rising too high. This large insulin response in turn tends to make the blood sugar fall to levels that are too low 3 to 5 hours after the candy bar or can of soda has been consumed. This tendency of blood glucose levels to fall may then lead to an adrenaline surge, which in turn can cause nervousness and irritability... The same roller-coaster ride of glucose and hormone levels is not experienced after eating complex carbohydrates or after eating a balanced meal because the digestion and absorption processes are much slower.

If you think about it, this is incredibly interesting because it shows that the foods you eat and the way you eat them can affect your mood and your temperament. Foods do that by affecting the levels of different hormones in your bloodstream over time.

Another interesting thing about this quote is its mention of insulin. It turns out that insulin is incredibly important to the way the body uses the glucose that foods provide. The functions of insulin are:

  • To enable glucose to be transported across cell membranes
  • To convert glucose into glycogen for storage in the liver and muscles
  • To help excess glucose be converted into fat
  • To prevent protein breakdown for energy

According to the Encyclopedia Britannica:

Insulin is a simple protein in which two polypeptide chains of amino acids are joined by disulfide linkages. Insulin helps transfer glucose into cells so that they can oxidize the glucose to produce energy for the body. In adipose (fat) tissue, insulin facilitates the storage of glucose and its conversion to fatty acids. Insulin also slows the breakdown of fatty acids. In muscle it promotes the uptake of amino acids for making proteins. In the liver it helps convert glucose into glycogen (the storage carbohydrate of animals) and it decreases gluconeogenesis (the formation of glucose from noncarbohydrate sources). The action of insulin is opposed by glucagon, another pancreatic hormone, and by epinephrine.

What you can begin to see from this description is that there are actually lots of different things happening in your body around glucose. Because glucose is the essential energy source for your body, your body has many different mechanisms to ensure that the right level of glucose is flowing in the bloodstream. For example, your body stores glucose in your liver (as glycogen) and can also convert protein to glucose if necessary. Carbohydrates provide the energy that cells need to survive.

For more information on carbohydrates, glucose and insulin, check out the links at the end of this article.


A protein is any chain of amino acids. An amino acid is a small molecule that acts as the building block of any cell. Carbohydrates provide cells with energy, while amino acids provide cells with the building material they need to grow and maintain their structure. Your body is about 20-percent protein by weight. It is about 60-percent water. Most of the rest of your body is composed of minerals (for example, calcium in your bones). Amino acids are called "amino acids" because they all contain an amino group (NH2) and a carboxyl group (COOH), which is acidic. Below you can see the chemical structure of two of the amino acids.

You can see that the top part of each is identical to the other. That is true of all amino acids -- the little chain at the bottom (the H or the CH3 in these two amino acids) is the only thing varying from one amino acid to the next. In some amino acids, the variable part can be quite large. The human body is constructed of 20 different amino acids (there are perhaps 100 different amino acids available in nature).


As far as your body is concerned, there are two different types of amino acids: essential and non-essential. Non-essential amino acids are amino acids that your body can create out of other chemicals found in your body. Essential amino acids cannot be created, and therefore the only way to get them is through food. Here are the different amino acids:


  • Alanine (synthesized from pyruvic acid)
  • Arginine (synthesized from glutamic acid)
  • Asparagine (synthesized from aspartic acid)
  • Aspartic Acid (synthesized from oxaloacetic acid)
  • Cysteine
  • Glutamic Acid (synthesized from oxoglutaric acid)
  • Glutamine (synthesized from glutamic acid)
  • Glycine (synthesized from serine and threonine)
  • Proline (synthesized from glutamic acid)
  • Serine (synthesized from glucose)
  • Tryosine (synthesized from phenylalanine)


  • Histidine
  • Isoleucine
  • Leucine
  • Lysine
  • Methionine
  • Phenylalanine
  • Threonine
  • Tryptophan
  • Valine

Protein in our diets comes from both animal and vegetable sources. Most animal sources (meat, milk, eggs) provide what's called "complete protein," meaning that they contain all of the essential amino acids. Vegetable sources usually are low on or missing certain essential amino acids. For example, rice is low in isoleucine and lysine. However, different vegetable sources are deficient in different amino acids, and by combining different foods you can get all of the essential amino acids throughout the course of the day. Some vegetable sources contain quite a bit of protein -- things like nuts, beans, soybeans, etc. are all high in protein. By combining them you can get complete coverage of all essential amino acids.

The digestive system breaks all proteins down into their amino acids so that they can enter the bloodstream. Cells then use the amino acids as building blocks.

From this discussion you can see that your body cannot survive strictly on carbohydrates. You must have protein. According to this article, the RDA (Recommended Daily Allowance) for protein is 0.36 grams of protein per pound of body weight. So a 150-pound person needs 54 grams of protein per day. The photo above is the Nutritional Facts label from a can of tuna. You can see that a can of tuna contains about 32 grams of protein (this can has 13 grams per serving and there are 2.5 servings in the can). A glass of milk contains about 8 grams of protein. A slice of bread might contain 2 or 3 grams of protein. You can see that it is not that hard to meet the RDA for protein with a normal diet.


Nutritional label from a bottle of olive oil
Nutritional label from a bottle of olive oil

We all know about the common fats that different foods contain. Meat contains animal fat. Most breads and pastries contain vegetable oils, shortening or lard. Deep fried foods are cooked in heated oils. Fats are greasy and slick.

You commonly hear about two kinds of fats: saturated and unsaturated. Saturated fats are normally solid at room temperature, while unsaturated fats are liquid at room temperature. Vegetable oils are the best examples of unsaturated fats, while lard and shortening (along with the animal fat you see in raw meat) are saturated fats. However, most fats contain a mixture. For example, above you see the label from a bottle of olive oil. It contains both saturated and unsaturated fats, but the saturated fats are dissolved in the unsaturated fats. To separate them, you can put olive oil in the refrigerator. The saturated fats will solidify and the unsaturated fats will remain liquid. You can see that the olive oil bottler even chose to further distinguish the unsaturated fats between polyunsaturated and monounsaturated. Unsaturated fats are currently thought to be more healthy than saturated fats, and monounsaturated fats (as found in olive oil and peanut oil) are thought to be healthier than polyunsaturated fats.


Fats that you eat enter the digestive system and meet with an enzyme called lipase. Lipase breaks the fat into its parts: glycerol and fatty acids. These components are then reassembled into triglycerides for transport in the bloodstream. Muscle cells and fat (adipose) cells absorb the triglycerides either to store them or to burn them as fuel.

You need to eat fat for several reasons:

  • As we will see in the next section, certain vitamins are fat soluble. The only way to get these vitamins is to eat fat.
  • In the same way that there are essential amino acids, there are essential fatty acids (for example, linoleic acid is used to build cell membranes). You must obtain these fatty acids from food you eat because your body has no way to make them.
  • Fat turns out to be a good source of energy. Fat contains twice as many calories per gram as do carbohydrates or proteins. Your body can burn fat as fuel when necessary.

For more information about fat in the diet, check out the links at the end of this article.


The Merriam-Webster Collegiate Dictionary defines "vitamin" as:

vi.ta.min: any of various organic substances that are essential in minute quantities to the nutrition of most animals and some plants, act esp. as coenzymes and precursors of coenzymes in the regulation of metabolic processes but do not provide energy or serve as building units, and are present in natural foodstuffs or sometimes produced within the body.

Vitamins are smallish molecules (Vitamin B12 is the largest, with a molecular weight of 1,355) that your body needs to keep itself running properly. In How Sunburns and Sun Tans Work, we learn that the body can produce its own Vitamin D, but generally vitamins must be provided in food. The human body needs 13 different vitamins:


Vitamin A (fat soluble, retinol) comes from beta-carotene in plants; when you eat beta-carotene, an enzyme in the stomach turns it into Vitamin A.

Vitamin B (water soluble, several specific vitamins in the complex)

  • Vitamin B1: Thiamine
  • Vitamin B2: Riboflavin
  • Vitamin B3: Niacin
  • Vitamin B6: Pyridoxine
  • Vitamin B12: Cyanocobalamin
  • Folic Acid

Vitamin C (water soluble, ascorbic acid)

Vitamin D (fat soluble, calciferol)

Vitamin E (fat soluble, tocopherol)

Vitamin K (fat soluble, menaquinone)

Pantothenic acid (water soluble)

Biotin (water soluble)

In most cases, the lack of a vitamin causes severe problems. The following list shows diseases associated with the lack of different vitamins:

  • Lack of Vitamin A: Night blindness, xerophthalmia
  • Lack of Vitamin B1: Beriberi
  • Lack of Vitamin B2: Problems with lips, tongue, skin,
  • Lack of Vitamin B3: Pellagra
  • Lack of Vitamin B12: Pernicious anemia
  • Lack of Vitamin C: Scurvy
  • Lack of Vitamin D: Rickets
  • Lack of Vitamin E: Malabsorption of fats, anemia
  • Lack of Vitamin K: Poor blood clotting, internal bleeding

A diet of fresh, natural food usually provides all of the vitamins that you need. Processing tends to destroy vitamins, so many processed foods are "fortified" with man-made vitamins.


Minerals are elements that our bodies must have in order to create specific molecules needed in the body. Here are some of the more common minerals our bodies need:

  • Calcium - used by teeth, bones
  • Chlorine
  • Chromium
  • Copper
  • Fluoride - strengthens teeth
  • Iodine - combines with tryosine to create the hormone thyroxine
  • Iron - transports oxygen in red blood cells
  • Magnesium
  • Manganese
  • Molybdenum
  • Phosphorus
  • Potassium - important ion in nerve cells
  • Selenium
  • Sodium
  • Zinc

We do need other minerals, but they are supplied in the molecule that uses them. For example, sulfur comes in via the amino acid methionine, and cobalt comes in as part of vitamin B12.


Food provides these minerals. If they are lacking in the diet, then various problems and diseases arise.


As mentioned above, your body is about 60-percent water. A person at rest loses about 40 ounces of water per day.

Water leaves your body in the urine, in your breath when you exhale, by evaporation through your skin, etc. Obviously, if you are working and sweating hard then you can lose much more water.


Because we are losing water all the time, we must replace it. We need to take in at least 40 ounces a day in the form of moist foods and liquids. In hot weather and when exercising, your body may need twice that amount. Many foods contain a surprising amount of water, especially fruits. Pure water and drinks provide the rest.


Fiber is the broad name given to the things we eat that our bodies cannot digest. The three fibers we eat on a regular basis are:

  • Cellulose
  • Hemicellulose
  • Pectin

Hemicellulose is found in the hulls of different grains like wheat. Bran is hemicellulose. Cellulose is the structural component of plants. It gives a vegetable its familiar shape. Pectin is found most often in fruits, and is soluble in water but non-digestible. Pectin is normally called "water-soluble fiber" and forms a gel. When we eat fiber, it simply passes straight through, untouched by the digestive system.


Cellulose is a complex carbohydrate. It is a chain of glucose molecules. Some animals and insects can digest cellulose. Both cows and termites have no problem with it because they have bacteria in their digestive systems secreting enzymes that break down cellulose into glucose. Human beings have neither the enzymes nor these beneficial bacteria, so cellulose is fiber for us.

I'm Starving

A normal person who is eating three meals a day and snacking between meals gets almost all of his or her energy from the glucose that carbohydrates provide. What happens if you stop eating, however? For example, what if you are lost in the woods, or you are purposefully fasting? What does your body do for energy? Your body goes through several phases in its attempt to keep you alive in the absence of food.

The first line of defense against starvation is the liver. The liver stores glucose by converting it to glycogen. It holds perhaps a 12-hour supply of glucose in its glycogen. Once you finish digesting all of the carbohydrates that you last ate, the liver starts converting its stored glycogen back into glucose and releases it to maintain glucose in the blood. Lipolysis also starts breaking down fat in the fat cells and releasing fatty acids into the bloodstream. Tissues that do not need to use glucose for energy (for example, muscle cells) start burning the fatty acids. This reduces the glucose demand so that nerve cells get the glucose.


Once the liver runs out of glycogen, the liver converts to a process called gluconeogenesis. Gluconeogenesis turns amino acids into glucose (see this article for more on gluconeogenesis).

The liver then begins producing ketone bodies from fatty acids being made available in the blood by lipolysis. Brain and nerve cells convert over from being pure consumers of glucose to partial consumers of ketone bodies for energy (see this article for information on ketone body metabolism).

Some of these alternative metabolic processes are actually used on a regular basis. For example, Eskimos eating a traditional Eskimo diet have virtually no carbohydrates on the menu. You may have also read about several recent weight-loss programs that try to take advantage of ketone metabolism to "burn fat" (this article offers a thorough description of the "ketogenic diet" as used in medicine, and this article talks about the "fad diets" that utilize the ketone effect). When you hear about these diets you will now have a better idea of what they're about!

Click on the play button to see how the body digests food.

If the animation above isn't working, click here to get the Shockwave player.

For more information on food, nutrition and related topics, check out the links that follow.