Vitamins: An Overview
The vitamin story really began hundreds of years ago when humans first recognized that specific diseases were related to eating habits. Accounts from as early as the seventh century described one such disease, beriberi, and scurvy was documented in the late thirteenth century. Although the names of some of these deficiency diseases seem comical and the diseases are rare in our population, they caused untold suffering and deaths throughout history. As time went on, people learned that certain foods could cure these diseases. But it wasn't until the turn of this century that scientists were able to identify the chemicals in foods which produced the dramatic cures.
The word vitamin came about from a Polish scientist, Casimir Funk, in the early 1900s. He discovered several substances that he termed vitamine, from "vital amine." An amine is a compound that contains nitrogen. Later, researchers found that not all true vitamins could be chemically classified as amines, so they dropped the "e" and kept vitamin. Here's the definition they decided on for a vitamin:
Their organic nature means that vitamins contain the element carbon, and this distinguishes them from another group of essential nutrients, the minerals; carbon is oxidized when combusted, while minerals retain their elemental character even if incinerated at high temperatures. This is another reason why vitamins are generally more readily destroyed by cooking and food preparation methods than their indestructible cousins, the minerals.
With this knowledge, vitamins became recognized as one of the six essential nutrients required for human life, along with water, carbohydrates, protein, fat, and minerals. As scientists discovered new vitamins, they named the compounds alphabetically: A, B, C, D, E, and K. The gaps in the list, which turned out to be rather lengthy, were due to some substances being dropped after they turned out not to be vitamins by the accepted definition. All the confusing numbers associated with the letters, like Bl and B2, are the result of the discovery that what initially was believed to be one compound in fact were several, each with a specific function in the body. At the present time, fourteen vitamins are recognized as being required by humans.
What Vitamins Do for You
Vitamins have specialized duties in the body, but many share similar types of functions. Some vitamins serve as chemical messengers, similar to hormones. Although the term hormone evokes colorful images of overpowering passions, hormones control the body's internal environment by keeping the innumerable activities of cells and organs in check. The process starts when specialized groups of cells, or organs such as the pancreas, release hormones in response to a situation in the body which needs correcting. The hormone travels in the blood away from its point of origin to the target, again a group of cells or an organ, and tells the target to do something.
The easiest example to understand is that of the hormone insulin, which the pancreas releases in response to an increase in blood sugar or glucose level, such as occurs after a meal. Insulin courses through the blood and relays the message "clear glucose from the blood." This simple message causes a variety of tissues to respond in different ways, but all with the goal of reducing blood glucose. To fat cells, whose purpose is to store excess glucose as fat, the reaction is to do just that. In contrast, other cells respond by allowing glucose to enter and perhaps be used for energy, and liver cells take up the glucose and make glycogen, a storage form of the sugar, for later use. The result is that in short order, the blood glucose level returns to normal.
For an example in the vitamin family, we turn to vitamin D, which some scientists say is more properly termed a hormone. While vitamin D has an active role in bone formation, one indirect way in which it fulfills this role is to regulate calcium levels in the blood. As with most compounds in the blood, the level of calcium must remain within a narrow range. If it doesn't stay in the right range, horrific results could ensue such as tetany, a condition marked by convulsions and muscle spasms. Vitamin D acts as a hormone by telling bone tissue to give up some calcium and release it into the blood in order to raise blood calcium levels.
Help the Helpers in the Meat Grinder of Life
Another common role for some vitamins is as part of enzymes. Enzymes are special proteins that jump-start the myriad reactions in the body, much as do jumper cables from a charged battery to a dead battery in your car. Without these important compounds, it would take considerably longer, in some cases virtually forever, to start chemical reactions vital to sustaining life. A vitamin that helps an enzyme is called a coenzyme, and unless it combines with that enzyme, the enzyme can't do its job. What is its job? The answer could encompass a college-level course in nutritional metabolism, but we'll synthesize it down to a few pages that start with a cheeseburger.
Although you may have really enjoyed the taste of the burger, the ultimate goal of eating it is to fuel your body so that it can keep doing work for you. Work requires energy, and the currency of the body's energy is a small compound you may remember from biology class called ATP (adenosine triphosphate). Your body's metabolic machinery makes ATP from the foods you eat, and the processes to do this are those of metabolism. Many processes involve breaking down larger compounds, such as the protein in the burger's meat, into smaller ones. Breaking things down is called catabolism, while making new compounds is called anabolism-the two aspects of metabolism.
After you eat a cheeseburger, your digestive system begins to dismantle it so that eventually the smallest nutrient cpmponents can be absorbed into the system. Vitamins and minerals generally are present in a form in which they can enter the blood, although they often need other molecules to carry them. However, the energy-yielding nutrients of fat, carbohydrate, and protein need to be broken down even further before they can give up their energy. Digestive enzymes start the process by doing just that: enzymes in the stomach and intestine render the energy nutrients absorbable and ready for the body to metabolize into usable energy.
The building blocks of the energy nutrients in our cheeseburger, components small enough to be ,absorbed, include amino acids, glycerol, glucose, and fatty acids. These building blocks enter into metabolic pathways which will break them down even further into their constituent atoms. A metabolic pathway is a cycle of chemical reactions into which compounds enter and are acted on by the cycle's enzymes, changing them into more useful forms.
You might think of the pathways as meat grinders, the old-fashioned kind with the crank handle that the butcher has to continually turn in a circle. He puts chunks of meat in (those are the nutrient building blocks), and after cranking the handle, ground meat comes out in even smaller pieces than before. Depending on what he's going to do with the meat, he can put the ground meat back in to make even smaller pieces. When he's done, our butcher can fry up the meat to eat right away (and get the energy from it!), or he can store it for later use in different forms, such as sausages in the fridge or a block of ground meat in the freezer. You might think of the B vitamins as butcher assistants who stand by and take turns at the crank.
In much the same way, glucose enters into one of the metabolic pathways and gets broken down into a smaller compound called pyruvate. This smaller compound can either continue on for further breakdown all the way to energy as ATP or be recycled back into glucose in a different pathway. All of the pathways are dependent on various B vitamins because of their role as coenzymes, or helpers to enzymes, that nudge the chemical reactions in the pathways. The very first step in getting energy from glucose, splitting it into pyruvate, requires niacin. Next, thiamin and pantothenic acid in concert with niacin, help spark the cascade of energy-producing reactions that follow. And in rapid succession or in other pathways, each B vitamin joins the symphony to allow the release of energy from the cheeseburger.
A glance at the myriad of metabolic pathways would make your head spin, and it probably takes a biochemist to appreciate the beauty of the complex reactions. But it's not the vitamins' "day jobs" that generate excitement among the average man on the street, it's the possibility that they can help prevent or treat a growing list of diseases. The upcoming chapters will review each vitamin In more detail, but shows the research highlights linking Specific vitamins to disease.