Let’s Talk Nutrition: The Six Essential Nutrients

A framework for understanding vitamins starts with a definition of the umbrella under which these vital compounds are but one piece of the puzzle-nutrition. Any number of textbooks can provide a working definition, some more inclusive than others, but most would agree on that provided by the Council on Food and Nutrition of the American Medical Association:

Nutrition is the science of food, the nutrients, and the substances therein, their action, interaction, and balance in relation to health and disease, and the process by which the organism ingests, digests, absorbs, transports, utilizes, and excretes food substances.

Quite a mouthful, and if you haven’t guessed, we are the organisms in question, along with any other species on earth that biologists consider to be alive. That definition is good for showing the scope of the science of nutrition, but it leaves out the more human factor of food behavior. And it is food behavior, or the food choices we make on a daily basis and why we make them, which determines how nutrition affects our health. Another way to say this is, once we know which foods and in what amounts are beneficial to our health, why don’t we eat those foods in those amounts?

Of course that’s the proverbial million-dollar question, and it doesn’t lend itself to easy answers. Cynics might respond that humans are inherently lazy and hedonistic, seeking after pleasure first and foremost. While that may explain it for some of us, a more enlightened response would be that our food behavior is influenced by culture, beliefs, heritage, socioeconomic status, lifestyle, and a host of other equally important factors. It’s no surprise, then, that simply knowing which foods are more health promoting doesn’t mean people actually eat those foods. But awareness is the first step, and only after acquiring correct information can a person choose to change his or her behavior.

A growing number of people are frustrated by what they perceive as conflicting information. Nutrition is a young science compared to other scientific disciplines, which means that nutrition studies got a late start and have continued to boom in the past few decades. In addition, as new reports came out, other researchers followed the scientific method and tried to repeat the same experiments. If the results of a study can be reproduced in similar studies, the conclusions are more reliable.

A good example is the recent flap about beta-carotene and lung cancer. Many studies have shown that people who eat lots of fruits and vegetables had lower rates of several different types of cancer (among these, lung cancer). Trying to figure out just what it is in fruits and vegetables producing the beneficial effect, however, is fairly complicated. Some subsequent studies have suggested that beta-carotene was the likely magic bullet, and researchers designed a special type of study to test that theory.

What they found was that smokers who took beta-carotene supplements actually had a higher risk for lung cancer! Since then, another study showed the same results. While even more studies would really convince everyone, the fact that the first results were reproduced in another study would suggest that smokers should avoid taking beta-carotene supplements. A look at the different types of studies that nutrition researchers do will help explain another reason for sometimes conflicting reports on diet and health, and will also prepare you for the upcoming chapters on vitamins, which focus on the latest research.

Why a Rat and Not a Person?

Another reason for the seeming contradictions in new nutrition information has to do with the different types of studies and the fact that the media tend to report results as soon as they are published, sometimes sooner. Because of this, no filtering occurs, which might help to put each single study in a broader context of what has come before and how to interpret the results. Research results can be difficult for experts to decipher, let alone a newspaper reporter who has no background in science. Yet this is how most Americans come by their nutrition information. Little wonder that each new report seems to add to consumer confusion instead of resolving it.

Scientists use four basic types of studies, each with its own strengths and weaknesses, to either prove or disprove a hypothesis, or a guess they may have about something. The first, and most familiar, is the laboratory study, which can include animals or simply test tubes. Lab studies can provide details on why a specific effect occurs, such as how vitamin C boosts immune function. One reason for doing this is that animals can be dissected after the study, in sharp contrast to human subjects.

Laboratory Study

Another strength of a lab study is the control that the researchers have over their experiments. They can use a specific breed of rat, knowing all the physiologic background of the strain, and are assured that all subjects will be extremely similar. In contrast, a group of seemingly similar humans presents many more variables than 100 “Sprague Dawley” rats Finally, researchers can use as many rats or rabbits as they can afford, and the greater the number of subjects, the more valid the results.

The major weakness of lab studies is obvious, however; just because a study works a certain way in a rat doesn’t mean the same applies to a human being. When the media report the latest research results, people often do not pay attention to the fact that it was an animal study and that applicability to humans remains to be proven. In addition, researchers have to be on their toes to use an appropriate animal model in a particular study because different species vary in how similar they are to humans with regard to a specific function. As an example, ferrets are the best animal for studying the absorption of beta-carotene, since their physiology most closely resembles humans in this regard.

Case Study

This type of study focuses on one individual, usually someone who has exhibited an interesting trait. A somewhat recent example comes to mind: a few years back, the New England Journal of Medicine reported that a man in his nineties, who had been eating an average of a dozen eggs a week, had the blood cholesterol levels of a healthy teenager. This obviously surprised his physician and countless nutrition professionals who read the published report, because age and male gender are both risk factors for high cholesterol. To add a dozen eggs a week, which conventional wisdom of the time implicated as raising blood cholesterol, was unthinkable! While these reports are intriguing and can lead to further research, the effect of diet on one individual is not significant.

Epidemiologic Study

So if one person isn’t enough, what about an entire population? Epidemiologic, or population, studies compare disease rates among groups of people around the world, and look for correlations between disease and dietary habits. It was this type of study that led researchers to the association between a Mediterranean-type diet and a lower risk for heart disease. Epidemiologic studies are useful in pointing to a possible connection between diet and disease, laying the groundwork for further studies. Unfortunately, these studies can only show a statistical association, but not cause and effect. In other words, just because a study reports that people who eat a high-fiber diet tend to get colon cancer less often, the study doesn’t prove that dietary fiber prevents the disease. It might just be that something else about the high-fiber diet, or the kind of people who eat that way, is protecting against colon cancer.

Intervention/Clinical Trial Study

An intervention study compares the effects of a treatment, or intervention, on a group of people or subjects, called the experimental group, to another group, who received no treatment, called the controls. The control group is sometimes called the placebo group, which simply means that they received a fake treatment or “sugar pill” instead of the real treatment. The intervention study is by far the most powerful of all research designs because it demonstrates the effect of a given treatment. Recent examples include the Physicians’ Health Study, which proved that a small daily dose of aspirin reduced the risk for heart attack in men.

So if this type of study is so good, why isn’t it used all the time? It’s very expensive to conduct clinical trials, partly because humans are involved, requiring longer periods of time for treatment than animal subjects. In addition, blood draws and other assessment methods require trained personnel, which adds to the cost. But even with powerful intervention studies, the results are only as good as another study’s ability to duplicate the results.

The Last Word?

The next time a news headline screams, “Broccoli Causes Cancer!” take a few minutes to evaluate the study. Look at a few simple criteria, such as whether it was a human or animal study. If it was a human study, how many people participated? Was it an intervention study or an epidemiologic study? And finally, if the results contradict research that has preceded this study, remember that reproducibility of those results will be necessary before taking the information too seriously. Researchers are just doing their job by continually asking questions; generally, they end up with many more questions after doing a study than answers to the first question they asked. Smart consumers will wait before acting on the results of a new study.

Six, Count ‘Em: The Essential Nutrients

Nutrition concerns itself with the study of foods and the numerous compounds they contain. However, some preferential tre,atment is in order for the essential nutrients. The word essential refers to the fact that we have to ingest these compounds through our diet; our body can’t make them. We need these substances to grow and maintain our bodies. Adults don’t tend to grow as do infants and children, but they still need to repair and replace body tissues.

As an example, red blood cells have a limited life span of four months. After that time, the body has to make new ones. To do that, it needs specific nutrients. You might consider nutrients to be building blocks for all forms of life. In some ways, we are not too different from an ear of corn or a bowl of rice. Although the comparison isn’t exactly flattering, it illustrates that the human body is made up of similar components as the foods we eat, which is why those foods are nutritious to us!

The six essential nutrient groups which we must obtain through our diet are water, carbohydrate, fat, protein, minerals, and vitamins. Within each group, we need several nutrients. For example, we need thirteen vitamins and fifteen minerals. Water is by far the most vital: you can get by without vitamin C for a few months, but a shortage of water will be life threatening within days. This is generally true for all the nutrients, with a deficiency of some adversely affecting us sooner than others. Another important principle is that only fat, carbohydrate, and protein provide energy in the form of calories. Each of the billion cells in our bodies is a small microcosm of the whole, and while each needs all the essential nutrients, the most critical in the short term are water and energy.

Each essential nutrient has a particular function in the body which can’t be performed by any other nutrient. Overall, water, vitamins, and minerals help convert the energy nutrients, fat, carbohydrate, and protein into energy the body can use. Aside from the obvious need for energy to fuel our physical activity, the body uses energy to build and maintain its components such as muscle, bone, and blood. A chemist once described the human body as a mini-chemistry lab forced to conduct millions of chemical reactions every day. A fairly apt metaphor, and the vitamins are among the industrious little chemists working to change the food we eat into fuel, glue, bricks, and other building materials to keep the main building in good repair.

Quick Notes on Each Nutrient Group

This book, and especially the next three sections, focuses on vitamins, but a quick summary of each essential nutrient group’s functions is in order.


Although this group has sometimes gotten a bad rap, it includes a variety of compounds which have implications for health, with the basic function of providing energy for the body’s needs. Our main source is green plants, which convert water and carbon dioxide in the air to carbohydrate, with the help of sunlight and chlorophyll. Carbohydrates are either simple or complex, with simple carbohydrates consisting of sugars. The most common form of sugar, glucose, is the energy currency that travels in our blood to keep thecells fed.

Complex carbohydrate refers to starch and fiber, and of the two, humans can only digest and derive energy from starch. Although we don’t have the needed enzymes to break down fiber, it helps the body in other ways. The fact that we can’t break it down means that it keeps moving along the intestinal tract, stimulating bowel function and keeping it healthy. There is good evidence that a high-fiber diet can lower the risk for colon cancer. Friendly bacteria that live in the colon can break it down, and the by-products appear to have a healthful effect on colon function and in lowering cholesterol.

You can see that the bad rap is undeserved: far from being “fattening” additions to the diet, a generous intake of complex carbohydrate may actually help with weight control by replacing higher-calorie fats. Aside from its potential health benefits, fiber also makes a person feel full (called satiety), which may reduce calorie intake. The key is to emphasize complex carbohydrates, and use sugars only moderately. And finally, complex carbohydrates are one of the best ways to obtain the B vitamins, with whole grains being an excellent source.

How much is the right amount? That depends on you and your total energy needs in a day. Remember, the type of carbohydrate is everything. Load up on food sources such as fruits, vegetables, whole grain breads, cereals, and other grain products. Cut back on sources which are mainly desserts such as cookies, cakes, pies, candy, and sweetened drinks. To ensure that you get enough fiber in your diet, you might even consider limiting your intake of added sugars and processed grain products that offer no fiber. These include white bread, presweetened cereals, and crackers, muffins, and bagels that are not whole grain. Shoot for 50 to 60 percent of your total calories to eat as carbohydrates. If your daily calorie intake is around 2,000, you should take in slightly more than half, or 1,000 calories, as carbs. Illustrates a healthy example focusing on carbohydrate choices.


Most people think of fat as something to avoid in their daily diets to prevent heart attacks and keep from becoming obese. But as with carbohydrate, the reputation is largely undeserved, with different types of fat serving many vital roles in the body. The type of fat we carryon our bodies, triglyceride, is the same type we eat in foods. While we may complain about how fat we are, if we had to store our energy reserves as carbohydrate, we’d be huge! Fat takes up roughly half the space, in chemical terms, as carbohydrate. The reason for fat being the perfect form of reserve energy in the body actually accounts for its higher calorie value-9 calories per gram, compared to carbohydrate and protein at 4 calories per gram.

Fat is also useful as an insulator against temperature extremes, and it protects vital organs by cushioning them. On a smaller scale, fat molecules make up the protective membranes surrounding individual body cells and other important structures. The fat in foods adds to flavor, texture, and mouth feel;’ which is a food technologist’s term to describe the creamy sensation in your mouth when you eat a food containing fat. Fat also contributes to a feeling of satiety, a feeling of fullness after a meal. This may be one reason why people who dramatically cut back on their fat intake can’t seem to get enough food.

Getting back to vitamins, several of them require the presence of fat in foods to be properly absorbed and used by the body. Some people who have conditions causing difficulty in fat absorption end up becoming deficient in the fat soluble vitamins (those requiring fat for absorption). In addition, certain fatty acids, the compounds which form triglycerides by combining in groups of three, are essential nutrients linoleic and linolenic acids, found in plant oils.

While everyone is still worried about reducing fat intake, the evidence suggests that it is the type of fat you eat, rather than the amount, which determines the impact on health. Currently, a diet that is moderate in fat, around 30 percent of total energy, and emphasizes monounsaturated fat is considered the healthy choice. Saturated fat, the type which predominates in animal products, is associated with higher blood cholesterol levels, which in turn increase the risk for heart disease. shows the sample diet we used for the carbohydrate count, adding in foods which add fat, and substitutions for those that usually do, in the recommended amounts.


This nutrient had been the darling of the diet world starting in the 1960s and continues to hold the interest of bodybuilders and other athletes. The basis for their interest is not too farfetched: lean body mass, and specifically muscle tissue, is composed primarily of protein. While we say that protein is an essential nutrient, it is more accurate to say that the body requires specific amino acids, the building blocks of protein. Different protein-containing foods provide a mix of the various amino acids.

Protein is different from its other two energy-yielding partners, fat and carbohydrate, in that it contains the element nitrogen. A protein can contain twenty different amino acids, and when we eat protein, we get a mix of them. The body can make eleven of them on its own, but we have to eat protein containing the other nine, which are called essential amino acids.

Protein in the body is, by and large, functional. In other words, we don’t have extra protein taking up space for reserve, as we do with fat. Body protein is structural, as in muscle, the inner parts of bone and hair, tendons, and other body components. But it also works, forming enzymes which pave the way for chemical reactions, immune system compounds such as antibodies, and carrier molecules (that ferry things such as vitamins in the blood). In addition to carrying nutrients in the blood, a major blood protein, albumin, is responsible for keeping the body’s fluid in balance.

It’s obvious, then, how important protein is. What you might not know is that the average American takes in twice as much protein as he or she needs. The recommended amount of daily protein intake is based on a person’s weight. Of course, you don’t add extra protein if you have excess weight. The recommended amount is 36.4 percent of body weight. A moderately active woman who weighs 120 pounds needs 43 grams of protein every day. The sample diet, at about 2,000 calories, probably works for many people and provides more than twice the amount this woman needs. This amount of excess shouldn’t pose a problem for the average person, unless there is kidney disease, but you can see how calls for higher protein intakes are off base.


These compounds are the stars of the nutrition world, and it was their discovery, probably more than anything else, that fueled the explosion of research into human nutrition. The current list of vitamins, thirteen to be exact, have been known to be essential for human life for several decades. However, scientists continue to study these intriguing nutrients to learn more about them-everything from how the body absorbs them from different foods to possible interactions within the body’s cells. One thing is certain: as new facts unfold, new questions arise.

Although nutrition researchers still study vitamins to learn about what they do, we know some of their general roles. Some act like hormones in the body, sort of chemical messengers. Others are a part of enzymes, vital compounds that control metabolic reactions in the body. Many of the enzymes vitamins team up with can’t function unless they combine with specific vitamins, called coenzymes.

A useful way to sort vitamins is into two groups: those that dissolve in fat (fat soluble) and those that dissolve in water (water soluble). This gives you some idea of how the vitamins function and how they’re,handled by the body. In addition, solubility determines if a vitamin can be stored in the body and how easily it’s lost from the body as well as from foods during processing or preparation. And finally, knowing whether a vitamin dissolves in fat or water can give you a rough idea of which foods contain it.

You don’t need much of any of the vitamins-it works out to about an ounce if you add up all the vitamins you need for a day. But scientists use different units of measure for vitamins, including mg and micrograms. Just to get an idea of how little a microgram is, one of these tiny units is equivalent to one-millionth of a gram, and a gram is about one-thirtieth of an ounce! Retinol equivalents (RE), and sometimes International Units (IU), are the measurements used in industry for vitamins A and E.

Since there are thirteen essential vitamins (fourteen, if you count the newcomer choline), our sample meal would get a bit complicated, so we’ll look at two key vitamins that many Americans don’t get enough of vitamins A and C. What you’ll notice is that with few exceptions, most of the significant sources of both vitamins are fruits and vegetables. One important distinction is that even in these foods, there is a wide variability in the amounts notice that a banana and an apple provide negligible amounts of either nutrient. But the sample meal is perfectly balanced and provides our reference female with almost four times the recommended amount of vitamin A and more than seven times the vitamin C.


Minerals are the most enduring of nutrients; that is, long after the human body has decomposed, and actually forever, the minerals in that human body will remain unchanged. Their indestructible nature tells us that cooking heat and other forms of food processing don’t lower the amounts of needed minerals in our foods. However, minerals are water soluble, so prolonged contact with water will leech them out of foods. In contrast to the complex forms of the other nutrients, minerals are the basic chemical elements familiar to us in useful things other than food, such as copper pipes and iron railings.

The minerals fall into two major categories, major and trace minerals. The difference between them is a matter of amount: the body contains major minerals in amounts greater than 5 grams and less than this amount of trace minerals. This gives you an idea of the amounts you’ll need to consume as well. As an example, compare the need for 800 mg of calcium, a major mineral, to that of 12 mg for the trace mineral zinc-a major difference! is a list of both groups.

The roles of the minerals are diverse, with many of these nutrients doing double and triple duty. As an example, sodium, familiar as the partner to chloride in table salt, helps to maintain the proper amount of acid in the blood and fluid balance outside the cells. In addition, sodium is involved in muscle contraction and nerve transmission, both vital functions considering that the heart is a muscle and has to keep beating.

As mentioned, the amount we need of each essential mineral varies depending on whether it is a major or trace mineral. Some minerals continue to prove challenging for some Americans to consume in the recommended amounts. As for toxicity, many minerals can prove fatal in excess amounts. A deadly example is copper, which because of its potential toxicity, was the drug of choice years ago for people in India wishing to commit suicide. One reason for easy toxicity has to do with the body’s handling of a particular mineral. Some minerals readily excrete into the urine when excess amounts arise, while others, such as iron, tend to accumulate in the liver, posing a considerable health risk.

If we go back to our sample day’s intake and consider two minerals which pose problems for many American women, calcium and iron, we’ll see that only certain types of foods provide significant amounts of each. And our sample menu again stands up to the test, providing our reference woman with about 20 percent more than the recommended amounts for calcium and iron. A final note that isn’t apparent just by looking at the numbers: for several minerals, and especially for calcium and iron, the body appears to better absorb and use those nutrients from animal sources compared with grains and vegetables.


Although water is so essential to life that even a few days without it can kill a person, most people don’t give much thought to this vital nutrient. And while everyone knows the importance of vitamins and minerals, people may not realize that water, along with those nutrients and fat, carbohydrate, and protein, together make up the six nutrients essential for human life.

Water is a very simple compound from a chemical standpoint, consisting of only two atoms of hydrogen and one of oxygen. However, its simplicity gives rise to functions which support every aspect of human physiology, the most basic being a near perfect solvent and the medium for most of the body’s chemical reactions. In addition, water brings nutrients to each cell and carries away the cell’s waste products. It is part of the chemical structure of cells, tissues, and organs. Water also acts as a lubricant and cushion for joints and lubricates the digestive tract and other mucosal tissues. Another important role is in body temperature regulation because of its ability to change temperature slowly.

The need for water is based on caloric intake, and therefore body size. Under normal conditions, the average adult needs about one milliliter for every calorie consumed, or about 13.6 milliliters per pound of body weight. However, fluid requirements are affected by factors such as internal and external temperature, physical exertion, and environmental humidity level. In addition, several compounds and certain conditions can act as diuretics to promote fluid loss or cause retention of body fluid.

Naturally occurring diuretic compounds include caffeine, alcohol, and chemicals found in certain vegetables such as asparagus. Compounds which promote retention of water include high salt intake and the body’s production of certain hormones, such as antidiuretic hormone and vasopressin. A person’s age also influences the need for fluid, with infants requiring a higher proportion because their body composition includes a higher amount of water.

Many people believe that they need to drink eight glasses of water every day to maintain proper fluid balance, which is an erroneous assumption based on overlooking the fluid contained in foods. Most foods consist of 50 to 90 percent water, which provides about 60 percent of the adult need for water. Foods such as fruits and vegetables contain the most fluid, so depending on intake, fluid provided by foods can vary. Additionally, although drinking plain water can be beneficial, it is not essential, since other beverages contain water. Exceptions include coffee and other caffeine-containing beverages and alcohol, which because of their diuretic effect tend to deplete fluid beyond the amount they provide.

Our reference woman who eats the sample menu is taking in about 2,000 calories. This means her fluid need is 2,000 mI, or 66.6 ounces. Looking at the water contained in just the beverages, two cups of milk and one cup of juice, she has already consumed 21.6 ounces, since both milk and juice are about 90 percent water. If we only give her credit for her apple, banana, and carrots, now she’s up to about 30 ounces, which doesn’t include the additional water from other foods. Another four and a half cups of any liquid, including water, puts her where she needs to be.

Anything Else in There?

By now, you may be wondering about some of the other substances you’ve heard of in the news lately: fiber, phytoestrogen, flavonoids, and many more. Scientists refer to these as nonnutrients, of which the phyto chemicals are one type. The word nonnutrient simply means that the compound is not one of the known nutrients which we need, while phytochemicals have some kind of activity in the body. There is increasing evidence that some of these compounds may be beneficial in preventing chronic diseases. But not all of these nonnutrients are helpful: cabbage and other plants contain compounds called goitrogens that interfere with the thyroid hormone, possibly causing a goiter. Fortunately, the heat from cooking destroys these compounds, so it’s not normally a problem. Scientists who study toxins (toxicologists) tell us that plant foods are rife with potentially toxic substances.

Most of the consumer excitement regarding phytochemicals may seem recent, but food scientists have known for some time of their existence. Other nonnutrients have been acknowledged for several decades, such as dietary fiber. Although evidence continues to mount that different types of fiber are beneficial for human health, from aiding normal bowel function to preventing colon cancer, strictly speaking, fiber is not an essential nutrient. In other words, humans could theoretically survive without it. Quality of life, however, might be another matter!

The consideration of nonnutrients seems to pose another question: How do nutrition scientists figure out if something is an essential nutrient? Part of the answer relates to the history of vitamin discovery, which we’ll save for upcoming chapters. But more recently, scientists have used a combination of the various types of studies to determine essentiality. Animal studies are the most obvious choice because of the method researchers use. To find out if a nutrient is essential, they feed the animal a diet, usually a formula, devoid of that compound but containing all the other nutrients they know to be essential. If the animal can grow, develop normally, and not suffer any ill effects, the compound is not essential. If the animal shows signs of deficiency, the compound is an essential nutrient.

Human case studies have also proved invaluable, usually the result of trial and error. In the not too distant past, say, forty years ago, scientists developed methods of feeding people who couldn’t eat normally. Eating normally means taking food through the mouth and all the way through the intestinal tract. Doctors had to figure out another route for feeding people who had a problem somewhere in the intestinal tract. Over the years, they’ve refined the methods by feeding people either through a tube inserted somewhere into the intestinal tract or, for people with a nonfunctioning intestine, directly into the bloodstream. You can see, then, that they learned fairly early on what nutrients had to be added to the formulas to prevent nutrient deficiencies. Although technology has advanced greatly, some questions remain about other compounds the formulas still might need.

How Does Your Diet Add Up?

To get the most out of this book, you may want to consider doing a bit of detective work about your diet. Completing the following form will enable you to do the “How Your Diet Adds Up” quiz at the end of each section relating to a specific vitamin. The first rule of thumb for keeping a food record is to do as many days as possible. It’s best to do a minimum of three, and consider two weekdays and one weekend day. The more days you evaluate, the more likely your intake for that time is representative of your diet in general. The reason for including a weekend day is that most people eat differently then. Try to avoid using a day that is not typical for you, such as a special occasion (a wedding or a party).

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