Summary: Epidemiological, clinical, and laboratory studies are all used in nutrition research. Most scientists believe each type of study has its strengths and limitations. Studies are the basis of nutritional recommendations, so recommendations change as knowledge about nutritional requirements is acquired.
Nutrition is a science, just as chemistry, biology, and physics are sciences. Since nutrition is a science, it involves research and the use of scientific methods to answer questions. The questions which nutrition researchers ask are often about which foods and how much of those foods people should eat. Research studies are designed to answer specific questions. Studies may differ in their design and in the method they use to answer a question.
For example, to answer the question "What can I eat to keep from getting cancer?" one scientist might collect information on the diets of groups of people for many years and see which people got cancer and which didn't. The scientist would then compare the diets of these two groups. Another scientist might conduct an intervention study where people are randomly assigned to either follow or not follow a certain diet or to use or not use a certain food or supplement. The scientist would then determine the effect of the intervention on the subjects' health. A third scientist might use cells or tissues or animals to evaluate the effects of a certain nutrient on cancer occurrence.
These types of studies are examples of epidemiological, clinical, and laboratory studies, respectively. Most scientists see advantages and disadvantages with each type of study.
Epidemiology is the study of populations. Often in epidemiological studies, people are asked to describe their diets, either current or past. The amounts of different substances in their diets are then correlated with incidence of certain diseases or conditions. Epidemiological studies may also measure the levels of substances such as vitamins or minerals in blood or urine and attempt to correlate these levels with diet or health.
The major strength of epidemiological studies is that they are able to look at large groups of people for a long period of time. People from more than one region or more than one country can be included. People of different ages, genders, and ethnic origins can be studied. Epidemio-logical studies look at people, so the results of these studies are directly applicable to humans and do not have to be extrapolated from cells or animals.
On the other hand, there are limitations to epidemiological studies, particularly those that are based on people's descriptions of what they eat. There are several different tools available to collect this information, but all have some limitations. They all rely on the subjects to describe accurately what they eat. This is difficult for many people. If you were a subject in a nutrition study, and you were asked to tell an investigator everything that you ate or drank yesterday, would you remember everything? Would you know how much of each food you had eaten? And how representative of your "typical" diet would that one day's intake be? Besides the limitations due to inaccurate reporting of subjects, another limitation is that investigators are unable to know all the factors affecting their subjects. For example, subjects may have a high rate of cancer because of exposure to radiation, not due to dietary factors at all.
Clinical studies also use human subjects, but these studies are often smaller than epidemiological studies. They modify people's diets or activity levels and then study the effects of these changes. These studies are also called randomized trials or intervention studies.
As with epidemiological studies, an advantage of clinical studies is that they use human subjects so the results of these studies are directly applicable to humans. An additional strength of clinical studies is that they are more controlled. Often in a clinical study, subjects are housed in a metabolic unit, where all food is provided and activity is closely monitored. This allows investigators to know exactly what is going on with their subjects.
A drawback of this type of study is that very few people live in a controlled setting so the results of these studies may not be the same as those in real life. Also, people are unwilling to live in a metabolic ward for a long time. If a scientist is trying to answer a question about something that takes many years to occur, the results of a clinical study may not be helpful.
Laboratory studies are used because they allow investigators to have more control than is possible in human studies. These studies are often less expensive than studies using human subjects. However, results of these studies can only provide preliminary information. Animals, tissues, and cells are not a substitute for humans. In addition, many people see moral reasons not to experiment on animals.
Often we are faced with newspaper headlines like "Coffee Drinking Leads to Pancreatic Cancer" or "Vegans Have a Deficiency of Vitamin X." These are frequently based on the result of one study. When you read a report like this or hear the latest nutrition news on radio or tele-vision, consider the strengths and limitations of each type of nutrition study. Usually, one study is not enough to answer a large question in an area as complex as nutrition.
Normally, when a controversial study is published, other scientists begin to do experiments to see if they can get similar results. If several scientists, using different methods, come to similar conclusions, more faith can be placed in their results.
Often the results of a variety of studies are used to make nutritional recommendations. Recommendations made by groups such as the National Academy of Sciences, the Dietary Guidelines Advisory Committee, the Academy of Nutrition and Dietetics (formerly known as the American Dietetic Association), the American Cancer Society, and the American Heart Association are usually made by a committee of scientists who have conducted a thorough review of the existing information on the subject. Recommendations change due to new information and to new ways of looking at previously collected information.
We do know a lot about nutrition and our knowledge base is growing daily. Nutrition studies can be used to make recommendations about what people should eat to achieve good health. In this nutrition section, the results of scientific studies will be used to make recommendations specifically aimed at people following vegan diets.