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Virtually all health care providers intuitively realize that nutrition is an important component—a cornerstone, actually—of treatment, particularly for the chronic conditions of this publication's focus: diabetes, obesity, and cardiovascular disease.
Stories on nutrition-related issues are not always as clear-cut or straightforward as we might like. If you follow nutrition and food matters, you probably realize that certain foods are like pendulums, swinging back and forth from "good" to "bad" to "good" again, depending on the latest study, trend, or fad diet.
In our society of instant news, nutrition and food research often hits the stands or arrives via Internet or e-mail alert without adequate context to examine study limitations, causing many consumers to stampede from one health fad to the next. It is interesting to note, however, that this phenomenon does have the ability to raise the number of research proposals and the funding available in a given research area.
UNDERSTANDING THE LANGUAGE OF NUTRITION
Before addressing the challenges of food and nutrition research, some definitions are needed. Nutrition is the term used for the process by which our bodies utilize food for energy production, for health maintenance, for growth, and for the normal functioning of every organ and tissue. At least 34 nutrients are needed for growth and normal body functioning. Foods contain not only various proportions of the macronutrients (e.g., carbohydrate, fat, protein) and water, but also various amounts of the well-known micronutrients (vitamins, minerals, and electrolytes) and perhaps one or more of hundreds of naturally occurring substances (e.g., carotenoids, flavonoids, and isoflavones).
Eating is not only a pleasurable experience, it is an essential function. An infinite number and variety of foods are available for our enjoyment, each with a unique nutrient composition. And more foods are being designed every day (e.g., functional foods, such as margarines with stanols or sterols). Thus, it isn't surprising that a wide variety of individual foods (e.g., herbs and spices, fish oil), food groups (e.g., dairy products, nuts, soy products), and nutrients (e.g., fat, protein, fiber, chromium) all have been implicated in reducing—or increasing—the risk, control, and/or complications of diabetes.
The classic example is sugar. In the pre-insulin era, sugar was a forbidden food for people with diabetes, as it was thought to cause diabetes or make it worse. Now sugar is considered a carbohydrate, just like any other carbohydrate food.
"Food concepts" also are making headlines. For example, the idea of the glycemic index has been around for at least 30 years. For a time, it was considered almost a dead issue (a 1997 commentary in Diabetes Care was entitled, "The Glycemic Index: Flogging a Dead Horse?"1), but now it is enjoying renewed interest.
FOOD STUDIES DIFFICULT TO DESIGN
Despite our growing knowledge about foods' properties, designing studies where food becomes the medium for delivering a nutrient treatment is not easy.2 Much of our evidence for or against nutrients and foods starts out as epidemiological in nature—retrospective or prospective cohort studies that track people with various conditions and what they say they eat. These studies, if well designed, can show trends, but they can't show cause and effect. For that, randomized, controlled trials are needed, and designing well-controlled clinical trials involving humans and diet can be extremely complicated.
The most well-controlled feeding trials are those conducted at a live-in metabolic unit where subjects are required to consume meticulously weighed amounts of foods and/or beverages. Variables such as physical activity are closely controlled. By their nature, these studies must be short-term (days or weeks rather than months or years), so the results cannot represent long-term consumption of the nutrients of interest, nor can they be translated to the real world, where people have free choice. However, they do provide a good idea of what the nutrient or food can do for an outcome of interest, and they provide rationale for longer-term outpatient studies.
Randomized, controlled clinical feeding trials on an outpatient basis are more acceptable to subjects and can be carried out over a longer term, but are compromised by a number of confounding variables—adherence to the diet, physical activity, and weight change, to name a few. However, when well designed and conducted, these studies can provide substantial evidence.
For example, the Dietary Approaches to Stop Hypertension (DASH) trial was a randomized, multicenter, controlled outpatient feeding trial comparing a typical American diet with one that was higher in fruits, vegetables, and low-fat dairy products; included whole grains, poultry, fish, and nuts; and was reduced in fats, red meat, sweets, and drinks containing sugar, yet had calories and macronutrients similar to the control diet.3 For 8 weeks all meals were provided to the 459 mildly hypertensive subjects. The DASH diet produced a significant drop in both systolic and diastolic blood pressure when compared with the control diet.
So, determining which nutrition studies produce good evidence is not easy;
on the other hand, it can be done! 
Where Can We Find the Answers?
The American Diabetes Association (ADA) has developed an evidence grading
system for clinical practice
recommendations,1
including nutrition recommendations. In 2006, ADA plans to begin updating its
nutrition recommendations annually (rather than every 4 or 5 years). Expert
consensus will provide concise statements of the best evidence, and the
reports also will identify issues where there is no evidence and issues where
more research is needed. DOC News will provide readers a synopsis as soon as
the next report becomes available.
In the meantime, the current ADA nutrition recommendations (from 2002) are available online at http://care.diabetesjournals.org/cgi/content/full/25/suppl_1/s50.
Nutrition resources for patients include credible newsletters that cover current food and nutrition issues, with factual evaluations as well as bottom-line opinions:
Footnotes
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References
2. Dennis BH, Ershow AG, Obarzanek E, et al. (Eds.): Well-Controlled Diet Studies in Humans: A Practical Guide to Design and Management. Chicago, The American Dietetic Association, 1997.
3. Appel LJ, Moore TJ, Obarzanek E, et al., for the DASH Collaborative
Research Group: A clinical trial of the effects of dietary patterns on blood
pressure. N Engl J Med 336:1117–1124, 1997.
4. American Diabetes Association: Clinical Practice Recommendations
2005. Diabetes Care 28 (Suppl.
1): S1–S2, 2005.
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