Dietary advice is everywhere, often dished out with absolute certainty.

Look for organic foods, avoid preservatives and stay away from seafood that may contain mercury, we are told. Avoid red meat, to be sure. And above all, go gluten-free.

Is this advice based on sound reasoning?

Dietary preferences vary from culture to culture. Concepts with regard to “healthy” eating tend to evolve.

Perhaps dietary advice should be taken with a grain of salt.

For example, avoiding butter and red meat seems a plausible approach to decreasing cholesterol levels. However, adhering to dietary restrictions may be challenging for some. Cholesterol-lowering medications typically lower lipids more effectively than dietary intervention alone.

The paradoxical popularity of high protein, high fat diets, advocated in the past for rapid weight loss, represents contrarian advice.

You may pick a diet to match your philosophy. As most of us know, diets rarely work.

Lifestyle choices are more conducive to good long-term health. Along those lines, many Americans are now pursuing gluten-free eating. Gluten refers to various protein compounds that are found in wheat, rye and other grains. Gluten contributes to the chewy texture of bread.

A recent American College of Rheumatology conference focused on gluten, with Mayo Clinic gastroenterologist Dr. Joseph Murray presenting his insights.

Thomas R. Collins reported in The Rheumatologist journal that gluten-induced illnesses do occur. There is a well-described gluten enteropathy, a condition in which the small intestine is damaged as a result of ingesting gluten.

Auto-immune diseases such as myositis, sarcoidosis and juvenile arthritis may be also be triggered by gluten.

Murray also tells us that among the 1.8 million Americans who may have gluten-related intestinal conditions, most remain on a regular diet.

Another large cohort seeks to avoid gluten in the absence of diagnostic reasoning for this approach.

Twenty times as many people are diagnosed with gluten-related gastrointestinal problems today, compared to the 1950s.

Has our digestive physiology changed significantly since that era?

Diagnosis of celiac disease typically requires a history and physical examination. The gastroenterologist, for example, may look for a history of so called “malabsorptive syndrome,” with weight loss, diarrhea and other symptoms.

If gluten-related enteropathy is suspected, there is a specific serology blood test called tTGA that may be employed, as well as sophisticated tests for genetic markers for gluten sensitivity.

The genetic markers, HLA-DQ2 and HLA-DQA, are fairly sensitive. Absence of those markers argues against gluten-related intestinal diagnostic conditions. A positive test, however, is not specific to such conditions, nor is a borderline tTGA result.

The gold standard for determining whether gluten-free eating is necessary is a biopsy of the small intestine, obtained by performing an upper endoscopy.

Adding to the complexity, a false-positive biopsy may occur in the setting of other rare diseases, including tropical sprue.

The take-home message is that the diagnosis of gluten-sensitive enteropathy is complex, and often requires the insight and expertise of a gastroenterologist.

If you are experiencing digestive difficulties, it is also worth noting that gastrointestinal textbooks describe hundreds of conditions that may be relevant, ranging from lactose intolerance to inflammatory bowel diseases, such as ulcerative colitis.

That is why gastroenterologists must train as subspecialists for several years after completing both medical school training and an internal medicine residency.

In a sense, the gastroenterologist is a detective, seeking to nail down a diagnosis based on logical reasoning.

Could gluten ingestion lead to obesity and diabetes? Perhaps. We need more research.

I agree with French statesman Charles Maurice de Talleyrand (1754-1838), who cautioned against political overconfidence, saying, “Surtout, pas trop to zele (Above all, not too much zeal).”

That goes double for dietary dogmatism.

Scott T. Anderson, M.D., Ph.D. (email [email protected]), is a clinical professor at the University of California, Davis Medical School. This column is informational and does not constitute medical advice.