Human beings are unique in the animal kingdom when it comes to nutrition. Besides being the only creatures that cook food, we are the only ones that make conscious decisions about what to eat.

The world’s animals—herbivores, carnivores, and omnivores—just seem to know instinctively what plants or prey to devour. Female monarch butterflies cannot be taught to lay their eggs in the milkweed plants upon which the larvae feed and ingest toxic cardiac glycosides as a defense against predators.

But they do it. The fantastic diversity and success of animal life affirm that nature has found ways to ensure proper nutrition.

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Human beings also know what plants and animals to eat—and avoid—perhaps through observation and trial and error over millennia. Eventually, we came to realize that too much or too little of some types of foods results in medical problems, and that a “balanced diet” is necessary for good health.

A balanced diet consists of the “right” proportions of this or that food item. Generally, this has come to mean, among other things, a diet emphasizing fruits, vegetables, and whole grains; increased intake of fish; and decreased consumption of red meat and foods high in saturated fat.

Medical societies recommend higher or lower intake of various foods based largely on scientific evidence showing what appears to be beneficial, harmful, or not helpful. The effort is commendable, but everybody needs to keep in mind that, any day, a new study can challenge the notion of what may or may not be good for you.

For example, the American Heart Association recommends eating fish at least twice a week, and cites recent research demonstrating that eating oily fish containing omega-3 fatty acids, such as salmon and herring, may help reduce the risk of death from coronary artery disease.

A study published recently in Journal of the National Cancer Institute, however, confirmed a previously reported association between high blood levels of long-chain omega-3 fatty acids and increased prostate cancer risk.

In addition, a meta-analysis published recently in Annals of Internal Medicine (2014;160:398-406) found no significant evidence to support eating a diet high in polyunsaturated fatty acids and low in saturated fats to lower the risk of coronary disease.

“Current evidence does not clearly support guidelines that encourage high consumption of polyunsaturated fatty acids and low consumption of total saturated fats,” the study authors concluded.

What constitutes healthy eating, as ascertained in studies, is always in flux, but that should not stop people from aspiring to good nutrition. Moderation in the amounts of food consumed would be a good beginning.