My wife, Catherine, and I eat food, not too much, mostly plants. For a variety of reasons, our diets exclude mammals altogether – although healthful diets certainly can include lean meats. Whether or not diets that are healthful for the planet can do so is the more debatable consideration. My particular problem with eating mammals is that some of my family members, and best friends in the world, have four legs.
And so it is that at a recent holiday party that offered a choice of three entrees, we chose the salmon. In case you care to know, it was too salty, otherwise not bad. But what our tablemates wanted to know was whether we were concerned about the mercury. When I eat in public, questions about nutrition are a predictable occupational hazard. Keeping Mom's counsel in mind, I try not to answer with my mouth full – but I do try to answer.
As it happens, mercury in salmon isn't much of a hazard. There is some, and apparently more in wild salmon than in farm-raised. There are other concerns with farm-raised salmon, however. At one time, PCBs were a particular problem with farm-raised salmon. Attention to that situation has improved it, but such industrial chemicals may still be at higher levels in farm-raised than wild fish.
Mercury is a heavy metal that accumulates in animal bodies through a process called bio-concentration. Small fish and marine animals that eat plants are exposed to some heavy metals, and their bodies retain them. Larger fish that eat fish that eat plants retain and accumulate the toxins from the flesh of their prey, concentrating them. Very large predatory fish that eat these smaller predatory fish repeat the process, concentrating heavy metals further. And so, predictably, mercury is at highest levels in very large, fish-eating fish: swordfish, large tuna, marlin, king mackerel and sharks.
So the concern about mercury in salmon was a bit misguided. But the underlying implication of the question was more general and perfectly legitimate: Is it OK to eat fish?
The answer is, unequivocally, yes. To date, all major epidemiologic studies that have compared the routine inclusion versus the exclusion of fish from omnivorous diets have found an overall health benefit when fish is included. Such studies have not, and could not, differentiate contaminant-free from contaminant-containing fish, so what the data indicate is that eating fish confers a net benefit despite the contaminants that come along for the ride. In general, what's good about eating fish outweighs what's bad, just as there is a health benefit from eating vegetables and fruits despite the risk of such contaminants as insecticide residues.
What is it that makes eating fish good for health? In the case of fatty fish such as salmon, the high concentration of omega-3 is certainly a factor. But we need to be careful to avoid the common mistake of looking at diets one food at a time.
There is another important implication of diets that include more fish: They probably contain less of something else to make room for that fish. In other words, people who eat fish as their main dish more often are probably eating less meat. Health relates more to the overall pattern of our diets than any one food, so when there is an association between a food or food group and health outcomes, it likely relates both to what that food adds to the diet and to what it displaces. This is true of fish and is likely just as true of nuts, soy, eggs and so on.
Some years back, a big fuss was made about the omega-6 fat content in tilapia, with wildly hyped media stories suggesting that donuts and bacon were somehow "safer" than tilapia. This was sheer nonsense. It's true that tilapia, a lean fish, provides virtually none of the anti-inflammatory omega-3 fatty acids commonly referred to as "fish oil" and does provide more omega-6 fat, which is pro-inflammatory. But that's not because tilapia is an important source of omega-6, but rather because it contains very little fat of any kind.
Tilapia is almost all lean protein, and again, may confer net health benefits by displacing less nutritious protein sources from the diet. The bottom line here is that if you want specific health benefits from adding fish to your diet, thenfish such as salmon, rich in omega-3s, are the best choices. But if you are inclined to include lean fish such as tilapia in lieu of meat, there is no reason not to do so.
When you do eat fish, there will be contaminants along for the ride as there will be with other foods. This is unfortunate but unavoidable. The sad fact is, we have done harm to our planet, and pristine air, soil and water are all but nonexistent now. On earth, perfect is clearly the enemy of edible good. If you want truly pristine food, it will have to come from another planet. Good luck with that. On this planet, including fish in an omnivorous diet is apt to confer net benefit.
Please note that I referred specifically to the inclusion or exclusion of fish from omnivorous diets. That's an important qualification. Generally, studies that have found benefits from eating fish have used prevailing, mixed diets as the basis of comparison. I am aware of no studies that have compared long-term health outcomes in vegetarians or vegans with balanced diets to counterparts who include fish. To my knowledge, there is no clear evidence of benefit from adding fish to a well-practiced vegetarian diet. In such diets, omega-3 is derived from plant sources such as walnuts and flaxseeds, and our bodies convert some of that alpha-linolenic acid into the kinds of omega-3 derived directly from fish.
So, if you are vegetarian, I see no reason to add fish to your diet. Keep on keeping on – just practice good vegetarianism. If, however, you eat a mixed diet, the weight of evidence clearly tips the scales in favor of including fish. Just go easy on the salt.
David L. Katz, MD, MPH, FACPM, FACP, is a specialist in internal medicine and preventive medicine, with particular expertise in nutrition, weight management, and chronic-disease prevention. He is the founding director of Yale University's Prevention Research Center, and principal inventor of the NuVal nutrition guidance system. Katz was named editor-in-chief of Childhood Obesity in 2011 and is president-elect of the American College of Lifestyle Medicine. He is the author of "Disease Proof: The Remarkable Truth About What Makes Us Well."