The dish on fish

Thursday, 02-01-2003

The topic has spawned plenty of research. We recently did a quick computer search of the medical literature for fish-consumption studies. Within minutes we found research papers on stroke in American women, prostate cancer in Swedish men, Alzheimer’s disease in French seniors, and leptin (an appetite hormone) levels in Tanzania. Not surprisingly, all came out swimmingly for the fish eaters.

Farm vs. wild

The glowing health reports have whet the American appetite for fish, and the millions of pounds of farm-raised fish produced each year help meet that demand. In addition to farm-raised catfish, salmon, and trout, we now have tilapia, striped bass, sturgeon, and walleye on the menu and at the store. In Australia, they’ve started tuna "ranching" — catching the fish in large nets and herding them into pens for several months of feeding.

Dilemmas abound. Farming fish makes a healthy food less expensive for consumers. The added supply almost certainly eases overfishing of dwindling stocks of some species. But some environmental groups are critical, especially of salmon operations on the West Coast, and want consumers to boycott farm-raised salmon. They say the "floating feedlots" harm fragile marine environments. There’s also an argument that raising carnivorous fish like salmon is wasteful of natural resources because it takes several pounds of wild fish like herring or anchovy to produce a pound of salmon. The industry says it has responded by cutting back on antibiotics, switching to low-phosphorous feeds that make fish waste less polluting, and experimenting with soy and other vegetable-based feeds. If you want to learn more about these environmental issues, visit our Web site at http://www.health.harvard.edu/article.cfm?id=137.

Nutritional issues

Coddled and cooped up, farm fish tend to be anywhere from two to five times fattier overall than wild fish, although the fat content of wild fish varies tremendously depending on the season and where the creature is in its reproductive cycle. That extra fat means more calories. But fattier (oilier) fish also tend to have more of the omega-3 fats that are the main reason fish is such a healthy food. A meal of an oily fish like bluefish will give you twice as many omega-3s as a like-sized serving of halibut, and four times as many as farmed catfish.

What is it about fish?

When you eat carbohydrates (sugar or starch) or protein, your body shows little respect for the artistry of those molecules. It tears them apart and reassembles them to suit its own purposes. Carbohydrates and protein — they’re just fodder.

But it’s different with fat. Some gets roughed up during digestion and metabolism. But some gets through more or less intact, becomes part of our cell membranes, and thus has considerable say-so over how cells behave. We are the fat that we eat.

Fish is a special food because it contains two important varieties of long-chain omega-3 fats that you won’t find anywhere else in a conventional diet. Long-chain refers to the number of carbon atoms, omega-3 to a position of a certain chemical bond that puts a 45-degree kink in that chain. Both attributes determine how a fat molecule is going to fit into cell membranes and what it’s going to do once it gets there.

As it turns out, long-chain omega-3 fats in fish are just the sort of fat molecules that any healthy cell should gladly welcome into its membranes. One of them, eicosapentaenoic acid, manages to displace molecules that would otherwise give rise to active prostaglandins, leukotrienes, and other inflammatory compounds. And inflammation seems to be a root cause of many diseases. Eicosapentaenoic acid also seems to be the omega-3 with the most pronounced cardiac benefits.

The other main omega-3 in fish is docosahexaenoic acid (DHA). It’s important to brain and vision development in infants and is added to infant formulas.

Sometimes there’s some confusion about where the alpha-linolenic acid in walnuts, flaxseed oil, and soy products fits in. It’s also an omega-3 fat, but has fewer carbon atoms and therefore isn’t a long-chain omega-3. Being shortchanged those few carbon atoms makes a difference because alpha-linolenic acid doesn’t have as many health benefits as the more carbon-blessed omega-3s in fish.

So farm-raised fish — simply because they’re fattier — tend to have more omega-3s than wild fish. But actual comparisons become complicated. Both the amount and type of fat in farmed fish depend on their feed, particularly the type of oil (fat) it contains.

When we looked up the omega-3 content of farmed and wild Atlantic salmon in a nutritional database compiled by the United States Department of Agriculture (USDA), they were the same. But wild Atlantic salmon is scarce and not commercially available very often. A more realistic comparison is farmed Atlantic with other wild salmon species. And according to the USDA database, wild coho salmon, for example, contains half the amount of omega-3s as farmed Atlantic salmon.

Researchers at Oregon Health & Science University have made their own comparisons. So far, their tests haven't shown any difference in the omega-3 content of farmed and wild salmon, according to Dr. William E. Connor, one of the researchers. But when they tested catfish, the omega-3 content of the wild fish was much higher than the farmed.

Fish feed

Fish feeds vary tremendously with the species. There is also continual experimentation with, for example, different sorts of enzymes to make the fish metabolize feed more efficiently and thus grow faster. British scientists announced last year that they had successfully added pheromones to feed to make it more appetizing. Red coloring in the form of synthetic carotenoids is added to salmon feed to give the flesh that rosy color that consumers have come to expect.

For consumers, the oil content of the feed is a key issue because it influences omega-3 levels. Currently, most of the oil used for fish feed comes from small fish like herring and menhaden — and it’s rich in omega-3s. But the industry is worried about dwindling supplies and rising costs and thus interested in plant-based alternatives. Researchers at the University of Stirling in Scotland have published several studies showing that replacing fish oil with plant-derived substitutes is feasible, but, not surprisingly, a high proportion of plant oil significantly reduces the omega-3 content of salmon.

Some experts we talked to said feed makers are more likely to switch from fish to vegetable (soy) sources of protein, not fat. For one thing, some species — notably salmon, trout, and steelhead — need omega-3 oil to flourish. The industry also has an interest in preserving the reputation of fish as a healthy food, which means keeping the omega-3 levels as high as possible.

As for farm vs. wild taste, we defer to the palate of Roger Berkowitz, CEO of Legal Sea Foods, a chain of seafood restaurants based in Boston. He says that wild fish, especially salmon, has a gamier, more intense flavor. It’s also more expensive. Berkowitz says farm-raised flounder has foundered because of poor taste and texture.

Mercury contamination

But an even bigger worry these days is that the fish we’re urged to eat for health may contain some very unhealthy contaminants, particularly mercury. Most research suggests that if the mercury in fish causes harm, the danger is primarily to the developing nervous systems of children, although studies have suggested a link between mercury and the atherosclerosis that underlies heart disease. Last spring, the FDA advised pregnant women and all women of childbearing age not to eat any shark, swordfish, king mackerel, and tilefish because of their high mercury content, and to limit consumption of all fish to 12 ounces (about two servings) per week. Harvard researchers recently published a study in the New England Journal of Medicine showing that Americans who eat more fish have higher levels of the metal in their bodies (more specifically, in their toenails), although they don’t believe the levels cause harm. No one is recommending routine mercury testing. But the contaminant does seem to pose a damned-if-you-do, damned-if-you-don’t problem for people who want to eat a lot of fish for health reasons. Mercury tends to accumulate in the food chain: the higher on the chain, the greater the concentration of mercury. But species rich in omega-3 fats also tend to be the food chain’s higher-ups, including swordfish, mackerel, and tuna.

Nutritional medicine

The dish on fish