About 15 years ago, when my cholesterol level began an inexplicable climb, I tried a slew of suggested dietary remedies that included eating more fish, along with a daily supplement of fish oil, which research at the time suggested would help prevent heart attacks and strokes.

Alas, my body disdained my efforts and I finally had to take a statin. But I continued to follow a heart-healthy diet laden with vegetables and fish and took 1,200 milligrams of fish oil every night.

I now suspect that those thousands of gel-covered capsules I’ve swallowed over the years may have done little more than enrich the pockets of supplement producers and sellers. A number of extensive analyses have been conducted, some supporting and others refuting the value of fish oils to the cardiovascular system, along with studies of other purported health benefits that also have had mixed results.

The supplements contain omega-3 fatty acids, the polyunsaturated oils prominent in fatty cold water fish like salmon, sardines and mackerel. In many observational studies, people who regularly consumed fish two or more times a week were less likely to suffer heart attacks, strokes and cardiovascular deaths than those who ate fish infrequently or not at all.

Enthusiasm for fish oils originated with reports decades ago that Eskimos, who consume lots of cold water fish, have surprisingly low rates of heart disease despite a diet very high in animal fat.

“This idea has since been pretty discredited; we really don’t know if the Eskimos got heart disease or not,” said Malden C. Nesheim, emeritus professor of nutrition at Cornell University, who chaired an Institute of Medicine committee assessing the risks and benefits of seafood in the early 2000s. “I’ve been an omega-3 skeptic since doing this study.”

Nonetheless, large population studies with solid data both on the participants’ diets and causes of disease and death bolstered the beliefs that eating fish often was a heart-healthy practice linked to reduced rates of cardiovascular disease. For example, a comprehensive analysis conducted by Dr. Dariush Mozaffarian and Eric Rimm of the Harvard T.H. Chan School of Public Health found that eating two servings of fatty fish a week — equal to about two grams of omega-3 fatty acids — lowered the risk of death from heart disease by more than a third and total deaths by 17 percent.

Omega-3s in fish clearly have effects that can account for such findings. They protect against abnormal heart rhythms, lower blood pressure and heart rate and improve the function of blood vessels. They may also lower heart-damaging triglycerides and counter inflammation, a known risk to the heart.

The question is whether the observed cardiovascular benefits often found among fish eaters is due solely to the oils in fish or to some other characteristics of seafood or to still other factors common to those who eat lots of fish, like eating less meat or pursuing a healthier lifestyle over all. Whatever the answer, it does not seem to be fish oil supplements.

In the latest report, published online on Jan. 31 in JAMA Cardiology, an international team headed by Dr. Robert Clarke of the University of Oxford analyzed the combined results of 10 trials of fish oil supplements involving 77,917 older adults at high risk for cardiovascular disease.

At doses ranging from 226 milligrams to 1,800 milligrams per day of omega-3 fatty acids, no significant protection against “major vascular events” was found overall among the participants or for any subgroup, like those with prior heart disease or diabetes.

While this does not necessarily mean the supplements are unhelpful, it does suggest a more nuanced consideration of who, if anyone, may benefit from taking fish oils and whether we all might be better off simply eating more fish, even though that too can have some downsides as well as benefits. (At the moment, I’m still doing both.)

For example, large predatory fish like shark, swordfish, king mackerel, tilefish and albacore tuna can contain high levels of methyl mercury, a toxin that would override any health benefit, especially for the developing brains of fetuses and young children as well as for adults, Dr. Nesheim and Marion Nestle, professor emerita of nutrition, food studies and public health at New York University, noted in 2014 in an editorial in the American Journal of Clinical Nutrition. (Levels of mercury and other contaminants in fish have since declined somewhat but are not negligible.)

However, in both observational studies and controlled clinical trials, eating fish was shown to foster optimal development of a baby’s brain and nervous system, prompting advice that pregnant women and nursing mothers eat more fish rich in omega-3s while avoiding species that may contain mercury or other contaminants like PCBs sometimes found in freshwater fish.

Another concern is the environmental cost that could result if people ate more fish, given that “many ocean fisheries are fully exploited or are in decline,” Drs. Nesheim and Nestle wrote. “In the face of limited supplies,” they added, the price of seafood would likely be “out of the reach of many consumers.”

The declining supplies and rising costs of wild-caught fish have spawned a worldwide explosion of fish farming, which also has its downsides. For example, marine organisms used to feed farmed fish can diminish this vital food supply for wild stock, and fish that escape from farms may change the gene pool of wild fish.

However, if the cost of wild fish is a concern, farmed salmon typically has as much or more omega-3s as wild-caught salmon, which can be three times as expensive.

Currently, fewer than one person in five in this country eats fish at least twice a week, as recommended in the Dietary Guidelines for Americans and by the American Heart Association, among others. Fish, after all, are high in protein, low in saturated fat and contain beneficial nutrients like vitamin D and selenium.

Other suspected health benefits of omega-3s and fish are less well established and need further study. They include suggestions of a reduced risk of breast cancer, colorectal cancer and possibly advanced prostate cancer, all related to eating fish rather than taking supplements. Some observational studies have associated omega-3s to a lower risk of cognitive decline, Alzheimer’s disease and dementia, as well as age-related macular degeneration.

Also not firmly established is the protective value of plant sources of omega-3s: alpha-linolenic acid found in flax seeds, walnuts and some vegetable oils, which could be an option for vegetarians and people allergic to fish.

By Jane E. Brody, www.nytimes.com