Keith Roach, M.D., syndicated columnist

DEAR DR. ROACH: Since I suffer from age-associated cognitive impairment, I read with interest an advertisement that claims that this condition may be caused by fish oil, which I have been taking regularly for many years.

That ad says that “a highly-acclaimed MD” states that today’s low-grade supermarket fish oil is causing the surge in America’s memory crisis. It claims that the brain needs DHA, but over the years, the DHA content in fish oil has plummeted and is being replaced with ALA.

This is being attributed to fish oil being produced by farmed, instead of wild, fish and that their formula will restore brain function. I’m desperate to try anything, but don’t want to substitute snake oil for fish oil. Is there any truth to that claim, or is this merely advertising fluff? — W.P.

ANSWER: It seems a bit of a stretch to say that dementia is being caused by fish oil supplements. The data on fish oil and prevention and treatment of dementia consists of two types of studies: observational studies and interventional studies.

Some of the first, observational, have shown that people who eat more fish or take in fish oil by supplements have a lower rate of developing dementia.

However, not all studies have shown this association; others suggested that only people with a genetic predisposition will benefit from fish oil.

Interventional studies, where one group is given fish oil and another is given a placebo, generally provide stronger evidence for benefit.

Unfortunately, five studies that looked at this possibility were unable to find a benefit from taking fish oil to reduce the risk of dementia.

This may be due to the trials being relatively small (although one followed over 4,000 people for five years), because the effect is too small to find or possibly because they used the wrong type of fish oil.

It is possible that a different fish oil preparation would have been effective.

I don’t recommend fish oil supplements for prevention or treatment of cognitive impairment or dementia. A better study showing that it is effective would change my mind. But especially for prevention, the evidence must be strong, and it just isn’t adequate right now.

DEAR DR. ROACH: Many of my friends and I are considering knee replacement surgery, and in my case, want to delay if possible. Are the nonembryo stem cell treatments effective in delaying surgery? These stem cell treatments use our own body fat to turn into fibroblasts, and these may repair our tendons and ligaments. Can we hope that these treatments work? — P.M.

ANSWER: We can certainly hope that these therapies work, but so far, the evidence of their safety and effectiveness is not yet at the point where I can recommend it.

The theory is that the cells, which are commonly obtained from a person’s own bone marrow, can help regrow a person’s own cartilage (not so much tendons and ligaments, just the cartilage that lines and protects the bone). That can delay or even prevent the need for a knee replacement.

The results so far have shown some benefit, but several reviews of all published studies have shown that the evidence is not high-quality. The Food and Drug Administration considers this an experimental treatment, meaning nearly all insurers won’t pay for it, and it is very expensive.

Many people get knee replacements. The most common regret I have heard is that they wished they hadn’t waited so long. More than 80% of knee replacements last 25 years.

Editor’s note: Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or send mail to 628 Virginia Dr., Orlando, FL 32803.