Correlation Versus Causation

According to a study of the nearly 140,000 women who were enrolled in the Women’s Health Initiative, women who breast-fed their babies had a lower risk of developing heart disease and diabetes later in life than women who did not breast-feed their babies. The headline of the New York Times article about the research read “Breast-Feeding Benefits Mothers, Study Finds”. Is this a correct summary of the research?

The answer is “NO”! This is a classic case of the common misunderstanding about the relationship between correlation and causation. Yes, there is a clear correlation between breast-feeding and a lower risk of diabetes and heart disease later in life, according to the study. But that is not proof that the act of breast-feeding is what reduces the risk. What if women who breast-fed their children are just more health conscious overall throughout life? What if they exercise more often, or have healthier diets?

A more correct headline would be “Breast-Feeding May Benefit Mothers, Study Suggests”. Indeed, the article itself goes on to say that some experts are cautioning that an association (between breast-feeding and health benefits) does not prove a causal relationship, and that more research would be needed to determine the exact cause of the effect (lower risk).

Access to the "Morning-After" Pill

The F.D.A. is lowering the age limit for obtaining Plan B, the “morning-after” contraceptive pill. Soon anyone over the age of 17 will be able to purchase Plan B over the counter at pharmacies and health clinics, simply by providing proof of age.

News articles on the upcoming change appeared in most major papers yesterday, including the Los Angeles Times.

Boosting Cardiac Repair Mechanisms

Conventional wisdom holds that a damaged heart cannot repair itself after a heart attack because the heart does not have the ability to produce new cardiac muscle cells throughout life. But now scientists have demonstrated that cardiac muscle cells are being replaced throughout life, though at a very slow rate - only about 1% of the heart muscle cells are replaced each year in young adults. The rate falls gradually to about half a percent per year by age 75. Over a lifetime, though, about 45% of the cardiac muscle cells present at birth will have been replaced.

One percent per year is not fast enough for the heart to repair itself under natural conditions after a heart attack. But the fact that it occurs at all is giving researchers new hope. If future research were to improve our understanding of how cardiac muscle cell replacement is regulated, perhaps new drugs or treatments could be developed that would jump-start the process after a heart attack.

It could be decades before any patients are actually helped by the new findings, but that’s the way science goes…..a little breakthrough here, a little breakthrough there, and pretty soon there’s real progress!

Global Warming Tipping Points?

One of the hottest controversies among global-warming watchers these days is whether or not global warming will ever pass a “tipping point”, setting off rapid, unstoppable, and catastrophic climate change. Do climate tipping points even exist? Or will climate change just continue slowly and inexorably for hundreds or perhaps even thousands of years?

No one knows for sure. But well-meaning climate scientists who believe in tipping points warn of doomsday scenarios in an effort to get people to do something about global warming before it is too late. You’ve heard it all before - the Amazon rainforest will become grasslands; the polar ice caps will melt; the seas will rise several meters, flooding most of Earth’s populated areas; ecosystems will be disrupted. Some climate scientists worry that talk of tipping points could backfire. If a tipping point doesn’t happen within the next couple of decades, will people quit worrying about global warming altogether and decide it’s not worth doing anything about? Yawn! I’m bored with this whole subject…….

The best climate models can’t tell us yet whether or not there are tipping points. And therein lies the danger - there may actually be tipping points. Perhaps more likely is that lots of little tipping points in Earth’s complex climate system will add up to acceleration of global warming the higher the temperatures get.

For a recent news article on the subject, see “Among Climate Scientists, a Dispute Over ‘Tipping Points’”, The New York Times, Mar. 29, 2009.

A DNA test for Cervical Cancer

It was bound to happen eventually. The annual Pap smear, the gold standard for over 40 years for detecting cervical cancer, may soon be replaced by a modern, much more specific DNA test for the human papillomavirus that is responsible for most cervical cancers. The cost of the test is currently around $20-30, but the test is so accurate and specific that women may only need the test once every 5-10 years.

A comparative study of the effectiveness of the new DNA test (HPV test) versus the Pap smear (cytologic test) in preventing cervical cancer deaths was conducted in India with funds provided by the Bill and Belinda Gates Foundation. After just eight years, the study showed that the DNA test reduced cervical cancer deaths by nearly 50% compared to the Pap test. Significantly, not one woman whose DNA test for HPV was negative died of cervical cancer during the study period.

Birth Dating Human Cells

How can scientists determine the age of human cells? How frequently are human cells replaced, if at all?

In 2005 scientists hit upon an ingenious method that takes advantage of a dark period in recent world history - the above-ground testing of nuclear weapons between the mid-1950s and 1963. Nuclear weapons testing resulted in a sharp spike in carbon-14 levels worldwide. The levels peaked in 1967 and have since declined as carbon-14 diffused and equilibrated with the atmosphere, the oceans, and the biosphere. Carbon is incorporated into the chemical components of all new cells, of course, including DNA. It turns out that the carbon-14 levels in nuclear DNA correspond very closely to the atmospheric levels at the time the DNA was synthesized. So by comparing the cells’ nuclear DNA carbon-14 levels to a chart of atmospheric cabon-14 levels each year, one can determine the cells’ birth date.

How does this help us determine cell turnover? Think about it: if all of the cells in a piece of tissue are the same age as the individual, then cells are not being replaced throughout life. But if the average cell age is much younger than the individual, then cell turnover must be relatively high. The scientists who developed the cell-dating technique report that neurons in the cerebral cortex (the most highly developed area of the brain) do not undergo significant replacement throughout life - you’re born with all the cortical brainpower you’re ever going to have. In contrast, cells lining the intestine are replaced frequently.

Earth's Dwindling Arctic Sea Ice

A news article in Science last week (Science Mar. 27, p. 1655) suggests that polar ice may disappear entirely from the Arctic Ocean during the summer in less than 30 years. According to the article, scientists narrowed 23 different climate models down to the six that best fit the data for the ups and downs of sea ice from winter to summer in the past. They then used the models to predict when the summer ice would disappear completely from the Arctic Ocean. The best estimate is somewhere around 2037.

You may live to see polar bears become nearly extinct in the wild. On the bright side (if there is one), marine shipping could occur between the Atlantic and Pacific Oceans through the Arctic Ocean during the summer months.