Readers write back.

Saturday, March 4, 2006

Missing from your otherwise comprehensive article on diabetes ["The Covert Plague," December] was any mention of studies showing that chemicals can produce insulin resistance in animals at levels of exposure lower than found in most people. A recent Centers for Disease Control study found that bisphenol A was present in the urine of 95 percent of people tested. This indicates a continuous and ubiquitous exposure of the population to bisphenol A from the linings of cans, polycarbonate plastic bottles and containers, and possibly dental sealants. Interestingly, bisphenol A also produces insulin resistance in animals. Perhaps we should think beyond diet and exercise.
Barbara McElgunn, R.N.
Health Policy Adviser
Learning Disabilities Association of Canada
Toronto, Ontario

While the causes of the epidemic of obesity and type 2 diabetes remain incompletely understood, epidemiological and physiological studies repeatedly demonstrate strong associations between obesity, insulin resistance, and the development of type 2 diabetes. Multiple recent studies demonstrate an important link between diet-induced obesity and the activation of the inflammatory pathway that may underlie and link insulin resistance to incident diabetes and cardiovascular disease. Although there is much data to support the role of diet-induced obesity in impairing insulin action, there is little data to clearly support a causative role of chronic low-dose exposure to specific environmental toxins. While additional studies are warranted, it should be made clear that there is much that individuals can do to reduce their risk of developing diabetes and improve their health status by making healthy food choices and increasing physical fitness. —Allison B. Goldfine, M.D., assistant director of clinical research, Joslin Diabetes Center, and assistant professor, Harvard Medical School

The diabetes article was a good one. However, two points require clarification. First, you state that cells "throttle back the processing of glucose during lean times by becoming insulin resistant." Starvation does not cause insulin resistance. The decline in glucose uptake observed when free fatty acid concentration rises, as it does in starvation, is instead attributed either to the Randle hypothesis [which links insulin resistance to fatty acid oxidation] or to a primary effect of free fatty acids in curtailing the transportation of glucose. Increased insulin resistance would be counterproductive in terms of survival. Starvation would be expected to enhance insulin sensitivity in an effort to use any available glucose with maximal efficiency. Second, the statement "mitochondria in people predisposed to developing type 2 diabetes produce less energy, causing cells to demand less fuel" is incorrect. The cells demand more fuel. The demand for ever increasing amounts of fuel, and the insulin needed to use it, logically leads to an increased demand for both fuel and insulin, which leads to insulin resistance.
Andrew Gerenyi, M.D.
Wexford, Pennsylvania

Unfortunately, Dr. Gerenyi is wrong on both accounts. First of all, increases in plasma fatty acids during starvation cause insulin resistance in skeletal muscle and the liver. Insulin resistance in both these major insulin-responsive organs during starvation preserves glucose for the brain, which requires the most glucose of any organ in the body for normal function. A relative increase in plasma insulin concentration also preserves protein mass during starvation by inhibiting protein degradation. Second, mitochondrial energy production in skeletal muscle is reduced by 35 percent in the young, lean, insulin-resistant offspring of parents with type 2 diabetes as well as in healthy, lean, insulin-resistant elderly volunteers. Recent studies have shown that the reduction in mitochondrial activity observed in the first group is most likely due to reduced mitochondrial content in their skeletal muscle. —Gerald Shulman, M.D., Ph.D., professor of internal medicine and cellular & molecular physiology, Yale University, and investigator, Howard Hughes Medical Institute

I enjoyed the article regarding recent galaxy surveys ["New and Old Galaxies Show Up in All the Wrong Places," Year in Science, January, page 61], but I'm puzzled by one of the reported discoveries. If the youthful galaxies located by the Galex telescope are 2 billion to 4 billion light-years from Earth but started forming less than 1 billion years ago, how can they be observed at all?
Lawson Henry Lowrance
Chapel Hill, North Carolina

Your question cuts right to one of the trickiest problems in cosmology: how to refer to the timing of events when there are many different ways to describe them. The conventional solution is to describe everything from the way we perceive it. In this case, that means that when we say that the galaxies started forming less than a billion years ago, we mean that the galaxies as we see them today appear to have started forming less than a billion years ago. Put another way, when their light started heading toward Earth 2 billion to 4 billion years ago, these objects were less than a billion years old. That convention may seem confusing, but the alternatives are even more puzzling. For instance, it would be more comprehensive to say that these galaxies, located 2 billion to 4 billion light-years from Earth, appear to have begun forming less than 3 billion to 5 billion years ago, and then their light spent 2 billion to 4 billion years traveling toward us. More comprehensive, yes, but even harder to follow! —The editors


The box at the bottom of "Astronomy at the Speed of Light" [Sky Lights, December] states "Rainbows and sunsets display many hues because each of the colors that make up light travels through matter at a different speed." The explanation is correct for rainbows, but the explanation for the color of sunsets is that shorter wavelengths of light (blue hues) are scattered by molecules in the air more often than longer wavelengths (red hues). A clear sky with the sun at the zenith appears blue because more blue light is scattered into your eyes than red light. A red sky with the sun on the horizon appears red because more blue light than red has been scattered away from your eyes after traveling through more of the atmosphere. 
Louis W. Adams Jr. 
Inman, South Carolina

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