Readers discuss marathoners’ myelin, menopausal chimps and more

Running on myelin

Marathon runners’ myelin levels in the brain temporarily dwindled in the days after a race. The finding suggests that this fatty tissue that insulates nerve cells may be an energy source for the athletes, Meghan Rosen reported in “Brain tissue may help fuel marathoners” (SN: 11/18/23, p. 16).

Reader Julian Young wondered whether myelin levels could also drop in people on a restrictive diet such as the low-carb keto diet or in people who experience low blood sugar levels for long periods.

It’s unknown whether ketogenic diets affect myelin levels. Such diets “generally don’t cause clinically relevant hypoglycemia,” so enough glucose remains in the body to fuel brain cells, says neurologist Russell Swerdlow of the University of Kansas School of Medicine in Kansas City. However, extreme calorie restriction, such as that experienced by people with anorexia nervosa, can decrease myelin levels in the brain, says Carlos Matute, a neurobiologist at the Achucarro Basque Center for Neuroscience and the University of the Basque Country in Leioa, Spain.

Reader Kimberly Barden asked whether the marathoners’ myelin loss might be connected to the phenomenon known as runner’s high.

Runner’s high has no obvious relationship with myelin loss, Matute says. The phenomenon “is associated with the release of certain neurotransmitters and endorphins in the brain. This feature activates reward brain circuits that generate a state of euphoria and well-being,” he says.

Beyond reproduction

Chimpanzees in Uganda are the first known example of nonhuman primates in the wild to experience menopause and live well past their reproductive years. The finding raises questions about how the hormonal changes evolved, Bruce Bower reported in “Menopause affects wild chimps too” (SN: 11/18/23, p. 8).

In humans, the ovaries typically stop releasing eggs during menopause. Reader Gerry Beard wanted to know whether the same is true for the chimps. Do the animals also go through symptoms such as hot flashes and mood swings?

Chimp ovaries also stop releasing eggs during menopause, says UCLA evolutionary anthropologist Brian Wood. It’s unknown whether the Ugandan chimps experienced hot flashes or mood swings. But the primates had low levels of estrogen and progestin, as well as high levels of follicle-stimulating hormone and luteinizing hormone — physiological changes that are associated with various menopausal symptoms in humans, Wood says.

Stem cell curiosities

A newfound type of stem cell in the spine secretes a protein that attracts cancer cells. The discovery might explain why cancers that spread to other body parts, such as breast cancer, preferentially target the spine, Meghan Rosen reported in “Spinal stem cells lure breast cancer” (SN: 11/18/23, p. 14).

An anonymous reader asked why this biochemical signaling exists.

It’s unclear why these stem cells in the spine are better at attracting cancer cells than stem cells in other parts of the skeleton, like the limbs, says pathologist Matthew Greenblatt of Weill Cornell Medicine in New York City. Presumably, the spine stem cells produce the tumor-attracting protein, called MFGE8, at high levels for reasons other than to attract cancer cells, he adds.

It is likely that the protein has multiple functions in the spine, Greenblatt says. “We do know that MFGE8 acts on skeletal cells to regulate bone mass, so perhaps the effect of MFGE8 to attract tumor cells reflects a co-option of this pathway,” he speculates.

“From another perspective, we know that the spine is very good at retaining blood-forming stem cells, even when chemotherapy or other factors deplete [the cells] at other skeletal sites,” Greenblatt says. It’s possible that the pathways that allow the spine to be a haven for blood cell formation are somehow co-opted by cancer cells.

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