For yeast life span, calorie restriction may be a wash

Decades of research have shown that severely limiting calorie intake can lengthen an animal’s life span. But for yeast, consuming fewer calories doesn’t always mean a longer life, a new study suggests.

The study uses a new technique that can keep track of thousands of yeast cells at once. Caloric restriction’s famed life extension disappeared in the high-volume experiment. But other researchers who study aging say caloric restriction works as advertised; it’s the new method that’s flawed.

Yeast is just one of the organisms, including mice, dogs and nematodes, for which caloric restriction seems to slow aging and prolong life. But as researchers delve into the mechanism behind caloric restriction, some are beginning to question whether cutting calories alone can really lengthen life span. Some recent studies have suggested that it’s the composition of the diet, not lower calories, that count for longevity (SN: 4/5/14, p. 6). Mixed results from studies of primates suggest that caloric restriction may make monkeys healthier but doesn’t prolong their lives (SN: 8/1/09, p. 8; SN: 10/6/12, p. 8). Those studies call into question whether caloric restriction will work for humans.

Researchers in Matthias Heinemann’s laboratory at the University of Groningen in the Netherlands used two different methods to examine the effect of caloric restriction on baker’s yeast, Saccharomyces cerevisiae. The single-celled fungus is favored for laboratory research because it allows scientists to easily study biological processes that also happen in human cells. The team first used the tried-and-true method of growing yeast in Petri dishes and painstakingly counting how many times an individual yeast cell produced a daughter cell. The researchers also used the new technique, called microfluidic dissection, in which large numbers of yeast are grown in a chamber bathed with liquid food containing a constant concentration of nutrients.

For yeast, reproduction is life, so both methods measure a yeast cell’s life span by the number of progeny it generates, rather than how long it remains metabolically active.

In Petri dishes, yeast grown on gel containing 0.5 percent of the sugar glucose produced, on average, about four more daughters than yeast grown with 2 percent glucose — a life span extension of about 15 percent. But when the team tried the same experiment in the microfluidics chamber, there was no difference in life span, the researchers report July 28 in the Proceedings of the National Academy of Sciences. Yeast bathed in both concentrations of glucose produced an average of about 26 daughters, or buds.

Life span extension “definitely exists with the classical method, and it definitely does not exist with the microfluidics method,” Heinemann says. “We now need to understand what is different.”

Heinemann has no explanation for the discrepancy but thinks there may be something that allows yeast to live longer on solid surfaces than they do in liquid cultures. Other experts on caloric restriction offered up other possible explanations. One is that the yeast may essentially drown in the constant liquid bath. Without enough oxygen, the yeast may be unable to produce the metabolic changes scientists think are necessary to prolong life, say MIT molecular biologist Leonard Guarente, whose lab pioneered the study of caloric restriction in yeast. But Heinemann says he’s confident the fungi are getting enough oxygen.

It’s also possible that microfluidic dissection is simply not a good way to measure the effect of caloric restriction, says Rozalyn Anderson, a researcher at the University of Wisconsin School of Medicine and Public Health in Madison who studies aging and caloric restriction. The age-delaying action of caloric restriction may stunt the growth of yeast cells so that the mother cells and daughter cells are easily flushed out of the chamber, Anderson speculates.

Hao Li, a systems and computational biologist at the University of California, San Francisco, has grown yeast in a slightly different microfluidics chamber. “In our hands, we see a robust life span extension,” he says. Li’s results aren’t yet published. He’s spotted a few small differences between the protocols his lab uses and Heinemann’s methods. Those tiny discrepancies could make all the difference in a very sensitive system, Li says. “It’s a subtle issue.”

Molecular biologist Matt Kaeberlein of the University of Washington in Seattle says the results are in line with work from his lab showing that slight differences in growth conditions, the genetic makeup of the yeast or other factors can change the outcome of the experiment. “It’s not particularly surprising, nor is it worrisome from my perspective,” Kaeberlein says.