In the 1920s, German chemist Otto Warburg discovered that cancer cells don’t metabolize sugar the same way that healthy cells usually do.

Since then, scientists have tried to figure out why cancer cells use this alternative pathway, which is much less efficient.

MIT biologists have now found a possible answer to this longstanding question.

In a study appearing in Molecular Cell, they showed that this metabolic pathway, known as fermentation, helps cells to regenerate large quantities of a molecule called NAD+, which they need to synthesize DNA and other important molecules.

Their findings also account for why other types of rapidly proliferating cells, such as immune cells, switch over to fermentation.

“This has really been a hundred-year-old paradox that many people have tried to explain in different ways,” says Matthew Vander Heiden, an associate professor of biology at MIT and associate director of MIT’s Koch Institute for Integrative Cancer Research.

“What we found is that under certain circumstances, cells need to do more of these electron transfer reactions, which require NAD+, in order to make molecules such as DNA.”

Fermentation is one way that cells can convert the energy found in sugar to ATP, a chemical that cells use to store energy for all of their needs.

However, mammalian cells usually break down sugar using a process called aerobic respiration, which yields much more ATP.

Cells typically switch over to fermentation only when they don’t have enough oxygen available to perform aerobic respiration.

Since Warburg’s discovery, scientists have put forth many theories for why cancer cells switch to the inefficient fermentation pathway.

Warburg originally proposed that cancer cells’ mitochondria, where aerobic respiration occurs, might be damaged, but this turned out not to be the case.

Other explanations have focused on the possible benefits of producing ATP in a different way, but none of these theories have gained widespread support.

In this study, the MIT team decided to try to come up with a solution by asking what would happen if they suppressed cancer cells’ ability to perform fermentation.