(The Hill) — The human body doesn’t age steadily throughout middle age and, instead, goes through bursts of rapid aging typically at around age 44 and again at 60, according to a new study published Wednesday in the academic journal Nature Aging.  

Stanford University researchers tracked age-related changes in more than 135,000 types of molecules and microbes in samples collected every three to six months from more than 100 adults between the ages of 25 and 75 years old.   

Researchers gathered more than 5,400 blood, stool, skin nasal swab and oral swabs as part of the study, and as a result were able to track more than 135,000 different kinds of chemical compounds, bacteria and cell parts affected by aging.  

Scientists found the abundance of these molecules and microbes did not change steadily over time but rather increased and decreased rapidly around two ages — first around the onset of a person’s 40s and again in their 60s.  

While researchers found evidence that cellular changes are more likely to occur around these ages, more work needs to be done to figure out why.  

“When people become old, the molecules in your body change,” Xiaotao Shen, a computational biologist at Singapore’s Nanyang Technology University and co-author of the study, told The Washington Post. “What we don’t know is what drives this change.” 

The findings could help researchers better understand age-related illnesses and why certain diseases such as cardiovascular and neurodegenerative diseases commonly pop up at certain ages, specifically at around age 40 and 65, respectively, researchers wrote in the study.