The human body does not age at a constant rate throughout adulthood — instead, it accelerates dramatically around ages 44 and 60, a new study finds.
The new research, published Aug. 14 in the journal Nature Aging,involved measuring more than 11,000 molecules in the adult body over time, and it revealed that 81% of them undergo dramatic changes at these two ages.
This type of aging research focuses on tracking "biological age," which refers to changes that occur in the body over a lifetime, affecting proteins, metabolites and gene activity. This concept is distinct from the "chronological age" that people celebrate each year on their birthdays.
Finding that biological aging accelerates at two points in midlife could help researchers understand why the risk of certain illnesses increases in fits and starts as chronological age rises. For example, approximately 6.5% of people ages 40 to 59 have coronary artery disease, but the prevalence rises sharply to 19.8% in people ages 60 to 79.
Related: Natural rates of aging are fixed, study suggests
For the study, researchers at Stanford University recruited 108 participants that were of diverse ethnic backgrounds and ranged from 25 to 75 years old. Every three to six months for several years — up to about seven years in total — the scientists collected blood samples from the participants to assess how different factors, such as gene activity and blood sugar levels, varied over time.
Many of the factors that shifted around age 44 and 60 were related to heart health. For example, a protein linked to atherosclerosis, or plaque buildup in arteries, increased in the blood of participants during their 40s and 60s. These age groups also showed declines in the ability to metabolize caffeine, which temporarily raises blood pressure, and alcohol, which initially lowers but then raises blood pressure.
The body's pathway for making unsaturated fatty acids, which lower "bad" cholesterol, also waned at these two ages.
Although the study's multiple links to cardiovascular health were only correlative, they point to potential reasons why heart disease becomes more common with age.
Aside from heart health, blood sugar levels peaked in participants in their 40s and 60s, suggesting a possible link to age-related type 2 diabetes.
Scientists don't yet know why body chemistry changes considerably at these ages, and the study didn’t account for the role lifestyle factors, such as diet or exercise, might play.
Juan Carlos Verján, who researches aging at the National Institute of Geriatrics in Mexico and was not involved with the study, told Live Science that "the 60-year inflection point, I believe, could be more due to inflammation." For instance, participants over 60 accumulated antioxidant enzymes in their blood. These enzymes neutralize chemical triggers for inflammation and suggests that inflammation could be accumulating in this age group
The aging boost at age 44 also coincides with the time some women start going through perimenopause. However, "we found the same trigger timepoints for women and men," suggesting sex-specific hormonal changes are not responsible for the aging boosts, said study co-author Xiaotao Shen, a computational biologist who's now at Nanyang Technology University in Singapore. Therefore, "there should be another reason to cause the same change in men and women." What that shared factor is remains a mystery.
The study was limited in that the participants ranged from ages 25 to 75, so the researchers could not assess sizable shifts that occur at other key moments in life, such as during puberty or at very advanced ages. The small sample of 108 participants from California was another limitation because the group is unlikely to represent all humans globally.
Generally speaking, "people in California have a lot of years of healthy life," Verján said. He proposed that the researchers could also explore aging in places where people have shorter lifespans, on average.
The team focused on changes to molecules in the blood, but this does not necessarily reflect all the organs in the body. "There are several papers that mention that aging is tissue-related," Verján said, rather than exclusively related to factors in blood. For example, one publication found that, in some people, the heart ages the fastest, whereas for others, it's the kidneys.
Shen's team discovered a number of changes that correlate with the timing of age-linked diseases, but they still need to confirm a causal tie to these factors. In other words, do the changes seen in blood actually drive disease, or are they more of a byproduct of the aging process?
"Animal experiments are a good option for us to study the reasons why there are two peaks for aging," Shen said. Verján speculated that epigenetic changes, which modify the activity of genes without altering their underlying code, might drive these dramatic shifts.
Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!
