As you age, you gradually lose muscle mass and gain visceral body fat, a type of fat deep inside your body that surrounds your heart, kidneys and other organs. Now, scientists say the ratio of visceral fat to muscle can uncover clues about your brain health.
People with higher muscle mass and a lower visceral fat-to-muscle ratio tend to have younger brains, according to research being presented next week at the annual meeting of the Radiological Society of North America.
“We know that the age of a person, in terms of their appearance, might not match with their chronological age,” said senior study author Dr. Cyrus Raji. “Turns out, the age of their organs might not match their chronological age either.”
One reason the findings are so important is because chronological age — and therefore an aging brain — is “by far” the top risk factor for developing Alzheimer’s disease and other forms of dementia, said Raji, an associate professor of radiology and neurology at the Mallinckrodt Institute of Radiology at the Washington University School of Medicine in St. Louis.
“Risk of disease crosses organ systems,” Raji said. “Disease doesn’t just neatly respect the anatomical boundaries of one organ system.”
Previous research, including Raji’s own, has studied the relationship between visceral fat and health outcomes such as brain volume loss, cognitive impairment and structural changes in the brain.
The study involved 1,164 healthy people whose average chronological age was 55.17. About 52% were women and 39% were nonwhite. All underwent a whole-body MRI that looked at brain, fat and muscle tissue.
To determine participants’ brain age, Raji and his colleagues used an algorithm that had been trained on the MRI scans of 5,500 healthy adults ages 18 to 89. The average brain age of study participants was 56.04 — older than their average chronological age. Researchers called this difference the “brain age gap.”
The average brain age gap was 0.69 years, meaning participants’ brains looked slightly older than they should. However, this metric was not statistically significant.
Just as higher muscle mass and a lower visceral fat-to-muscle ratio corresponded to a younger brain age, lower muscle mass and a higher visceral fat-to-muscle ratio corresponded to an older brain age.
This link between body fat and brain age only held true for visceral fat, which is also known as hidden fat, active fat or deep belly fat. Subcutaneous fat, the layer of fat just below the skin’s surface, wasn’t associated with brain age, the study found.
“Visceral fat, which is the really terrible fat, [is] linked to a higher rate of diabetes, insulin resistance, prediabetic states, high cholesterol,” Raji said. “That leads to a higher inflammatory state in the body, which over time affects the brain. That’s the main mechanism by which we believe obesity can affect risk for Alzheimer’s disease.”
BMI reveals little of body composition
From a clinical perspective, the study results are on par with what Dr. Zhenqi Liu, the James M. Moss professor of diabetes at the University of Virginia School of Medicine, would expect.
“When people are very young and healthy, they tend to have more muscle mass and they will have [a] younger brain age,” said Liu, who wasn’t involved in the research.
Older adults, on the other hand, especially those living with chronic diseases such as diabetes or obesity, usually have lower muscle mass, Liu added. “I wouldn’t be surprised if their brain volume is decreased.”
Even so, the study reinforces that muscle health is vital to overall health, Liu said.
The limitations of body mass index, or BMI, as a measure of health are also on display in this research, Liu said. BMI is a standardized, nearly 200-year-old body fat metric calculated using your height and weight. But it’s fallen out of favor in recent years, namely because it doesn’t take into account how your fat — neither visceral nor subcutaneous — is distributed throughout your body.
A BMI of 30 or higher falls into the obese category. For Raji, his latest work demonstrates how a low BMI could disguise poor brain health.
“If you have more visceral fat and less muscle, you can have an older-looking brain,” Raji said. “That makes sense in terms of Alzheimer’s being a big risk factor — regardless of whatever the BMI would show.”
Liu favors two other approaches to assessing body composition. The first is waist circumference. Measurements beyond 35 inches for women or 40 inches for men increase your risk of heart disease and Type 2 diabetes, according to the National Heart, Lung, and Blood Institute. The second metric is the waist-hip ratio, which involves dividing your waist circumference by your hip circumference. According to the World Health Organization, this ratio shouldn’t exceed 0.85 for women or 0.9 for men.
Still, you can’t accurately measure your own visceral fat unless your doctor refers you for an MRI, which can assess the amount of fat under the skin and surrounding the organs. An elective full-body scan can cost up to $5,000.
The good news is, there are steps you can take to increase muscle mass and lower visceral fat without spending a dime.
How to build muscle and burn visceral fat
If it’s been ages since you’ve touched a dumbbell or gone for a long walk, fear not, said Siddhartha Angadi, a colleague of Liu’s and an associate professor of kinesiology at UVA’s School of Education and Human Development.
“Anyone can exercise — regardless of their age,” said Angadi, who wasn’t involved in the research. “There’s excellent data across the lifespan, across the health span, across multiple diseases showing that exercise is incredibly safe.”
Angadi recommends following the American College of Sports Medicine’s physical activity guidelines. At least twice a week, healthy adults 65 and younger are encouraged to do muscle-strengthening activities that work all major muscle groups.
“Do 10 to 15 different exercises,” Angadi said. “You want to do one to three sets; in each set, you want to have eight to 12 reps. Use machines if you’re not familiar with resistance training, which is a fancy way of saying weightlifting.”
Aerobic exercise is particularly effective at targeting visceral fat, Angadi said. The guidelines suggest 150 minutes of moderately intense aerobic activity every week.
“There’s nothing that can reverse brain aging,” Angadi said. “You can just slow the rate of it.”
Glenn Gaesser, a professor of exercise physiology at the Arizona State University College of Health Solutions, said baby steps can help build muscle and burn visceral fat.
“A lot of people think that it takes a lot to produce some sort of health benefit from exercise, and that is not necessarily true,” said Gaesser, who wasn’t part of the study. “The biggest return on investment is with the initial few minutes of exercise.”
For example, if you’re striving for the recommended 150 minutes of aerobics per week, your body will benefit most in those first 30 minutes, Gaesser said.
“Same with resistance training,” Gaesser said. “A lot of people may not want a gym membership, they may not want to go and lift weights, but you can do resistance training even with the weight of your own body.”
While Raji’s research doesn’t conclude that higher muscle mass and a lower visceral fat-to-muscle ratio guarantee better brain health, it spotlights the relationship between the brain and the musculoskeletal system, Gaesser said. When you work out, your muscles release chemical signals thought to positively influence the brain and other tissues.
“This is why individuals who perform regular exercise tend to have lower risk of cognitive decline, dementia, Alzheimer’s and the like,” Gaesser said. “If you want a healthy brain, you need to have healthy muscle.”
Gaesser added, “It’s not surprising that the single best predictor of whether or not you’re going to spend the last years of your life in a nursing home or assisted living is fitness.”
Strength training crucial for healthy weight loss
In October, Michael Snyder turned 70, an age when muscle mass and strength are naturally in decline. He’s also taking a GLP-1 medication to help maintain a healthy weight.
As GLP-1 usage has surged in recent years, so too has awareness of the decline in muscle mass that can accompany significant weight loss, said Snyder, the Stanford W. Ascherman professor of genetics at the Stanford University School of Medicine, who wasn’t part of the study.
“If you’re on [GLP-1s], you should be strength training,” Snyder said. “I lift weights every day.”
Dr. David D’Alessio, chief of the division of endocrinology, metabolism and nutrition at the Duke University School of Medicine, stressed that muscle loss isn’t unique to people taking GLP-1s.
“If you lose weight by restricting calories — that is, going on a diet — or if you lose weight by bariatric surgery, or if you lose weight by taking Ozempic, you’re going to lose some fat mass and some lean mass,” said D’Alessio, who wasn’t involved in the study. “It’s going to be about 30% lean, 70% fat. About half of lean mass on these measures is muscle.”
Even for people who aren’t trying to lose weight, strong muscles are necessary for a long, healthy life, Snyder said.
“There’s a ton of interest in longevity these days,” Snyder said. “Everybody wants to live forever, and to do that, you’re going to want to keep your mass up.”