Subtle increases in blood glucose – below the threshold for diabetes – may silently harm the hearts of young adults, new research shows. Despite appearing healthy, some adolescents already show early signs of cardiac strain linked to elevated glucose. The findings suggest that routine screening of adolescents and young adults, especially girls, could help to prevent future heart disease by catching these hidden risks before symptoms emerge.
Scientists know a lot about two blood glucose territories: the normal range and levels high enough to be diagnosed with type 2 diabetes. But between the two lies a borderland – slightly elevated blood glucose that does not necessarily ring alarm bells for healthcare providers if it does not exceed locally established thresholds for prediabetes. New research, published in Diabetes Care, suggests that changes to the heart associated with diabetes are already well in progress among adolescents with slightly elevated blood glucose – even those who appear perfectly healthy.
Only screening adolescents can detect these invisible cases of high blood glucose and insulin resistance and ward off damage to the heart, says lead and senior author Andrew Agbaje, a physician and Professor of Clinical Epidemiology at the University of Eastern Finland in Kuopio. “Waiting until the age of 40 years to do random checks of blood glucose is a deliberate decision to throw away a 20-year opportunity for prevention.”
A rare peek under the hood
Since most people do not seek care for heart problems until middle age, thinking that young adulthood is smooth sailing for the heart is tempting. But a cohort study in the United Kingdom provided Andrew Agbaje and his colleagues the opportunity to peek under the hood.
The Avon Longitudinal Study of Parents and Children (ALSPAC) birth cohort tracked the development of thousands of children born between 1991 and 1993. When the participants reached 17 and 24 years, researchers measured the blood glucose and insulin levels of 1,595 participants and took echocardiograms – ultrasound scans to map the anatomy of the heart. Seeing the heart at two separate times in young adulthood is very valuable, Andrew Agbaje says, because it enables researchers to assess the heart’s growth trajectory. “Having the heart measured when it is not part of treatment for a disease is very uncommon,” he explains.
Heart to heart
Andrew Agbaje and his colleagues aimed to learn whether young adults with blood glucose that is elevated but not quite high enough for being diagnosed with diabetes experience the heart damage associated with diabetes. “In the general population of apparently healthy people, we do not have a sense of whether elevated glucose can damage anything, even if we can see any signs before the full-blown diabetes begins.”
Indeed, different countries draw the line for worrisome blood glucose differently: some set the cut-off for prediabetes at 5.7 mmol/L, whereas others do not see cause for concern until 6.1 mmol/L.
“You can travel from one country to another and change from having prediabetes to having normal blood glucose, depending on the threshold,” Andrew Agbaje says.
The researchers examined key features of the heart’s anatomy, comparing young adults with elevated blood glucose to their peers with normal glucose.
“We wanted to see the degree of the intensity of the destructive capability of high blood glucose,” he explains. They found evidence that, even at the more conservative cutoff of 5.7 mmol/L, young people’s hearts can already be struggling.
Nearly five times worse for young women than for young men
Diabetes and insulin resistance are associated with cardiac hypertrophy, in which the muscular walls of the heart grow thicker. This can reduce how much blood the heart can hold, leading the heart to beat faster to keep the body supplied with fresh, oxygenated blood.
These hallmarks of hypertrophy were present in the hearts of adolescents with blood glucose exceeding 5.7 mmol/L. “But blood glucose of 6.1 mmol/L almost doubled the adverse effects,” Andrew Agbaje says.
Young people with blood glucose exceeding 6.1 mmol/L were three times more likely to develop hypertrophy in the heart’s left ventricle than their peers with normal blood glucose.
He emphasises that many of the adolescents with heart damage seemed healthy and would not have been flagged for concern by body-mass index (BMI), a metric Andrew Agbaje describes as deeply flawed and a “troublesome measure of obesity”. Fat mass, and not heaviness, is a proven driver of insulin resistance and high blood glucose. Meanwhile, muscle mass, which can contribute to heaviness and is lumped in with fat in the BMI, has a protective effect, hoovering up excess blood glucose.
This means that relying on the BMI can lead to unnecessary interventions among young people with healthy muscle mass and enable people with normal weight but high fat mass to fall through the cracks, he explains.
But high blood glucose has divergent effects on young people’s hearts. Andrew Agbaje and his colleagues were “shocked” to see that high blood glucose affected the heart nearly five times greater for teenage girls than for teenage boys. “The magnitude of that difference was not expected in the young population,” Andrew Agbaje says. “That is really frightening, because being an adult female over 60 years old was identified as a risk factor for diabetes-associated heart failure four decades ago.”
This does not mean that the young women in the study need to lose fat to the levels of young men, Andrew Agbaje underscores – fat mass is important for healthy menstruation and other reproductive functions, and on average, women have a higher proportion of fat mass than men and a lower proportion of protective muscle mass than men do.
An ounce of prevention
The findings demonstrate just how effective screening for blood glucose could be for young people, Andrew Agbaje says – intervening in adolescence or young adulthood could fend off heart disease that otherwise would not be detected for decades.
“Having a blood sample profile once every 10 years does not cost much,” he says. “It may detect certain people who are already on the trajectory of injury.”
Clinicians should be particularly vigilant about young women, Andrew Agbaje says. “If we find elevated blood glucose in a female teenager, we should quickly intervene – much earlier even than with the males, because we find that the progression might be more rapid for females,” he explains.
We know that the starting-point of the vicious cycle of insulin resistance is excess fat. “But we also know the solution,” Andrew Agbaje says. “We can advise people to increase their muscle mass – go to the gym and build muscle, and that muscle will protect you, pulling glucose from the blood and burning it for energy.”
