In a group of cognitively normal adults age 55 and older in Singapore, those with certain cardiovascular risk factors were more likely than others to develop mild cognitive impairment or dementia over a six year period.
The risk factors - known collectively as the metabolic syndrome - include large waist circumference, high triglyceride levels, low HDL or "good" cholesterol levels, high blood pressure and high fasting blood sugar, which together increase the risk of heart disease and other health problems.
"The results are in one sense not surprising but they provide for the first time strong evidence of the link between the metabolic syndrome and the pre-dementia syndrome (mild cognitive impairment)," said lead author Dr. Tze Pin Ng of the National University of Singapore.
"Such supporting data have long been sought, but surprisingly have eluded us, probably because virtually all investigations so far have studied older (people) in their seventies and above at highest risk for dementia."
"Because we recruited a younger cohort of older persons aged 55 and above (average age about 65), we were in a better position to observe two . . . events occurring early in the course of dementia development: the metabolic syndrome and (mild cognitive impairment)," Ng told Reuters Health by email.
In five communities in Singapore between 2003 and 2009, the researchers studied more than 1,500 adults. More than half were female. Overall, 340 had the constellation of risk factors consistent with metabolic syndrome.
Over time, 141 participants developed mild cognitive impairment, including 14 percent of those with metabolic syndrome and 8 percent of those without it.
Those with type 2 diabetes, central obesity, unhealthy cholesterol levels or metabolic syndrome at the beginning of the study had an increased risk of developing mild cognitive impairment over time compared to others.
Diabetes was tied to a more than tripled increase in risk, while metabolic syndrome, central obesity and unhealthy cholesterol increased the risk of cognitive impairment by about 50 percent.
But for the risk of mild cognitive impairment progressing to dementia, metabolic syndrome increased the risk more than fourfold, even more than type 2 diabetes.
Of 425 participants who had mild cognitive impairment at the beginning of the study, 14 progressed to dementia, including about 8 percent of people with metabolic syndrome compared to only about 2 percent of those without metabolic syndrome, as reported in JAMA Neurology.
"These observations should be replicated in more similar studies in other populations," Ng said.
Physical inactivity, poor diet, smoking, and mental stress are known to increase the risk of metabolic syndrome and are now increasingly acknowledged to increase the risk of dementia as well, Ng said.
Insulin plays a vital role in brain metabolism and memory, Ng said, and people with diabetes or the risk factors leading to diabetes may be resistant to insulin.
"Moreover, (metabolic syndrome) and some of its individual components (particularly diabetes and obesity) are associated with the production of inflammatory substances that can have long-term toxic effects on all the cellular structures of our brain," said Dr. Paola Forti of the University of Bologna in Italy who was not part of the Singapore study.
Metabolic syndrome was associated with cognitive impairment before but not after age 75, Forti noted.
"Physically active and more educated people are more likely to be more attentive to health issues; affluent people are more likely to have better lifestyle habits and get better medical care," Forti told Reuters Health by email. "So, it cannot entirely be excluded that the association of metabolic syndrome and cognitive impairment just reflect the effect of other factors associated with poor health."
"We do not as yet have much evidence to support the possibility that reducing weight in middle age will reduce the risk of developing dementia later," Ng said. "Nevertheless, given the weight of the evidence so far, it is advisable for overweight people in their middle age to reduce weight."