The nuts help raise levels of magnesium and alpha tocopherol, or vitamin E, two nutrients that help protect against a range of age-related health problems such as Alzheimer's disease.
Eating a handful of the nuts every day for four months raises levels of both nutrients, say researchers from Oregon State University, who monitored 32 older adults—aged 55 and older—for four months.
As well as raising levels of the two nutrients, the hazelnuts also lowered levels of glucose, or blood-sugar, and LDL lipoproteins, often described as the 'bad' cholesterol.
Many people in the US are deficient in both nutrients, partly because they "don't like taking multivitamins, but hazelnuts represent a multivitamin in natural form," said Maret Traber, one of the researchers.
Hazelnuts are also a source of healthy fats, copper and B6.