The study looked at how quickly dietary sources of vitamin E in leafy greens are absorbed from the blood stream into tissue.
Maret Traber, lead researcher for micronutrient research at the College of Public Health, Oregan, said that the research raises particular concern for people who are obese or have metabolic syndrome.
“Every tissue in the body is under oxidative attack, and needs more vitamin E. But [for people with high cholesterol] the vitamin E needed to protect these tissues is stuck on the freeway, in the circulatory system. It's going round and round instead of getting to the tissues where it's needed. People with elevated lipids in their blood plasma are facing increased inflammation as a result."
"In simple terms, we believe that less than one third the amount of vitamin E is actually making it to the tissues where it's most needed," Traber said.
The researchers, who published their study in the American Journal of Clinical Nutrition, suggest that methods for measuring vitamin levels using only blood levels do not give an accurate idea of vitamin E bioavailability because they do not show how much is actually absorbed by tissue.
According to the study, 90% Americans who do not take supplements are vitamin E-deficient. While there have been calls for the recommended daily allowance to be lowered, Traber et al. say that in light of their findings, the current level of 15 mg should be maintained.
The study
41 male and female participants aged 20 to 82 were given a controlled breakfast which included 120g of collard greens. The greens contained 4.7mg of a-tocopherol and were deuterium-labeled, allowing researchers to track it in the body. Meals and beverages were provided to reduce variability and to ensure a total of 9.2mg dietary a-tocopherol each day during the five-day study.
The scientists measured a-tocopherol using liquid chromatography-mass spectrometry from blood samples which were collected before breakfast and 24, 48 and 72 hours afterwards.
They found that while maximum and minimum plasma a-tocopherol concentrations did not vary with age or sex, they did for serum lipids.
“As serum lipids increase, a-tocopherol remains in circulation for a longer time likely because higher lipid concentrations are associated with slower lipoprotein catabolism and uptake by tissues,” says the study.
“These findings have important public health consequences because they highlight a limitation in assessing vitamin E status by using only plasma a-tocopherol concentrations.”
Using isotope methods, which they claim provide more accurate results than using radioactive a-tocopherol, they scientists suggest that only 24% of vitamin E is absorbed by body tissue, instead of previous estimates of 81% measured by radioactive vitamin E.
According to Traber, while vitamin E in the bloodstream is still beneficial by preventing oxidation of low-density lipoprotein (LDL) and high-density lipoprotein (HDL) cholesterol, there are concerns that insufficient levels reach tissue and organs where it is needed.
Source: American Journal of Clinical Nutrition
"A-Tocopherol disappearance rates from plasma depend on lipid concentrations: studies using deuterium-labeled collard greens in younger and older adults"
Published ahead of print March 4 2015, doi: 10.3945/ajcn.114.100966.
Authors: Maret G Traber, Scott W Leonard et al.