Metabolic syndrome affects 34.7% of Americans (1).
It is a collective term for several adverse health conditions commonly associated with abdominal obesity.
Studies suggest that people with metabolic syndrome have lower levels of antioxidants, such as vitamin E, but the reason is not entirely clear.
A recent study examined vitamin E bioavailability in people with metabolic syndrome.
Today’s review provides a detailed summary of its findings.
Vitamin E is a group of fat-soluble antioxidants, also known as tocopherols and tocotrienols.
Only one of them — alpha-tocopherol — is thought to meet the body’s vitamin E requirements. In this review, alpha-tocopherol will simply be referred to as vitamin E.
Like other fat-soluble vitamins, vitamin E is believed to be better absorbed into the bloodstream when eaten with fat (2).
In the body, its strong antioxidant properties protect cell membranes from oxidative damage.
Vitamin E levels are generally lower in people with metabolic syndrome, compared to healthy individuals (5).
Oxidative stress and inflammation are believed to explain these low levels, but it’s unclear how metabolic syndrome affects vitamin E bioavailability or the proportion of active vitamin E entering the blood circulation.
This study examined the effects of metabolic syndrome and dairy fat on vitamin E bioavailability.
The purpose of this randomized, crossover trial was to examine vitamin E bioavailability in people with metabolic syndrome, compared to healthy people. It also examined whether dairy fat had any effects on its absorption.
A total of 20 men and women were recruited — 10 with metabolic syndrome and 10 healthy individuals. These two groups were age-matched and contained an equal number of men and women.
All participants received 15 mg of vitamin E on 4 separate study visits, each of which was separated by a 2-week washout period.
On each visit, the participants arrived at the research unit in the morning after an overnight fast and took the vitamin E supplement with 240 ml of milk.
The fat and carb content of the milk differed between visits, and on one occasion they got soy milk instead of cow’s milk, as shown in the chart below.
All of the participants were provided with standardized food three days prior to, as well as during the first 24 hours of, each study visit.
Throughout each study visit, which lasted for 72 hours, the researchers collected blood at regular intervals. They then measured various blood variables, including vitamin E and C, blood sugar levels and inflammatory markers.
Researchers also conducted a test-tube experiment, examining the effect of dairy fat on the potential absorbability of low (15 mg) and high (268 mg) doses of vitamin E.
Bottom Line: This was a randomized, crossover trial investigating the bioavailability of vitamin E in people with metabolic syndrome. It also examined the effects of dairy fat on vitamin E bioavailability.
Finding 1: Metabolic Syndrome Was Associated with Lower Vitamin E Levels
At the start of the study, circulating levels of vitamin E were similar between healthy participants and those with metabolic syndrome.
However, when vitamin E levels were corrected for plasma total lipids (µmol/mmol lipid) as recommended, the levels were lower in participants with metabolic syndrome, compared to those who were healthy (6).
These findings are shown in the chart below.
The results are supported by previous studies (5).
The lower levels of vitamin E may be explained by the chronic inflammation and oxidative stress associated with metabolic syndrome.
Accordingly, the study showed that participants with metabolic syndrome had higher levels of oxidized low-density lipoprotein (O-LDL), a marker of oxidative stress, and lower levels of vitamin C compared to healthy participants.
They also had higher levels of the inflammatory markers, interleukin-6 (IL-6), interleukin-10 (IL-10) and C-reactive protein (CRP), as shown below.
High levels of oxidized LDL and inflammatory markers were associated with lower levels of vitamin E.
Yet, vitamin E depletion was slower in participants with metabolic syndrome, compared to those who were healthy.
Slower vitamin E depletion may indicate lower requirements, but vitamin E metabolism is likely more complicated than that.
Bottom Line: People with metabolic syndrome had lower circulating levels of vitamin E, compared to healthy individuals.
Finding 2: Vitamin E Bioavailability Was Lower in People with Metabolic Syndrome
Estimated vitamin E bioavailability was significantly lower in participants with metabolic syndrome, compared to healthy individuals.
Also, average and maximum levels of vitamin E in blood and lipoproteins were lower over the whole measurement period.
Why the bioavailability of vitamin E was lower in people with metabolic syndrome is unclear.
However, lower levels of vitamin E were associated with higher levels of oxidized LDL and inflammatory markers, suggesting that inflammation or oxidative stress might be involved.
The authors also speculated that metabolic syndrome might be associated with disrupted vitamin E metabolism in the liver or that vitamin E might be temporarily trapped in enterocytes, which are the cells lining the small intestine.
However, further studies are needed before any hard conclusions can be reached.
Bottom Line: The bioavailability of vitamin E was lower in people with metabolic syndrome, compared to healthy individuals.
Finding 3: Dairy Fat Did Not Affect the Absorption of Low-Dose Vitamin E
Dairy fat had no significant effects on vitamin E absorption, regardless of the participants’ health status.
The average vitamin E levels over each 72-hour study visit, presented as area under the curve (AUC), are shown in the chart below. These values represent both vitamin E bioavailability and depletion.
The results are inconsistent with two previous studies. One of them showed that vitamin E was better absorbed when taken with milk containing 1% fat, compared to orange juice (7).
Another study found that vitamin E absorption was greater when it was taken with whole milk, compared to low-fat milk (8).
However, these studies used very high amounts of vitamin E, or 150–200 mg. In comparison, the participants in the present study supplemented with 15 mg of vitamin E, which is equal to the Recommended Daily Allowance.
In the present study, the researchers also simulated milk digestion in a test tube. Their findings supported the human results – the fat content of milk did not affect the absorbability of 15 mg of vitamin E.
Yet, when the dose was increased to 268 mg, higher amounts of dairy fat significantly increased the potential absorbability of vitamin E.
Bottom Line: Dairy fat did not affect the absorption of vitamin E at low doses (15 mg). However, a test-tube experiment revealed that it may affect the potential absorbability of higher doses (268 mg) of vitamin E.
The study didn’t appear to have many limitations, and the study design was excellent.
However, it didn’t allow the researchers to determine the absolute bioavailability of vitamin E. As a result, it may have been underestimated (9).
Summary and Real-Life Application
In short, this study showed that vitamin E bioavailability was lower in adults with metabolic syndrome, suggesting that a higher dietary intake of vitamin E might benefit them.
This is potentially explained by elevated inflammation and oxidative stress in these individuals.
Additionally, although fat is thought to increase vitamin E absorption, the bioavailability of low-dose vitamin E supplements (15 mg) seems to be unaffected.