Wednesday June 16 2010
We cannot tell if diet directly influenced blood vitamin levels
A diet of meat and potatoes “reduces the risk of lung cancer by half”, according to The Independent. The newspaper says that people with above average levels of vitamin B6 in their blood had a lower risk of developing lung cancer.
This news story was based on a large and well-conducted study comparing the levels of various nutrients with the risk of developing lung cancer. It found that having higher levels of vitamin B6 or the chemical methionine in the blood was associated with a lower risk of the cancer, regardless of smoking status. However, current smokers also tended to have the lowest levels of B6 and methionine.
Although B6 and methionine are found in meat and potatoes, the researchers say it is not yet known whether consuming more of these nutrients would make a difference to the risk of developing lung cancer. This is because lower concentrations of these substances in the blood of smokers could reflect poorer absorption by the body. The vitamins are also found in a range of foods, such as wholegrains, leafy green vegetables and fruit.
Where did the story come from?
The study was carried out by researchers from the International Agency for Research on Cancer in France, and was funded by the World Cancer Research Fund and the European Commission. It was published in the peer-reviewed Journal of the American Medical Association.
This research was generally covered accurately by newspapers. However, The Independent placed emphasis on the potential for dietary supplementation to lower the risk of lung cancer. Further follow-up studies are needed to assess whether this is the case.
What kind of research was this?
It is thought that B vitamins regulate the expression of genes and maintain the integrity of DNA in cells. The researchers suggested that this may affect cancer risk. This was a case-control study that compared the blood levels of B vitamins in a large cohort of participants against their risk of developing lung cancer.
What did the research involve?
The participants were part of the European Prospective Investigation into Cancer and Nutrition (EPIC) cohort. As part of EPIC, a total of 519,978 individuals across Europe were asked a standardised questionnaire including questions on their diet between 1992 and 2000. Of these participants, 385,747 provided a blood sample.
In this study, participants who developed lung cancer after the date on which their blood sample was taken (between 2002 and 2005) were put into the “case” group. The control group consisted of participants that had not developed lung cancer but had similar baseline characteristics to the cases (for example country, gender, age and date of blood collection).
The researchers measured the amount of B2 (riboflavin), B6, B9 (folate), B12 (cobalamin), homocysteine and methionine in the blood samples. They also measured a chemical called cotinine as an indicator of recent smoking intensity at the time at which the blood sample was taken.
In certain analyses, the researchers adjusted for the influence of some factors that may affect the likelihood that an individual would develop lung cancer, such as body mass index (BMI), educational status, whether they were a smoker and alcohol consumption at the time of recruitment.
What were the basic results?
Within the entire EPIC cohort, the age-standardised incidence rates of lung cancer were calculated in those who had never smoked (never smokers), former smokers and current smokers. There were:
- 6.6 cases per 100,000 person years in never-smoking males
- 44.9 cases per 100,000 person years in former-smoking males
- 156.1 cases per 100,000 person years in currently smoking males
- 7.1 cases per 100,000 person years in never-smoking females
- 23.9 cases per 100,000 person years in former-smoking females
- 100.9 cases per 100,000 person years in currently smoking females
There were 899 cases who had provided a blood sample and later developed lung cancer. The researchers then selected 1,815 matched controls. Out of the cases with lung cancer, 11% were never smokers, 29% were former smokers and 39% were current smokers at the time of their blood sample. Cases were mostly male (62%) and their average age when blood samples were taken was 59 years.
The blood levels of the B vitamins were similar between never and former smokers, but lower in current smokers. Current smokers who smoked a lot had lower B12, B6 and B9 than current smokers who smoked fewer cigarettes.
Looking at all of the blood samples, the researchers split the results into quartiles of blood levels of each of the four B vitamins, homocysteine and methionine. They found that there was a lower risk of lung cancer with increasing levels of B6. For example, people whose B6 levels were in the highest quartile were 54% less likely to have lung cancer compared with people whose B6 levels were in the lowest quartile (odds ratio OR 0.44; 95% CI, 0.33 to 0.60, P<0.000001). Similar analysis between the highest and lowest quartiles showed that a lower risk was associated with increasing methionine (OR, 0.52; 95% CI, 0.39 to 0.69 P<0.000001).
The researchers found that when they analysed the never, former and current smoker groups separately they found a similar pattern in which higher levels of B6 and methionine were associated with a lower risk. Making adjustments for BMI, educational attainment and alcohol consumption did not modify the results.
They found that smokers consumed fewer fruits and vegetables than former smokers, and there was a weak correlation between dietary vitamin measures and serum levels of B2, B6 and B12.
How did the researchers interpret the results?
The researchers concluded that “above-median serum measures of both B6 and methionine, assessed on average five years prior to disease onset, are associated with a reduction of at least 50% in the risk of developing lung cancer”.
This large, well-conducted study found an association between higher levels of vitamin B6 and methionine, and a lower risk of developing lung cancer.
However, the researchers highlight the following considerations, which they say warrant further research:
- They took blood samples on only one occasion, when the individuals were recruited to the study. Therefore, the measurements may not reflect the day-to-day, seasonal or long-term variation in vitamin levels.
- The researchers did not find an association between dietary consumption of vitamin B6 and cancer risk. They say the blood B6 levels may vary between cases and controls because of differences in absorption of the vitamin by the body rather than dietary intake. This is important to address as, if the effect is due to poorer absorption, changing the diet would not be a suitable means of reducing cancer incidence.
- This study did not look at how smoking affected absorption of vitamins directly.
Smoking is the greatest risk factor for developing lung cancer, and stopping smoking can greatly reduce the risk. This study warrants further research into a possible protective role of B6 and methionine in lung cancer, as well as into whether dietary changes can or cannot influence this risk.