Carrot chemical 'may cut cancer risk'

Behind the Headlines

Tuesday November 23 2010

Carrots are a good source of nutrients and antioxidants

Eating carrots can reduce the risk of suffering from heart disease and protect against cancer, The Daily Telegraph has reported.

The story comes from a large 14-year study that found participants with the highest blood levels of alpha-carotene, an antioxidant found in many orange vegetables, were at lower risk of dying from any cause, and from conditions specifically related to cardiovascular disease and cancer.

However, the study only measured people’s alpha-carotene levels. It did not measure blood levels of other antioxidants, so it is difficult to know if alpha-carotene alone is associated with health benefits or if other constituents were also involved. The study also used only one blood sample to measure people’s alpha-carotene levels, and levels may well have changed during the years of the study. These are important limitations.

Like other antioxidants, alpha-carotene is found in several vegetables, including carrots, and this study may be considered to support the health benefits of a diet high in fruit and vege.

However, the health benefits of antioxidant supplements are far from well established and the use of antioxidant supplements requires further research. The public should be aware that there are many different brands of antioxidants on the market and these may not all be subject to the same safety and efficacy regulations as conventional medicines.

 

Where did the story come from?

The study was carried out by researchers from the Centers for Disease Control and Prevention in Atlanta, Georgia and the University of California in the US. No external financial funding was reported. The study was published in the peer-reviewed medical journal, Archives of Internal Medicine.

Both the Telegraph and Daily Express reported on the study. While their message - that fruit and vegetables have health benefits – was correct, their stories overstated the findings from this particular study. The study did not measure the health effects of people’s diets, only of blood levels of one particular antioxidant. The Daily Express confusingly linked the study to other research on orange juice, concluding that both carrots and orange juice can cut the risk of heart disease and cancer.

 

What kind of research was this?

This was an analysis of data from a long-running cohort study of over 16,000 US adults, looking at health and nutrition. This particular study aimed to find out if there was any relationship between blood levels of alpha-carotene, an antioxidant found in many vegetables, and the risk of death due to any cause as well as from specific causes including heart disease, stroke and cancer.

The researchers point out that oxidative damage is now suspected of playing a role in the development of chronic disease and that antioxidants may help protect against this process. While many studies have linked high consumption of fruits and vegetables to lower risk of several chronic diseases, uncertainty remains about the specific constituents that may contribute to these health benefits.

The researchers also say consumption of beta-carotene supplements has been shown to have no effect on risk, suggesting the possibility that other carotenoids (such as alpha-carotene) may contribute to the reduction in disease risk. The results of studies on possible health effects of alpha-carotene are mixed and this subject warrants further investigation, they argue.

Cohort studies are useful to follow up large groups of people over many years and are often used to investigate the possible effects of lifestyle measures such as diet and exercise. This was a prospective cohort study, which means it tracked people over time. This is considered to be more reliable than retrospective studies, which look at a person’s history.

However, it can be difficult to prove cause and effect with this type of study due to the influence of possible confounding factors and also because it is hard to regulate participants’ diets and ensure that they have remained the same throughout the study.

 

What did the research involve?

For this study, the researchers used data from a national health and nutrition survey undertaken between 1988-1994. The study had recruited US adults aged 20 years and over, and was designed to be representative of the US population. Participants were interviewed in their homes, attended a medical centre for examination and provided a single blood sample.

From the 16,573 adults who attended the examination centres, a total of 15,318 (92.4%) were included in the study. The rest were excluded for various reasons, for example, failing to have a blood test or because important data was missing.

After taking blood samples, researchers used standard laboratory techniques to measure the amount of alpha-carotene, reported in terms of μg/dL (micrograms per 100ml of blood). They also measured cholesterol levels. Researchers then divided participants into five categories, depending on their blood levels of alpha-carotene, ranging from 0-1 μg/dL to 9 or more μg/dL.

In the years following to December 2006, they matched participants to the National Death Index, to determine their survival status. They used a standard classification of diseases to determine underlying cause of death. They divided the causes of death into three major categories: cardiovascular disease, cancer and all other causes.

They used standard statistical methods to assess the relationship between blood levels of alpha-carotene and the risk of death during the follow-up period, which was 14 years on average. The findings were adjusted to take account of other things that might have affected the results (called confounders), such as age, lifestyle, education, blood pressure and cholesterol measurements.

 

What were the basic results?

Of the 15,318 participants, 3,810 died during the follow-up period.

Overall, the researchers found that people with higher blood levels of alpha-carotene were at lower risk of dying from any cause over the 14-year follow-up period than people with low levels. People with higher levels were also at lower risk of specifically dying from any cardiovascular disease and from cancer (P < .001 for linear trend). This lowered risk was independent of factors such as lifestyle habits, blood pressure, age and sex.

The following are the risk reductions for death from any cause (adjusted for potential confounders):

Compared with participants with alpha-carotene concentrations of 0 to 1 µg/dL

  • those with alpha carotene levels of 2 to 3 µg/dL, were 23% less likely to die from any cause (relative risk 0.77, 95% confidence interval 0.68 to 0.87)
  • those with levels of 4 to 5 µg/dL were 27% less likely to die from any cause (RR 0.73, 95% CI, 0.65 to 0.83)
  • those with levels of 6 to 8 µg/dL were 34% less likely to die from any cause (RR 0.66, 95% CI, 0.55 to 0.79)
  • Those with levels of 9 µg/dL or higher were 39% less likely to die from any cause (RR 0.61, 95% CI 0.51-0.73).

Researchers also found a significantly lower risk between higher alpha-carotene levels and risk of death from cardiovascular disease (P for trend 0.007) and cancer (P for trend 0.02). Associations between death from individual cancers and from specific cardiovascular causes (e.g. heart attack, stroke) were, however, non-significant.

 

How did the researchers interpret the results?

The researchers say that their results suggest further research is needed into the possible health benefits of alpha-carotene. Although chemically similar to beta-carotene, some studies suggest it may be more effective as an antioxidant in protecting against some diseases. They also say that blood levels of alpha-carotene, which is not currently found in supplements, is a reliable biomarker for fruit and vegetable consumption. Therefore, their study supports previous findings that fruit and vegetable consumption is beneficial to health.

 

Conclusion

Overall, this study supports other findings that high consumption of fruit and vegetables is beneficial to health. Its strengths include its large size and relatively lengthy follow-up period. However, it also has some limitations, which make the results less reliable:

  • Researchers only took one measurement of blood levels of alpha-carotene. Blood levels may change for various reasons, including variations in diet.
  • In particular the researchers did not measure blood levels of other substances (such as other antioxidants), which might have had an effect on results.
  • The results may been affected by other unidentified factors (confounders) despite efforts to adjust for some of these.
  • The researchers carried out a large number of statistical comparisons between levels of alpha-carotene and mortality from an extensive number of causes. This increases the possibility of chance findings.
  • Of note is the fact that alpha-carotene levels were associated with both death from any cause and from cardiovascular causes and cancer. However, when it was subdivided into specific types of disease, the association was no longer significant.

Also, the study provided no indication as to how many vegetables (and of what type) would need to be consumed to match the blood level measurements used in the study.
 
There is no doubt that carrots and other vegetables have health benefits. Whether this is due to the alpha-carotene they contain, to other antioxidants or to a complex balance of nutrients and vitamins they contain, remains uncertain.

Of importance is the fact that the health benefits of antioxidant supplements are far from well established. Other large reviews have found no evidence to support any benefit from taking antioxidant supplements, and have instead identified potentially harmful effects of taking certain supplements, including beta-carotene. As such, the use of antioxidant supplements requires further research.

The public should be aware that there are many different supplement brands on the market and these may not all be subject to the same safety and efficacy regulations as conventional medicines.

Links to the headlines

Carrots 'help protect against cancer'. The Daily Telegraph, November 23 2010

How orange juice can help to ward off heart disease. Daily Express, November 23 2010

Links to the science

Li C, Ford ES, Zhao G et al. Serum -Carotene Concentrations and Risk of Death Among US Adults. Archives of Intern Medicine, [Published online] November 22, 2010.

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Analysis by Bazian

Edited by NHS Choices