N05023: Influence of genetic mutations on iron status linked to dietary intake - from the UK women's cohort study

Study Duration: April 2000 to May 2003

Contractor: University of Leeds

Public health policy to prevent iron deficiency may disadvantage those who are susceptible to high levels of iron storage. Haemochromatosis is a common, genetically inherited condition which leads to high levels of iron in the body, and if left untreated can have serious consequences including liver damage, heart disease and cancer. The genetic material found in cells, known as DNA, is collected in paired strands, the chromosomes. These are made up of thousands of pairs of genes which tell the body to make proteins. A gene has been found called HFE, which has been linked to haemochromatosis and is involved with iron absorption and storage. Some people have altered HFE genes which makes them more likely to have high iron levels. The two most common alterations (mutations) are called C282Y and H63D. Since each cell has two copies of the HFE gene it is possible for a person to have one or two copies of these alterations. No studies so far have considered the influence of dietary iron intake in relation to these mutations and levels of iron storage in the body. Iron in the diet is found in two forms: haem iron which is found in haemoglobin in meat and fish and non haem iron found mainly in cereals and vegetables. There is a need to know how diet in the long term may affect iron absorption and status, particularly in the light of new knowledge about genetic factors.

Objectives

This study set out to determine the relationship between iron intake, iron status and the risk of iron overload in subjects according to genotype. i.e. comparing women with one copy (heterozygous) or two copies (homozygous) of the two genetic mutations (C282Y and H63D) associated with haemochromatosis with normal women. The study objectives were: to measure dietary iron intake (both haem and non haem) and to see how common the occurrence of the gene mutations of interest were in a population of adult British women; to measure in a blood sample, markers of iron transport and iron storage in a sub-group of 2,000 women and compare these with their iron intake and level of mutations; to explore these issues in vegetarian and non-vegetarian women; and to make recommendations concerning food policy aimed at reducing the level of iron deficiency

This is the first study to characterise genotype, blood iron levels and dietary iron intakes in a large population based sample of women.

The UK Women’s Cohort formed the population for study. Detailed dietary information was available from a questionnaire on 35,372 women aged 35-69 years at recruitment. The women were contacted for a second time and asked for more detail on diet by filling in a 4-day food diary. Approximately 15,000 women were sent a request at the same time to provide cheek cell samples from which the gene mutations could be measured. In addition, 3,000 women who had already provided a food diary and had not been asked to provide a cheek cell sample were asked to provide a blood sample to act as a control group. DNA was available from 7233 subjects and blood iron levels on 2,528 of these. Key dietary information was captured from the food diary of 6,726 women and of these, 430 women had their full diary coded.

Key findings are summarised below:

• A fast effective screening method for the two commonest HFE mutations, C282Y and H63D has been developed using a highly sensitive technique.
• The method used to extract DNA from cheek cells is cheap, fast and effective.
• Collecting cheek cell DNA by subjects using cytology brushes followed by second class postal delivery to the laboratory produces poor quality DNA. Used cheek brushes should be sent by first class post and stored at 4°C before DNA extraction.
• A reliable assessment of haem and non haem iron in the diet has been obtained from both questionnaire and food diary data.
• 0.5% of the sample was homozygous (two copies) and 13% heterozygous (one copy) for the C282Y mutation. 2% were homozygous and 26% heterozygous for the H63D mutation.
• 45% of the women who were homozygous for the C282Y mutation had clinically high iron levels.
• Women who are C282Y homozygotes are clearly different from the other women in terms of their iron storage and handling.
• A number of factors are associated with the main measure of iron storage, serum ferritin concentrations. These are age, body mass index, blood donation, menopausal status and polymorphism (DNA mutation) status. Ferritin concentration is strongly positively associated with haem iron intake and not with non haem or total iron intake. This means that the more haem iron you eat the higher will be the ferritin concentration. Weaker positive associations were seen with vitamin C, alcohol and calcium supplements. Higher calorie intakes were associated with lower ferritin concentrations. Higher serum ferritin concentrations were associated with lower white and brown bread intake and fruit juice; and higher red and white meat intake.
• The effect of genotype on ferritin concentrations primarily occurs post-menopausally. There is a strong interaction between genotype and haem iron intake which is seen predominantly postmenopausally. Haem iron having a much stronger influence on serum ferritin amongst C282Y homozygotes.
• Meat eaters have a 35% higher ferritin concentration than strict vegetarians. This is despite the fact that strict vegetarians have twice the odds of having a high total iron intake than omnivores.
• Women with the highest serum ferritin concentrations were post-menopausal, C282Y homozygotes and eating a diet rich in haem iron.
• Nutrients from food diary data on a subsample compares well with the questionnaire measures of nutrients.
• Factors associated with serum ferritin concentrations using the sub group with fully coded food diary data show similar results to the full sample. Age, blood donation, menopausal status and polymorphism status were associated. Higher haem iron and iron supplement intakes were associated with higher ferritin concentrations.
• There were not more anaemic women in the normal group (ie. those without the gene mutations being investigated) than expected.
• The risk of being anaemic was significantly related to being premenopausal.
• Genetic status did not appear to be a strong predictor of anaemia defined by extremely low iron stores.

Many of the current recommendations to reduce the risk of iron deficiency anaemia, such as high intake of fruit and vegetables, avoiding polyphenol rich beverages (such as tea) with meals, consumption of iron fortified breakfast cereals and taking iron supplements were not consistently protective and were not strongly related to serum ferritin concentrations in the cohort. Advice to reduce the risk of iron deficiency anaemia should be directed at younger, pre-menopausal women.

Regarding iron overload, women who are homozygous for the C282Y mutation should be advised to reduce their meat (haem iron) intake, particularly post menopausally. In premenopausal women, poor regulation of iron absorption may be masked by menstrual blood loss, and iron overload may still be possible later in life. These women may need to modify their iron intake as they grow older and iron requirements decrease. From a public health perspective, the large group of women who are heterozygous for the C282Y mutation appear to respond to haem iron similarly to the normal women with regard to their serum ferritin concentrations. These women do not need to be concerned about altering their diet to avoid increasing ferritin levels.

Contact: Ms Mamta Singh
Tel: 020 7276 8919
Email: mamta.singh@foodstandards.gsi.gov.uk