N05015: The bioavailability of iron, zinc and copper in meat-containing and vegetarian diets in the UK

Study Duration: January 1999 to February 2002

Contractor: Institute of Food Research

Data from the Dietary and Nutrition Survey of British Adults demonstrated that iron deficiency is present in up to one third of fertile women in the UK, caused mainly by an imbalance between menstrual iron losses and iron supply from the diet.

The bioavailability of iron and zinc in a Western-type diet, containing significant quantities of meat and vitamin C, is assumed to be high (approximately 15%). However, with the current trend towards vegetarianism and the substitution of red meat by poultry, it is possible that iron and zinc bioavailability may, in fact, be somewhat lower. Copper bioavailability is not well-understood and published figures for copper absorption range from 35 to 70%. The primary aim of this project is to determine the efficiency of absorption of iron, zinc, and copper from vegetarian and non-vegetarian diets.

90 women of childbearing age (18-45 years) were recruited and split into 3 groups of either lacto-ovovegetarians, poultry/fish eaters or red meat eaters, 30 in each group. The aims of the study were to:

• measure the extent to which menstrual losses and iron status influence iron absorption
• determine zinc, non-haem iron and copper absorption from the test meals fed to subjects by measuring stable isotopes of zinc, iron and copper in fecal samples.

Analysis of the three diet types (meat, vegetarian and poultry/fish) consumed by the volunteers in terms of their zinc, copper and iron content and the analysis of the fecal samples was carried out by the Central Science Laboratory under project N05014.

The first aim of this project was to investigate how menstrual blood loss and amount of iron in the diet affected body iron levels in women of childbearing age consuming the three main types of diet in the UK; those containing either red meat, poultry and fish but no red meat, or lacto-ovovegetarian (no meat or fish at all). A second aim was to investigate how much iron, zinc and copper was absorbed by women eating the three types of diet. This information will help us understand the nutritional significance of current dietary changes in women of childbearing age in the UK. It will also assist in making recommendations about dietary intakes of minerals.

Ninety healthy, pre-menopausal women aged 18-45 years took part in a study at the Institute of Food Research, Norwich. Subjects were allocated into one of three dietary groups, red meat, poultry/fish or lacto-ovovegetarian according to their normal eating patterns. Each group contained 30 volunteers. The levels of iron, zinc and copper in their normal diets was estimated by analyzing duplicate meals collected by the volunteers over a period of seven days. Menstrual blood loss was measured during one menstrual period. The levels of iron stores were measured by analyzing blood samples taken over three consecutive days. Finally, an assessment of iron, zinc and copper absorption was made on the second and third days of the iron status blood sampling days. The amounts of iron, zinc and copper absorbed from the three types of diet were measured using stable isotope labels. Stable isotopes allow minerals to be traced through the body to calculate how much is absorbed from food.

The results of this study indicated that a large proportion of the women were mildly iron deficient in all of the dietary groups (70%). Iron stores were the lowest in the red meat group, but the levels in all groups were considered to be less than adequate.

Iron absorption from the diet and the amount of iron lost in menstrual blood were the two most important factors affecting iron levels in the body. When iron stores were low, efficiency of iron absorption increased, presumably to maintain homeostasis. However, despite these adaptive changes, body iron stores were lower in women with higher menstrual losses, indicating that the capacity to absorb sufficient iron to match iron losses was not unlimited, being restricted by dietary iron availability.

The levels of zinc in the diet and in the blood were lowest in women consuming a poultry/fish-based diet. However, the amount of zinc estimated to be absorbed from the diet was significantly greater in this group compared with the red meat and vegetarian groups. The amount of copper estimated to be absorbed from the diet was similar for each of the three dietary groups. The levels of copper in a vegetarian diet were significantly greater than in the red meat group, which suggests that at the mean dietary copper levels consumed, one of the mechanisms by which the body maintains balance is through changes in copper absorption.

Main findings:

• Menstrual iron loss correlated with iron status; high blood loss resulted in low iron stores.
• Iron absorption was negatively correlated with iron stores; low iron stores resulted in increased absorption.
• Iron intake was highest from the lacto-ovovegetarian diet but fractional absorption was lower, resulting in similar quantities of iron being absorbed from each diet.
• Zinc intake and serum zinc concentration were lowest in the poultry fish group, and zinc absorption was highest.
• Copper intake was highest and fractional copper absorption lowest in the lacto-ovovegetarian group.

In Summary:

• Iron stores are low in women of childbearing age in the UK, regardless of type of diet; thus women are at risk of developing iron deficiency anaemia with inappropriate diets or an increase in requirements, such as in pregnancy.
• Vegetarians are at no greater risk of iron deficiency than omnivores.
• High menstrual losses are associated with low iron stores.
• Zinc nutrition may be inadequate in women consuming a poultry/fish diet.

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