The iron intakes and likely bioavailability of iron in vegetarian and non-vegetarian adolescent women in New Zealand

Abstract

Background: Adolescent women are vulnerable to iron deficiency, including iron deficiency anaemia, and those who follow a vegetarian diet may be at even greater risk due to the lower bioavailability of iron in vegetarian diets. To date, no study has explored the iron intakes or likely bioavailability of iron in the diets of vegetarian adolescent women in New Zealand.

Objective: To compare, in vegetarian and non-vegetarian adolescent women 15-18 years of age in New Zealand: intakes of dietary and total iron; the prevalence of inadequate iron intakes; intakes of haem iron, non-haem iron, and selected iron absorption modifiers; intakes of estimated available iron; and the main food sources contributing to iron intake.

Design: The SuNDiAL (Survey of Nutrition Dietary Assessment and Lifestyle) study is an ongoing cross-sectional survey of adolescent women 15-18 years of age from eight cities in New Zealand. Participants in the current study were enrolled between 18 February 2019 and 4 September 2019. Dietary intakes were assessed using two 24-hour recalls, and were adjusted to reflect ‘usual’ intake using the Multiple Source Method. Supplement iron intakes were estimated using information from an online questionnaire. The prevalence of inadequate iron intakes was estimated using the full probability approach, using information on total iron intakes (from dietary sources and supplements). Intakes of available iron were estimated using the Monsen and Balintfy algorithm. Comparisons were made by self-defined vegetarian status.

Results: The current study comprised 31 vegetarians and 219 non-vegetarians. Only 59% of the vegetarian participants reported never consuming flesh foods currently. When compared to non-vegetarians, vegetarians had a higher estimated mean total iron intake (difference [95% CI]: 21% [2 to 42] (reported as a percentage because data are log transformed); P=0.026), and a lower estimated prevalence of inadequacy (12% vs 21%). However, intakes of several dietary factors that influence iron bioavailability differed between groups: vegetarians reported lower mean intakes of haem iron (-0.99 mg/d [-1.20 to -0.79]; P<0.001) and ‘Meat, Fish, and Poultry’ (-102.8 g/d [-118.5 to -87.1]; P<0.001), and higher mean intakes of fibre (as a proxy for phytate; 5.25 g/d [1.55 to 8.95]; P=0.006), and also vitamin C (by 26% [1 to 58] (reported as a percentage because data are log transformed); P=0.04), than did non-vegetarians. The estimated mean iron bioavailability of vegetarian diets was statistically 20% lower than that of non-vegetarian diets (7.7% vs 9.6%; P<0.001), and the estimated mean intake of available iron for vegetarians was numerically 11% lower (0.99 mg vs 1.11 mg) than for non-vegetarians (-0.12 mg/d [-0.28 to 0.03]; P=0.105). Vegetarians obtained a higher proportion of iron from plant-based food sources (particularly ‘legumes, nuts and vegetarian products’ and ‘grains and pasta’), and a lower proportion from flesh foods sources, when compared to non-vegetarians.

Conclusion: Although the vegetarians had higher total iron intakes than the non-vegetarians, the bioavailable iron content of their diets was slightly lower, suggesting that self-defined vegetarian adolescent women may be at greater risk for iron inadequacy than non-vegetarians. Further research is required to determine what impact this may have on biochemical iron status, and to generate findings that are more representative of adolescent women in the wider New Zealand population, including those who adhere more closely to a vegetarian diet.