A total of 75 subjects will be recruited for this study (25 normal weight, 25 overweight, 25 obese). In each subject blood volume will be determined using the minimally invasive carbon monoxide-rebreathing method (substudy 1) and iron absorption will be measured from a test meal labeled with stable iron isotopes. The effect of ascorbic acid on iron absorption will further be determined using a second labeled test meal (substudy 2). In addition, inflammation, hepcidin and iron status will be determined. In order to study the effect of body fat content on blood volume and iron absorption weight and height will be measured, and body fat will de determined by dual energy X-ray absorptiometry.

Obesity has repeatedly been shown to be a risk factor for iron deficiency. The reason for this is believed to be reduced iron absorption as a result of increased circulating hepcidin concentrations. However, the direct interactions between body fat mass, iron absorption, inflammation and hepcidin have not been studied to date. Whether the low iron absorption in obese subjects could be overcome by conventional methods to increasing iron absorption, e.g. the addition of ascorbic acid and whether a reduction of inflammation could improve absorption is further unclear. For the calculation of fractional iron absorption from oral doses of stable iron isotopes an estimation of blood volume is required. Several equations to estimate blood volume from weight and height or calculated body surface area or volume are available, but have rarely been validated or used in obese subjects. The aim of the described study is therefore: 1) to determine blood volume using the carbon monoxide-rebreathing method over a large BMI range and to use those values to validate existing blood volume equations; 2) to measure iron absorption, hepcidin and inflammation in relation to BMI and body fat and 3) to assess the effect of ascorbic acid on iron absorption.

Inclusion Criteria:

• Women
• Age 18-45 years
• Pre-menopausal (no absence of a menstrual cycle in the past 12 months)
• BMI 18.5-39.9 kg/m2 (n=25 18.5-24.9, n=25 25-29.9, n=25 30-39.9)
• Apparently healthy (no significant medical conditions that could influence iron or inflammatory status other than obesity (i.e., cancer, HIV/AIDS, inflammatory bowel disease, gastrointestinal bleeding, and rheumatoid arthritis, renal disease or hemochromatosis) (judged by study physician)

Exclusion Criteria:

• Moderate/sever anemia (Hb<11 g/dl)
• Use of medication which interferes with study measurements (judged by the study physician)
• Smoking
• Pregnancy and lactation; planning to become pregnant during the course of the study (pregnancy test will be conducted at visit 2 and subjects will be asked about contraception*)
• Consumption of vitamin- or mineral supplements unless they consent to discontinue intake two weeks before the start of the study until the final blood sampling
• Prior participation in studies using stable iron isotopes
• Food allergies/intolerances (gluten, lactose, milk protein)
• Blood donation during the two months prior to the study
• Major injuries during the 6 months prior to the study
• Illness during the 4 weeks prior to the study (judged by study physician)
• Administration of recombinant erythropoietin (rhEPO)
• More than 5 days at altitudes above 1600 m above sea level during the 3 weeks prior to the study
• Long distance flight (>8 h) during the 3 weeks prior to the study
• Chronic alcohol abuse