Leafy vegetables are important sources of provitamin A carotenoids. Information on their ability to provide vitamin A is often misleading because of the methodology used to assess bioefficacy. Mongolian gerbils were used to evaluate the bioefficacy of provitamin A carotenoids in tropical leafy vegetables (i.e. Solanum nigrum, Moringa oleifera, Vernonia calvoana and Hibiscus cannabinus) that are indigenous to Africa. Gerbils (n 67) were vitamin A-depleted for 5 weeks. After a baseline kill (n 7), the gerbils were weight-matched and assigned to six treatment groups (n 10; four vegetable groups; negative and positive controls). For 4 weeks, the treatments included 35 nmol vitamin A (theoretical concentrations based on 100 % bioefficacy) in the form of vegetables or retinyl acetate. In addition to their diets, the control and vegetable groups received daily doses of oil, while the vitamin A group received retinyl acetate in oil matched to prior day intake. Serum and livers were analysed for vitamin A using HPLC. Serum retinol concentrations did not differ among groups, but total liver vitamin A of the vitamin A and vegetable groups were higher than that of the negative control group (P < 0·0001). Liver β-carotene 15,15′-monooxygenase-1 expression levels were determined for two vegetable groups and were similar to the positive and negative controls. Conversion factors for the different leafy vegetables were between 1·9 and 2·3 μg β-carotene equivalents to 1 μg retinol. Small quantities of these vegetables maintained vitamin A status in gerbils through efficient bioconversion of β-carotene to retinol.

Inflammation influences the assessment of nutritional status. For example, inflammation reduces plasma retinol concentrations and vitamin A deficiency is overestimated. Conversely inflammation increases plasma ferritin concentrations and Fe deficiency is underestimated. Blood samples were obtained from 163 free-living HIV-1-infected adults, not on continuous medication, anti-retroviral drugs or micronutrients, not unwell and who had not reached WHO stage IV of HIV/AIDS. We used four markers of inflammation, C-reactive protein (CRP), α1-acid glycoprotein (AGP), α1-antichymotrypsin and erythrocyte sedimentation rate but mainly CRP and AGP were used to separate the subjects into four groups: ‘healthy’ where both CRP and AGP were normal; ‘incubation phase’ where CRP was elevated; ‘early convalescence’ where AGP and CRP were elevated and ‘late convalescence’ where only AGP was elevated. Correction factors were calculated to remove the influence of inflammation from each biomarker and group where inflammation was present and the data are shown before and after recalculation. The correction increased median plasma retinol concentrations of the whole group from 1·16 to 1·33 μmol/l, comparable with values (mean 1·29 μmol/l) in HIV-negative Kenyan women. Median ferritin concentrations fell by about 50 % in both sexes and the number of women with plasma ferritin concentrations ≤ 12 μg/l increased from eleven to twenty. The correction also increased plasma carotenoids and Hb but not α-tocopherol concentrations. We suggest that the method described to remove the influence of inflammation from nutritional biomarkers should be generally applicable in apparently healthy people and prevents discarding valuable data because of mild inflammation. The method does now need to be tested in other populations.

Vitamin A deficiency is a major global public health problem. Among the variety of techniques that are available for assessing human vitamin A status, evaluating the provitamin A nutritional values of foodstuffs and estimating human vitamin A requirements, isotope dilution provides the most accurate estimates. Although the relative expense of isotope dilution restricts its applications, it has an important function as the standard of reference for other techniques. Mathematical modelling plays an indispensable role in the interpretation of isotope dilution data. This review summarises recent applications of stable isotope methodology to determine human vitamin A status, estimate human vitamin A requirements, and calculate the bioconversion and bioefficacy of food carotenoids.