A Study of the Biophotometer as a Means of Measuring the Vitamin A Status of Human Adults

Abstract

Provided the worker is equipped with suitable means for frequent calibration of the intensity of the light emanating from the illuminated central dot of the test screen and from the large diffusing plate used for light adaptation, the biophotometer may be adequate for detecting marked dysadaptation in adults. The method used by the National Bureau of Standards to calibrate the biophotometer used for the work reported here is described in the text. Measurements of the critical threshold values (taken within a 20-second interval following the light adaptation period) are difficult to reproduce because of the extremely rapid decrease in the light threshold value at this time. It is, therefore, recommended that this measurement be verified for each subject. The threshold value at the end of the 9-minute recovery period is an important factor in the final interpretation of the test since it should reflect the same trend in threshold value as the measurement taken within the 20-second interval following the light adaptation period. A critical threshold value (taken within a 20-second interval following the 3-minute light adaptation period) numerically greater than 3.0 microlamberts, together with a threshold value at the end of the 9-minute recovery period numerically greater than around 0.20 microlamberts, would appear to be fairly definite evidence of a subnormal power of dark adaptation in an adult. In terms of the calibrations furnished by the manufacturers of the biophotometer these borderline thresholds would be on the order of 0.70 and 0.06 ‘millifoot-candles,’ respectively. These borderline regions, however, probably are not sharply defined. There is at least a suggestion that these threshold values may be somewhat different for different adult age groups. Moreover, the normal range being quite widespread, a subject having a critical threshold value in the highest part of a so-called normal range might not have a sufficiently high level of vitamin A intake to maintain the lowest critical threshold of which he is potentially capable. Five subjects with critical threshold values in the region of 4 to 8 microlamberts (or about 0.8 to 1.3 ‘millifoot-candles’) showed definitely lowered thresholds, commensurate with normal critical thresholds, following daily administration of 5000 to 8500 international units of vitamin A in the form of halibut liver oil within a period of 1 to 2 weeks. The threshold values at the end of the 9-minute recovery period reflected the same trend of change.