Vitamin A Requirements and Reference Intakes

Very few direct studies have been performed to determine human vitamin A requirements. In the Sheffield study (Hume and Krebs, 1949), 16 subjects were depleted of vitamin A for 2 years only three subjects showed clear signs of impaired dark adaptation. One of these subjects was repleted with 390 /rg of retinol per day, which resulted in a gradual restoration of dark adaptation the other two subjects received /3-carotene. On this basis, the minimum requirement was presumed to be 390 /rg, and the reference intake was set at 750 /rg. 

European Union FAO, Food and Agriculture Organization WFIO, World Flealth Organization. 

Sources Department of Health, 1991 Scientific Committee for Food, 1993 Institute of Medicine, 2001 FAOAVHO, 2001. 

Since then, eight more subjects have been studied (Sauberlich et al., 1974 Hodges etal., 1978). From these studies, the reference intake for adult men was set at 1,000 /rg of retinol equivalent, with a minimum physiological requirement of 600 /rg per day. Because the signs of deficiency only resolve slowly, it is possible that depletion/repletion studies overestimate requirements. 

An alternative approach to determining requirements is to measure the fractional rate of catabolism of the vitamin by use of a radioactive tracer, then determine the intake that would be required to maintain an appropriate level of liver reserves. As discussed in Section2.2.1.1, when the liver concentration rises above 70 /rmol per kg, there is increased activity of the microsomal oxidation of vitamin A and biliary excretion of retinol metabolites. The fractional catabolic rate is 0.5% per day assuming 50% efficiency of storage of dietary retinol, this gives a mean requirement of 6.7 /rg per kg of body weight and a reference intake of 650 to 700 /rg for adult men (Olson, 1987a). Reference intakes for vitamin A are shown in Table 2.4. 

Although there is some evidence that /3-cttrotene tmd other carotenoids may have actions in their own right, aptuT from their provittunin A activity (Section 2.6.3), there is no evidence on which to base any recommendations 

---

Tolerable Upper Levels of Intake A number of the vitamins are known to be toxic in excess. For most, there is a considerable difference between reference intakes that are more than adequate to meet requirements and the intake at which there may he adverse effects, although for vitamins A (Section 2.5.1) and D (Section 3.6.1) there is only a relatively small margin of safety. 

The total body pool of vitamin is of the order of 2.5 mg (1.8 Jmol), with a minimum desirable body pool of about 1 mg (0.3 J.mol). The daily loss is about 0.1% of the body pool in subjects with normal enterohepatic circulation of the vitamin on this basis requirements are about 1-2.5 Jg/day, and reference intakes for adults range between 1.4 and 2.0 Jg. 

Dietary vitamin D makes little contribution to status, and the major factor is exposure to sunlight, a conclusion that is supported by the two-fold seasonal variation in plasma calcidiol in temperate regions (see Table 3.2). There are no reference intakes for young adults in the United Kingdom and Europe for house-bound elderly people, the reference intake is 10 /rg per day, based on the intake required to maintain a plasma concentration of calcidiol of 20 nmol per L (see Table 3.5). This will almost certainly require supplements of the vitamin, because average intakes are less than half this amount. The U.S./Canadian adequate intake is 5 /xg per day up to age 50, increasing to 10 /xg between 51 to 70, and 15 /xg over 70 years of age (Institute of Medicine, 1997). 

Horwitt (2001) has criticized this high reference intake, noting that it was based on reinterpretation of the same data as had been provided to the committee in 1998, which set a lower RDA. Data were from studies that he had performed (Horwitt, 1960), andhe notes that to provide a diet rich in polyunsaturated fatty acids and low in vitamin E, the oils had been oxidized to remove vitamin E, and therefore contained large amounts of oxidized lipids that would increase apparent vitamin E requirements. 

More recent depletion/repletion studies, using more sensitive indices of status in which subjects were repleted with either a constant intake of vitamin Be and varying amounts of protein, or a constant amount of protein and varying amounts of vitamin Bg, have shown average requirements of 15 to 16 /xg per g of dietary protein, suggesting a reference intake of 18... 

Carr and Frei (1999b) reviewed studies of vitamin C intake associated with reduced risks of cancer and cardiovascular disease and suggested that, by this criterion, the average requirement was 90 to 100 mg per day, giving a reference intake of 120 mg per day. 

There is a school of thought that human requirements for vitamin C are considerably higher than the reference intakes discussed previously. Pauling (1970) measured the vitamin C intake of gorillas in captivity, assumed that this was the same as their intake in the wild (where they eat considerably less fruit than under zoo conditions), and then assumed that because they had this intake, it was their requirement - an unjustified assumption. Scaling this to humans, he suggested a requirement of 1 to 2 g per day. He also quoted the rate of... 

Pyridoxine (vitamin Be) requirements vary with protein intake the daily Average Requirement is 13 micrograms/ g protein intake and the Population Reference Intake is 15 micrograms/g protein intake. In adults this translates into a daily Average Requirement of 1.3 mg/day for men and 1.0 mg/day for women, and a Population Reference Intake of 1.5 mg/day for men and 1.1 mg/day for women. 

---