More recently, immune function has been used as a dependent variable to help to determine proper vitamin E nutriture. In a rat model, Bendich et al. (1986) showed that vitamin E concentrations required for optimal T- and B-lymphocyte responses to mitogens were greater than 50 mg·kg-1 of diet, whereas 7.5 mg·kg-1 and 15 mg·kg-1 of diet were sufficient for normal rates of growth and prevention of red-cell hemolysis, respectively. In a randomized, double-blind, placebo-controlled intervention study in healthy elderly human subjects fed a placebo or vitamin E at 60, 200 or 800 mg·d-1 for 235 days, Meydani et al. (1997) were able to demonstrate that at least 200 mg·d-1 were needed to enhance in vivo indexes of T-cell-mediated function. That dosage is about 10-12 times higher than the 15-19 mg·d-1 currently recommended for adult humans (Institute of Medicine, 2000). A careful examination of the immune response, as reflected in a dose-response experiment with vitamin E, has not been conducted in nonhuman primates.
A number of studies have provided evidence that vitamin E metabolism or requirements might vary among species. The New World monkey Cebus albifrons appeared to develop vitamin E deficiency twice as fast as an Old World species Macaca fascicularis when the two species were fed identical diets (Ausman and Hayes, 1974). The cause of the greater sensitivity of the cebus monkey than the cynomolgus monkey to vitamin E deficiency in this study was not established. Ghebremeskel et al. (1990) observed that common marmosets exhibit higher erythrocyte hemolysis and lower plasma a-tocopherol:cholesterol ratios compared to humans at equivalent plasma a-tocopherol concentrations of 10 mg·L-1. Some karyotypes of owl monkeys (Aotus trivirgatus) developed a hemolytic anemia and cardiomyopathy that were ameliorated with intramuscular vitamin E and selenium injections (Sehgal et al., 1980; Beland et al., 1981; Meydani et al., 1983). Further investigations into the mechanism of this apparent vitamin E-deficiency anemia indicated that susceptible Aotus had no change in activity of the glutathione peroxidase system (Brady et al., 1982; Meydani et al., 1982). However, they did have decreased concentrations of PUFAs and increased cholesterol concentrations in their erythrocytes, leading to a markedly increased free-cholesterol:phospholipid ratio in red-cell membranes (Walsh et al., 1982). That presumably made the erythrocytes more susceptible to hemolysis. Susceptible Aotus monkeys suffered from chronic enteritis and inflammatory bowel disease (Meydani, 1983), which might have led to decreased absorption of PUFA, vitamin E, and cholesterol and later abnormal cholesterol metabolism and decreased cholesterol esterification (Mickel et al., 1975). The anemia observed in some Aotus might also be secondary to genetically determined dietary allergies and an associated malabsorption.
Table 7-1 is a summary of individual studies in which nonhuman primates were fed one or more diets in an attempt to assess vitamin E requirements. Studies in which only a deficiency was produced without an estimation of requirements are omitted. Vitamin E requirements are reported or calculated as a-tocopherol both in mg·kg-1 dietary DMand in mg·BWkg-1·d-1.
For the Old World macaques and African green monkeys fed diets that did not contain large amounts of n-3 fatty acids (fish oils), minimal dietary requirements were variously estimated to be 3.2, 5-10, 12, less than 50, less than 60, or 87 mg·kg-1 of DM. Dinning and Day (1957) showed that 333 mg·kg-1 of dietary DMwas more than enough to cure vitamin E-deficiency anemia. In the short term, with one exception, a-tocopherol at 50 mg·kg-1 dietary DM appears to be a reasonable estimate of the requirement on the basis of published data. In relation to body weight, the vitamin E requirement appears to be about 3.0 mg·BWkg-1·d-1.
The New World monkeys that have been studied include Cebus albifrons and Callithrix jacchus. The minimal dietary requirements of the former were estimated to be about 3.0 mg·kg-1 of DMand of the latter 4-48 mg·kg-1 of DM. When fish oils were included in the diet, vitamin E requirements appeared to be greater than 95 mg·kg-1 of DMbut certainly less than the one dose of 1,600 mg·kg-1 of DMthat was used. In relation to body mass, Cebus albifrons appeared to require -tocopherol at least at 0.165 mg·BWkg-1·d-1, and Callithrix jacchus at 0.4-4.7 mg·BWkg-1·d-1. When fish oils were added to the diet, the estimate increased to something less than 14 mg·BWkg-1·d-1.
All the above estimates should be used with caution because of uncertainty about the relative biologic activity per unit of weight of all-rac-a-tocopherol vs RRR-a-tocopherol and because the forms of tocopherol used in some of the published studies were not identified. In addition, many observations in other animals have shown that vitamin E requirements for support of optimal immune function are higher than for prevention of clinical signs of deficiency.
Vitamin K is the collective name for compounds with a 2-methyl-1,4-napthoquinone nucleus and a lipophilic side chain (attached at carbon 3) that have antihemorrhagic activity. The principal active compound in higher plants is phytylmenaquinone (phylloquinone, or vitamin K1) with a 20-carbon phytyl side chain. Prenylmenaquinones (menaquinones, or vitamin K2) are compounds with polyisoprenyl side chains of varied length, generically designated menaquinone-n (MK-n). Those produced by bacteria have side chains with seven to 13 unsaturated isoprenyl units and are designated menaquinone-7 to menaquinone-13 (MK-7 to MK-13). The synthetic provitamin menadione (formerly known as vitamin K3) has no side chain but can be alkylated