In humans as in animals, genetic variation contributes to the basal plasma levels of lipids and lipoproteins (Friedlander et al. 2000).


Serum cholesterol is significantly below normal in infants with severe protein-calorie malnutrition (Schendel and Hansen 1958), and a relationship between clinical condition and the nonessential-to-essential amino acids ratio has been established (Whitehead and Dean 1964a, 1964b, 1964c). Terpstra et al. (1983a) have summarized the results of the first ten experiments investigating the effect of the amount of proteins, highlighting the fact that these data are sometimes conflicting because of the variety of experimental conditions, especially different dietary fat content and fatty acid composition and the presence or absence of cholesterol (Keys and Anderson 1957; Olson et al. 1958a; Furman et al. 1958; Lutz et al. 1959; Albanese et al. 1959; Léveillé et al. 1962b; Beveridge et al. 1963; Wilcox et al. 1964; Prather 1965; Elson et al. 1971). One study reported that, in middle-aged people, supplementing dietary proteins (from 8.6% to 17% of daily energy intake) with skimmed milk powder produced no effect (Keys and Anderson 1957), whereas in another study, if milk concentrates were added to the diet of elderly people, a moderate increase in body weight was observed. Furthermore, a reduction in serum cholesterol concentrations was observed more frequently than any increase (Albanese et al. 1959). In men, a diet containing 25 g of vegetable proteins increases the molar ratio of nonessential to essential amino acids from 1.82 to 2.90. Plasma alanine, glycine, and asparagine-glutamine concentrations are increased, whereas plasma lysine, threonine, valine, and leucine levels are decreased (Garlich et al. 1970), although to a lesser degree than in kwashiorkor (Holt et al. 1963).