THE EFFECT OF DIETARY CASEIN ON THE INDUCTION OF LUNG TUMOURS BY THE INJECTION OF 9,10-DIMETHYL-1,2-BENZANTHRACENE (DMBA) INTO NEWBORN MICE

Protein restriction (PR) and caloric restriction (CR) compared: effects on DNA damage, carcinogenesis, and oxidative damage

Protein restriction (PR) and caloric restriction (CR) similarly impinge upon various physiological factors that can significantly inhibit the growth of DNA-damaged tissue and, therefore, carcinogenesis. Whether this effect is largely, or only in part, due to simple inhibition of body weight gain is examined. Among their many other health-improving effects, PR and CR delay the onset of puberty. It has been suggested that animals have developed mechanisms to cope with lean periods and that, when food is limited, resources are diverted from those physiological functions that offer no benefit for immediate survival (e.g., reproductive capacity) to thereby support an increase in the maintenance functions that prolong life. PR has also been shown to affect numerous other varied mechanisms that can affect carcinogenesis, including gene expression and metabolism of xenobiotics. The effects of PR on initiational and promotional growth of DNA-damaged tissue is also discussed. PR also seems to boost antioxidant defenses and inhibit the accumulation of oxidative damage (as does CR). Protein restricted animals have been shown to accumulate more calories, but develop fewer preneoplastic lesions and tumors than their high-protein counterparts. This observation seems quite counter to most ideas about dietary restrictions and CR. Despite the fact that both PR and CR induce many beneficial physiological effects in common, it is possible that PR is the more feasible option for human consideration. The levels of PR likely to improve health without negative side effects are discussed.

Diet and nutrition research

The interest in nutrition and cancer, which was high in the 1940s and 1950s, was rekindled in the 1970s and is now more intense than ever. There has been considerable experimental work on dietary fat, but delineation of the precise role(s) of the essential fatty acids is still lacking. There have been few studies on protein or carbohydrate effects and only Visek has adequately addressed the important area of nutrient interactions. More work is needed in the fiber field with regard to the influences of short chain fatty acids, and the standardization of protocols is needed to make the various findings comparable. Currently, fiber data have been accrued using rats of different strains and gender, commercial and semipurified diets, a variety of colon carcinogens, and different routes of administration. The effects of energy restriction (in the literature since 1909) have not been widely studied as to mechanism and influence of energy expenditure (i.e., exercise) and merit more attention.

Influence of level of dietary protein on tryptophan-induced promotional activity in induction of gamma-glutamyl transpeptidase-positive foci of rat liver

The influence of varying the dietary protein content on the emergence of gamma-glutamyl transpeptidase (GGT)-positive foci in the livers of male rats fed elevated (2%) L-tryptophan (TRP) after being exposed to a hepatocarcinogen was investigated. Subtotal hepatectomies were performed, and 18 hr later the rats were treated with diethylnitrosamine (30 mg/kg). Ten days later four dietary groups were followed for 10 weeks: (1) control diet containing 21% protein (C); (2) control diet containing 5.3% protein (C-LP); (3) C + TRP; and (4) C-LP + TRP. Rats fed the C-LP diet developed heavier livers but fewer and smaller GGT + foci than did rats fed the C diet. Rats fed elevated TRP diets (C + TRP and C-LP + TRP) developed more and larger GGT + foci than did rats fed the regular control diets (C and C-LP), indicating that the promotional effect of elevated dietary TRP was similar at the two levels of dietary protein.

Dietary protein and experimental carcinogenesis

This review summarizes selected information about the influence of proteins, protein-fat interactions, and calorie intake on carcinogenesis. Most of the definitive studies concerning protein and cancer have utilized protein underfeeding and feed restriction. Optimal or less than optimal protein intakes have generally inhibited spontaneous and chemically induced tumor growth as well as the growth of transplantable tumors. Studies have focused on the quantity of protein and its amino acid supply rather than its source. Raising protein intake increases carcinogen metabolizing capacity, and the incidence of tumors depends upon the biologic activity of the metabolites that are formed. The few published studies dealing with the effects of protein on chemically induced colon, mammary, and liver cancers show that the incidence varied with the carcinogen and the level of protein fed at the time of carcinogen administration. With 1,2-dimethylhydrazine, a colon cancer-inducing agent, the toxic and tumorigenic responses have varied with the route of administration, the level of protein fed, and the level and duration of exposure to the carcinogen. In some instances, high protein diets may have led to a lower incidence of tumors because of depressed feed intake, a known confounding factor. The existing data about the relation of protein to cancer make generalizations about mechanisms hazardous because experimental models and protocols have varied widely. Some early studies undoubtedly used diets that lacked nutrients now known to be essential. Unfortunately, some recent studies have overlooked established nutritional principles and the known nutritional requirements appropriate for the age and species of animals used as models.

The carcinogenicity of polycyclic hydrocarbon epoxides in newborn mice

Benz(a)anthracene injected subcutaneously during the first 3 days of life caused a dose related increase in the incidence of liver and lung tumours in Swiss mice but over a similar dose range, the K region epoxide of benz(a)anthracene was less effective. Neonatally injected 7-methylbenz(a) was considerably more active than its K region epoxide in increasing the incidence of liver tumours in males. Both the parent compound and the epoxide slightly raised the incidence of lung tumours. Both chrysene and its K region epoxide increased liver tumour incidence but not lung tumour incidence. The K region epoxides of dibenz(a,h)-anthracene and 3-methylcholanthrene were without apparent effect on the incidence of liver, lung or other tumours despite indications from previously reported studies that the parent hydrocarbons are active at the same dose levels. The K region epoxide of phenanthrene had no effect on the incidence of any kind of neoplasm.