Diet, cholesterol metabolism, and atherosclerosis

Plasma cholesterol levels in rabbits fed low fat, low cholesterol diets: effects of dietary proteins, carbohydrates and fibre from different sources

Rabbits become hypercholesterolemic when transferred from commercial feed to a low fat, cholesterol-free semisynthetic diet. The role of different dietary components in mediating this effect was investigated by varying the composition of the semisynthetic diet and of the commerical feed. Addition of alfalfa to the semisynthetic diet prevented the normal hypercholesterolemic response, but other plant products, including several with high fibre content, were ineffective. Increasing the content of powdered cellulose appeared to enhance the response. A commercially formulated diet in which the alfalfa and soybean meal components were replaced by ground corn and oats did not produce a significant elevation of plasma cholesterol. Substitution of different sugars or starches for the dextrose in the semisynthetic diet gave variable results, but of those tested, only potato starch prevented the hypercholesterolemia. The type of protein used in the semisynthetic diet had a marked influence on the level of plasma cholesterol. Semisynthetic diets containing proteins from animal sources gave higher plasma cholesterol levels than those containing proteins from plant sources. Very low levels were obtained with a low choline semisynthetic diet containing soy protein isolate, and supplementation with choline and methionine only raised the level to that normally obtained with commercial feed. Replacement of the salt mixture in the semisynthetic diet by one specially recommended for rabbits made no significant difference to the hypercholesterolemic response. Prevention of coprophagy did not significantly affect plasma cholesterol levels in rabbits on either commerical or semisynthetic diets. Growth performance was generally better on commercial feed than on semisynthetic diets but there was no direct correlation between weight gain and level of plasma cholesterol in rabbits on the different semisynthetic diets.

The effects of some dietary changes upon the concentrations of serum lpids in rats

1. The responses of serum lipids of weanling and adult male rats to different diets have been examined. Total cholesterol, phospholipids and triglycerides were measured. The proteins used in the diets were casein and wheat gluten, the carbohydrates were starch and sucrose, and the main lipid source was groundnut oil supplemented with either gluten oil or maize oil.

2. In general, the type of carbohydrate did not influence the experimental results, whereas the type of protein had a marked effect. Compared with casein, gluten caused an increase in concentration of serum triglycerides, phospholipids and cholesterol in weanling rats, but there was no change in adult animals. Much of this ‘gluten effect’ in weanlings was not ob- served when the diet was supplemented with 0.7% L-lysine hydrochloride.

3. When 1% cholesterol was added to the gluten diets, the serum cholesterol concentrations were only slightly and variably affected in weanlings. Serum triglycerides and phospholipids were not altered in adult rats by the dietary cholesterol, whereas there were reductions in these lipid components in the weanlings, whether the protein was supplemented with L-lysine hydrochloride or not.

4. Gluten oil was not shown to be responsible for any major portion of the differences observed between gluten and casein in their effects upon the lipids of rat serum.