Dietary protein effects on lipoproteins and on sex and thyroid hormones in blood of rhesus monkeys

We studied plasma lipoprotein and hormone concentrations in rhesus monkeys that had consumed either a low protein (3.8% of kilocalories) or a control protein (13.9%) purified diet since birth (6-10 yr before the beginning of this experiment) in order to test the hypothesis that chronic deficiency could influence plasma lipoproteins through an effect on the hepatic metabolism of gonadal or thyroid hormones. Protein-deficient monkeys had greater plasma concentrations of very low density lipoproteins (VLDL) plus high density lipoproteins (HDL) than controls. The also had lower serum albumin and greater alkaline phosphatase levels than controls. Plasma thyroxine (T₄) and free T₄ concentrations were lower and the triiodothyronine (T₃) levels tended to be greater in the protein-deficient group than in controls. This effect was apparent at two widely different levels of dietary iodide. Plasma T₃ concentrations were elevated in other adult rhesus monkeys that were fed the low protein diet for only 6 wk. Monkeys injected with estradiol benzoate (100 μg/kg body weight) for 4 d had a marked reduction of VLDL concentrations. VLDL triglycerides were depressed more and plasma estrone levels were greater in deficient monkeys than in controls at 24 h after the last injection. In the control monkeys the T₃ level rose and T₄/T₃ fell in response to estrogen injection, whereas the deficient monkeys did not responsed.