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Proc Soc Exp Biol Med 2000 Sep;224(4):256-63

Low-protein diet changes thyroid function in lactating rats.

Ramos CF, Teixeira CV, Passos MC, Pazos-Moura CC, Lisboa PC, Curty FH, de Moura EG.

Departamento de Ciencias, Faculdade de Formacao de Professores de Sao Goncalo, Universidade do Estado do Rio de Janeiro, 20550-030 Rio de Janeiro, RJ, Brazil.

Lactating rats were fed with free access to an 8% protein-restricted diet (PR); the control group was fed a 23% protein diet (C). An energy-restricted (pair-fed) group was given the same food as the animals in the control group, but the amounts of food consumed by both PF and PR were about the same. The body weight and serum albumin concentration of PR and PF dams were significantly (P < 0. 05) lower than that of the controls. The PR group had a significant increase in serum-free triiodothyronine (FT3) concentration, 24-hr mammary gland and milk radioiodine (I131) uptake (67%, 278%, and 200%, respectively) as compared with the controls. On the other hand, those animals had a significantly lower serum-free thyroxine (FT4) concentration and 2- and 24-hr thyroid I131 uptake (67%, 64%, and 74%, respectively). Protein malnutrition during lactation did not alter thyroid or liver 5'-deiodinase activity significantly. However, PF dams had a significantly lower (25%) thyroid 5'-deiodinase activity. These data suggest that protein-restricted lactating dams had an adaptive change in the thyroid function, which could be important to increase the transference of iodine or triiodothyronine through the milk to their pups and prevent sequelae of neonatal hypothyroidism.
Proc Soc Exp Biol Med 1997 May;215(1):82-6

Dietary carbohydrate and fat do not alter the thyroid response to protein deficiency in chicks.

Carew LB, Alster FA

Department of Animal and Food Sciences, University of Vermont, Burlington 05405, USA.

Consumption of low-protein diets consistently causes elevations in circulating levels of triiodothyronine (T3) in several species of animals. In chicks this is often accompanied by lower levels of circulating thyroxine (T4). Since low-protein diets are usually formulated by replacing the detected protein with carbohydrate, the question arises as to whether the changes in thyroid hormones are a result of the lower protein or higher amounts of carbohydrate in such diets. Male broiler chicks, 13-26 days of age, were fed experimental diets that contained either an adequate level of protein (24%) or levels that were slightly (17%) or moderately (10%) deficient in protein. The deleted protein was replaced, isocalorically, with either glucose, soybean oil, or hydrogenated coconut fat. Though the level of protein and source of energy differed among diets, all diets contained identical amounts of all nutrients and energy, and were of similar weight densities. Circulating levels of thyroid hormones were measured from blood samples taken at the end of the study. Plasma T3 was elevated to a similar degree in all protein-deficient animals compared with control. Plasma T4 decreased in all protein-deficient chicks and was lowest with 10% dietary protein. Changes in circulating levels of thyroid hormones occurred independently of the source of dietary energy. Therefore, it is concluded that alterations in circulating levels of thyroid hormones that occur in chicks fed low-protein diets are a specific effect of the protein deficit and are not a related to the amounts of carbohydrate or fat present in the diet.

High Protein Diet Found Beneficial
From Dr. Mercola's site,

Contrary to what many conventional medical authorities, vegetarians, and other promoters of low-fat diets say, consumption of very high levels of protein may not have adverse effects and may in fact boost antioxidant levels, new research from Germany has found.

Since "The maximum dietary protein intake that does not cause adverse effects in a healthy population is uncertain," as the researchers note, they decided to test their theory that increased protein consumption would induce greater oxidative stress in order to determine this threshold of protein consumption at which adverse effects could be seen.

Researchers performed tests on laboratory rats, splitting them up into groups receiving one of three different levels of dietary protein:


  • 14% of total calories
  • 26% of total calories
  • 51% of total calories

After 15 weeks of feeding on the specific protein level diets, various parameters of antioxidant status were measured.

Much to the authors surprise, it was found that the groups consuming the higher protein diets had better antioxidant parameters than the lower protein diets, such as reduced lipid peroxide levels.

"Long-term intake of high protein diets did not increase variables of oxidative stress, in contrast to our initial hypothesis," the authors concluded. "An unexpected finding was that adequate (14%) protein feeding may in fact induce oxidative stress," they add.

Journal of Nutrition 2000; 130: 2889-2896