Soy protein, thyroid regulation and cholesterol metabolism.

The effects of dietary protein on plasma cholesterol concentrations are well documented: animal proteins (casein) are hypercholesterolemic compared with plant proteins (soy protein). Although this effect of protein source on plasma cholesterol has been shown in many species, the mechanism is not completely understood. This paper reviews the relationship between dietary protein source and plasma thyroxine concentration. The basic premise is that feeding soy protein lowers plasma cholesterol concentration by causing an increase in plasma thyroxine concentrations. The metabolic changes involving cholesterol that occur when soy protein is fed are discussed. These changes are consistent with changes induced by elevating thyroxine. Data are presented from animal studies showing that feeding soy protein to laboratory animals consistently elevates plasma thyroxine concentrations. Furthermore, this elevation in plasma thyroxine concentrations precedes the change in plasma cholesterol concentrations: a necessary requirement for hypothesizing a causative effect. Possible mechanisms as to how a dietary protein source affects plasma thyroxine are also presented.

Dietary proteins, cholesterol and thyroxine: a proposed mechanism.

The effects of dietary protein sources on plasma cholesterol concentrations are well-documented: animal proteins are hypercholesterolemic relative to plant proteins. While this effect of dietary proteins has been shown in many species, the mechanism is not known. This review will explore the relation between dietary proteins and plasma thyroxine concentrations. Data will be presented showing that feeding dietary animal proteins consistently depresses plasma thyroxine levels. Furthermore, the effects of thyroxine on cholesterol metabolism is consistent with the effects of dietary proteins on cholesterol metabolism. Although evidence is not conclusive, data support the hypothesis that dietary proteins may affect plasma cholesterol levels through changes in plasma thyroxine concentrations. To elucidate the mechanism by which this happens will be the basis for future research.

Neuroendocrine responses of type A individuals to exercise.

Ten Type A and 10 Type B individuals exercised for 20 minutes on a bicycle ergometer at 40%, 60%, and 80% of maximal capacity to determine if differences in neuroendocrine reactivity exist. Pre-exercise plasma concentrations of beta-endorphin and epinephrine were similar for Type As and Type Bs. Pre-exercise plasma levels of norepinephrine tended to be higher for the Type As (p less than 0.07). Post-exercise plasma epinephrine concentrations were similar for As and Bs for all trials. The 40% and 60% trials resulted in no differences in post-exercise norepinephrine and beta-endorphin levels for the Type As and Bs. Conversely, the 80% trials resulted in significantly greater norepinephrine and beta-endorphin concentrations for the Type As (p less than 0.05). Plasma serotonin levels at rest and during exercise were always lower for the Type As (p less than 0.05). These results suggest that our Type As had a greater neuroendocrine response to high-intensity exercise than our Type Bs. The greater reactivity and analgesia may allow the Type A person to suppress feelings of fatigue, thus enduring higher levels of exertion for longer periods of time.

Exercise intensity-related responses of beta-endorphin and catecholamines.

Ten men and 10 women exercised on a bicycle ergometer for 20 min at 40, 60, and 80% maximal oxygen uptake (VO2max) to determine the relationship between plasma beta-endorphin, catecholamines, and exercise intensity. Compared to rest, plasma beta-endorphins were not significantly elevated during the 40 and 60% workloads (4.8 +/- 1.0 pmol.l-1 vs 3.8 +/- 0.7 and 6.3 +/- 0.9, respectively). In contrast, the 80% exercise significantly elevated endorphins to 16.1 +/- 4.0 pmol.l-1. Plasma norepinephrine concentrations were 0.30 +/- 0.04 ng.ml-1 at rest and increased with exercise intensity (40% = 0.60 +/- 0.05, 60% = 0.93 +/- 0.07, 80% = 2.00 +/- 0.14, VO2max = 2.55 +/- 0.14 ng.ml-1). Plasma epinephrine followed the same trend (rest = 0.07 +/- 0.01, 40% = 0.33 +/- 0.03, 60% = 0.49 +/- 0.02, 80% = 0.88 +/- 0.07, VO2max = 0.95 +/- 0.06 ng.ml-1). Norepinephrine was found to significantly correlate to endorphins (r = 0.499; P less than 0.02). Conversely, epinephrine was not correlated with beta-endorphin (r = 0.309; P greater than 0.05). The low correlation suggests a weak relationship between beta-endorphin and catecholamine responses during exercise. The results of this investigation suggest that the relationship between beta-endorphin and exercise intensity is curvilinear, with anaerobic activity producing the most significant endorphin response. It was also noted that the beta-endorphin response was not related to gender, but the amine response to exercise was gender-related, being greater for the men.

Influence of caffeine on exercise performance in habitual caffeine users.

The effect of caffeine on the exercise responses of six women habituated to caffeine (greater than 600 mg/day) was examined during 1-h running at 75% VO2 max on a motorized treadmill. Each subject completed a placebo (PL) and a caffeine ingestion (CC) trial while maintaining normal caffeine intake. The subject then abstained from caffeine for 4 days and again ran after receiving caffeine (CW). Caffeine dosage for all trials was 5 mg/kg body weight. Ingestion of caffeine after withdrawal (CW) resulted in the greatest physiologic effects. Exercise oxygen uptake was significantly elevated by 0.17 l/min over the PL and CC trials (P less than 0.05). The CW trials resulted in an overall R value of 0.79 +/- 0.04 compared with 0.85 +/- 0.08 for the PL and 0.83 +/- 0.04 for the CC trials. Caffeine had its greatest effect on the resting free fatty acid levels after withdrawal: 1104 +/- 425 mu Eq/l compared with 543 +/- 288 for the PL and 839 +/- 526 for the CC. Postexercise lactates were similar for all trials. Post-exercise plasma norepinephrine and dopamine were the highest after the CW trials. The results suggest that habitually high caffeine users acquire a tolerance to caffeine which reduces its effects during prolonged exercise. Furthermore, to magnify the effect of caffeine, habitual users should withdraw from caffeine use for about 4 days.

Comparison of dietary casein or soy protein effects on plasma lipids and hormone concentrations in the gerbil (Meriones unguiculatus).

The effects of dietary animal protein (casein) or soy protein (soy isolate) on plasma lipids and hormones were investigated in the gerbil. Diets, fed to male gerbils (initial weight, 60 g) for 4 wk, contained either 18% casein or soy isolate as the protein source. The dietary fat sources were lard (16%) and safflower oil (1%). The cholesterol content of the diet was 0.1%. Plasma total cholesterol concentrations were lower in gerbils fed the soy protein diet (159 mg/dl) than in the gerbils fed the casein diet (190 mg/dl). Absolute HDL-cholesterol concentrations were unaffected by the protein source, but LDL-cholesterol concentrations were lower in the soy-fed gerbils. Thus, the ratio of LDL to HDL cholesterol was lower in the soy-fed gerbils (0.42) compared with the casein-fed gerbils (0.70). Plasma insulin levels were higher in the soy-fed gerbils as were plasma thyroxine and thyroid-stimulating hormone levels. The results indicate that the gerbils can be used to study dietary effects on cholesterol parameters. These data also suggest that changes in plasma thyroxine levels may in part account for the hypocholesterolemic effect of soy protein.

Dietary protein effects on cholesterol and lipoprotein concentrations: a review.

Different dietary proteins exert different effects on plasma cholesterol concentrations. Animal studies have shown that animal proteins, most notably casein, increase plasma total cholesterol concentrations compared with vegetable proteins, such as soy. Soy protein has been shown to be hypocholesterolemic in rats, swine, primates, and rabbits. Epidemiologic studies have disclosed that vegetarians have lower mean plasma cholesterol concentrations than populations consuming diets of mixed proteins, but it is unclear whether this effect results specifically from the animal or vegetable nature of the protein. In human clinical experiments, substituting soy protein for mixed protein reduces plasma total cholesterol concentration in hypercholesterolemic subjects, but it causes only a small, nonsignificant change in persons with normal plasma cholesterol concentrations. The mechanism responsible for the effects of different proteins on plasma cholesterol concentrations has not been established. One hypothesis suggests that animal proteins, which have a greater content of phosphorylated amino acids than vegetable proteins, interfere with bile acid reabsorption. Another hypothesis suggests that the amino acid content of the protein affects cholesterol absorption, tissue storage, synthesis, and excretion. The dietary protein may also alter cholesterol metabolism by affecting plasma hormone concentrations, either postprandially or over weeks to months. Among the hormones thought to be affected by dietary protein source are insulin, glucagon, and thyroid hormones. Gastrointestinal hormones, such as gastrointestinal inhibitory polypeptide, may also be affected by dietary protein.

Responses of endurance-trained subjects to caloric deficits induced by diet or exercise.

The purpose of this study was to examine the effect of 7 d of caloric deficit (1,000 kcal X d-1) induced by diet or exercise on weight loss and exercise performance of six endurance-trained males. The diet of each subject was controlled during the weeks preceding the dietary restriction and exercise to normalize nitrogen balance. Weight and blood chemistries were monitored daily. Submaximal and maximal exercise responses were tested at the end of each week. Weight loss during the exercise week was 0.76 kg, significantly less (P less than 0.05) than in the dietary restriction week (-2.16 kg). Seven d of cumulative nitrogen loss was greater (P less than 0.05) during the diet week (-24.5 g) than the exercise week (-11.1 g). Resting hematocrit, hemoglobin, total plasma protein, and albumin were significantly reduced during the exercise week compared to all other weeks. Maximal exercise capacity, measured as VO2 (61 ml X kg-1 X min-1) or duration of exercise, was not affected by either method of caloric deficit, but R values and lactates were lower than controls for both dieting and exercise deficits. The results suggest that endurance-trained individuals lose weight more slowly and conserve more protein when using exercise to induce a caloric deficit compared to dietary restriction. The results also suggest the possibility of the sequestering of blood proteins to supplement muscle protein synthesis during periods of exercise-induced caloric deficits.

Effects of dietary protein and fat sources on plasma cholesterol parameters, LCAT activity and amino acid levels and on tissue lipid content of growing pigs.

Young male pigs were used to examine effects of dietary protein and fat sources on plasma cholesterol parameters. Diets providing 16 and 42% of metabolizable energy from protein and fat, respectively, were fed for 12-14 weeks. Protein was derived either from plant sources (50% from soybean meal and 25% each from corn and wheat) or from animal sources (90% from casein and 10% from lactalbumin). The polyunsaturated to saturated fat ratio in the diets averaged 3.0 in the polyunsaturated fat diets and 0.3 in the saturated fat diets. Cholesterol content of the four experimental diets (plant protein-polyunsatured fat; plant protein-saturated fat; animal protein-polyunsaturated fat; and animal protein-saturated fat) was 0.6 mg/kcal. Consumption of diets containing plant protein rather than animal protein reduced total plasma cholesterol levels by 50 mg/dl; high density lipoprotein (HDL) cholesterol levels were also lowered in pigs fed plant protein. Similarly, plasma cholesterol levels were approximately 40 mg/dl lower in pigs fed the polyunsaturated fat diets than in pigs fed the saturated fat diets. HDL cholesterol levels, however, were unaffected by source of fat fed. These results show that the hypocholesterolemic action of the plant proteins was as great as the hypocholesterolemic action of polyunsaturated fat and that consumption of plant proteins rather than animal proteins resulted in lower plasma cholesterol levels regardless of whether polyunsaturated or saturated fats were fed.