The effect of dietary energy concentration on the hormone profile and lipid metabolism in growing pigs

Growing pigs, as a model for humans, (2 groups × 7) were fed diets with high or low levels of energy and protein. The diet low in energy and protein decreased insulin, leptin, thyroxin levels but did not decrease concentrations of glucose, triacylglycerols or cholesterol. A reduced supply of energy and protein can decrease the levels of anabolic hormones in a growing organism.


INTRODUCTION
The life span of animals and humans can be increased by energy restriction operating via decreasing the metabolic rate and reduction in body fat content.The quantity and quality of body fat regulates the metabolism of fatty acids, which determines expression of genes encoding the enzymes involved in the ß-oxidation of fatty acids, lipoprotein and hormone-dependent lipolysis, synthesis of cholesterol, as well as proliferation of thyroid gland cells and secretion of triiodothyronine (T 3 ) (Raclot and Oudart, 1999).Food restriction modifies the endocrine and nervous system, influencing the metabolism of the entire organism.However, a suf-ficient energy supply is necessary for ATP synthesis required for optimal growth and development.The question is to what extent the pro-aging dietary effects decrease genetically defined capacities for growth and development and whether increasing the life span of an adult requires a reduced rate of growth and development of a young organism.
The aim of this study was to evaluate the influence of high and low energy and protein supply on the level of selected anabolic hormones and indices of lipidcarbohydrate metabolism in growing pigs as a model for humans.

MATERIAL AND METHODS
Pigs (Landrace × White Large, 30 kg body weight) were divided into two groups (2 × 7 animals) and housed in individual cages for 100 days.The pigs were fed ad libitum with diets with high (H) and low (L) energy and protein concentrations (Table 1).At the end of the experiment the animals were fasted for 12 h and blood was sampled from the heart.Triacylglycerols (TG), total

RESULTS
The levels of insulin, leptin, and thyroxin (T 4 ) increased significantly and there was a tendency towards increasing T 3 in pigs fed the high energy and protein diet (Table 2).The levels of TG and total cholesterol were not significantly different between diets.However, diet L decreased the concentration of HDL cholesterol and increased the concentration of VLDL cholesterol in serum.As expected, the body gain of pigs fed diet L were lower than on diet H.

DISCUSSION
The concentration of insulin in blood serum depends on the intake of dietary energy and protein.A high feeding level increases the insulin concentration in the serum of pigs (Fernandez-Figares et al., 2003).Insulin elevates utilization of glucose for TG synthesis in adipocities and consequently, increases the release of leptin from them.In this investigation, feeding restriction was the reason for decreasing the concentration of insulin and leptin in the serum of pigs.
Da Costa et al. (2004) found that diets with reduced energy and protein contents increased potentials for substrate (protein, glycogen and lipid) turnover and mitochondrial function in growing pigs.This mechanism is probably responsible for sustaining a constant level of glucose and TG despite decreased insulin and leptin concentrations.A decreased level of glucose has often been discussed as an important factor in the anti-aging action of food restriction (Fu at el., 2004).However, it is likely that the amount of glucose available for cell metabolism, but not its serum level, plays a key role in pro-aging activity.Consequently, the level of insulin may be an important marker of aging processes.The low level of dietary energy and high level of fibre in diet L did not decrease TG and cholesterol concentrations (total and LDL), but decreased the T 3 , T 4 and HDL-cholesterol concentrations in growing pigs.A key role in changing the serum lipid profile is played by the synthesis of apolipoproteins, which can be modified by the concentration of insulin and leptin (Tso et al., 1999).

CONCLUSIONS
The restriction of energy and protein supply for growing pigs (as a model for young humans) had negative effects on insulin and leptin levels in serum without altering glucose and TG concentrations.The insulin level may be considered a key marker of the pro-aging properties of a diet.

Table 1 .
Composition and nutritional value of the experimental diets, %

Table 2 .
Level of hormone and lipid metabolism indices in blood serum of pigs fed diets with low