Appetite responds to changes in meal content, whereas ghrelin, leptin, and insulin track changes in energy availability

Abstract

Context: It is uncertain how between-meal variations in energy availability and physiological changes in ghrelin, leptin, and insulin affect appetite.

Objective: The aim of the study was to examine the influence on human appetite of the meal size and its nutrient content or changes in energy availability and concentrations of ghrelin, leptin, and insulin.

Design: We conducted a crossover study manipulating meal size and energy availability through exercise energy expenditure and iv nutrient replacement (TPN).

Setting: The study was performed at a Clinical Research Center.

Participants: Ten healthy postmenopausal women (age, 59.7 +/- 1.5 yr; mean body mass index, 26 kg/m(2)) were studied.

Interventions: We conducted trials based on different morning meal size (418 vs. 2090 KJ), presence or absence of exercise energy expenditure (2273 to 2361 KJ), energy replacement by TPN (1521 to 1538 KJ), and a midday ad libitum meal.

Main outcome measures: Changes in hunger, fullness, midday ad libitum food consumption, and concentrations of ghrelin, leptin, insulin, and metabolic fuels were measured. We also performed midday meal tests for the presence of caloric compensation.

Results: Appetite was influenced by the size and energy content of the meals, but not by variation in energy availability which also did not trigger consummatory compensation. Exercise reduced hunger and increased fullness. Ghrelin, leptin, and insulin responded to changes in energy availability but not to meal size. Appetite was unaffected by physiological changes in ghrelin, leptin, or insulin.

Conclusions: During rest, appetite is influenced by the size and energy content of meals, but it bears no homeostatic relationship to between-meal changes in energy availability due to small meals, exercise, or TPN, or concentrations of ghrelin, leptin, and insulin.

Figure 1

The effects of variable meal size (left) and energy availability (right) on the psychophysical ratings of hunger (top) and fullness (bottom) in 10 postmenopausal women subjected to a sedentary trial with a large morning meal (SED-AL), or a small morning meal (SED-R), 2 h of moderate intensity after a large morning meal (EX), and iv nutrient infusion (TPN) as a replacement of energy withheld from a morning meal (SED-R-TPN) or expended through exercise (EX-TPN). Meal size had a negative effect on hunger (F_df4,36 = 39.3; P < 0.0001) and a positive effect on fullness (F_df4,36 = 115.3; P < 0.0001). Exercise energy expenditure had a negative effect on hunger (F_df4,36 = 25.5; P < 0.0001) and a positive effect on fullness (F_df4,36 = 42.8; P < 0.0001). TPN had no effect on the psychophysical ratings.

Figure 2

Effects of the morning energy availability (top) on the energy consumed during the midday meal (center) and the residual postmeal energy balance (bottom) in 10 women subjected to small (SED-R) or large (SED) morning meals, exercise (EX), and TPN (SED-R-TPN and EX-TPN). Midday meal did not compensate for the significantly lower energy balance in SED-R and EX trials (F_df4,45 = 77.13; P < 0.0001), which remained uncorrected after the meal (F_df4,45 = 10.17; P < 0.0001).

Figure 3

The effects of variable meal size (left) and energy availability (right) on the plasma concentrations of insulin (top), GIP (center), and glucose (bottom) under the experimental conditions described in Fig. 1. Insulin and glucose showed significant postprandial increases to meal size (left) and TPN (right) (F_df4,36 = 25.71; P < 0.0001), whereas GIP responded only to meal size (F_df4,45 = 42.29; P < 0.0001). Neither hormone responded to exercise energy expenditure. Postprandial increases in glucose concentration were unaffected by meal size or exercise.

Figure 4

The effects of variable meal size (left) and energy availability (right) on the plasma concentrations of total ghrelin (top) and leptin (bottom) under the experimental conditions described in Fig. 1. Plasma ghrelin concentration was rapidly influenced by energy availability as the increases in the small-meal (F_df4,36 = 13.23; P < 0.0001) and exercise trials (F_df4,36 = 5.28; P < 0.002) were abolished by TPN. Plasma leptin concentration slowly and progressively increased in response to reduced energy availability caused by small meal (F_df4,36 = 48.06; P < 0.0001) and exercise (F_df4,36 = 39.08; P < 0.0001), and this response was abolished by TPN.

Figure 5

The effects of variable meal size (left) and energy availability (right) on the plasma concentrations of FFAs (top) and β-hydroxybutyrate (bottom) under the experimental conditions described in Fig. 1. FFA and ketone body concentrations increased to exercise energy expenditure (F_df4,36 = 10.1, P < 0.0001; and F_df4,36 = 19.23, P < 0.0001, respectively), but not to the different meal size, and these increases were abolished by TPN.