Effect of Lamb and Soy Protein on Postprandial Glycaemic, Insulin and Appetite Responses to Potato, Pasta and Rice in Macronutrient-Equalised Meals—A Randomised Pilot Human Intervention Study

Background: Plant and animal proteins have been shown to affect postprandial glycaemic, insulin, and appetite responses to carbohydrate meals. Here we compared lamb protein and soy protein effects on glycaemic response, insulin secretion and appetite, examining whether the protein source modified the relative glycaemic effects of different carbohydrate foods, and whether lamb and soy protein differed in their effects. Methods: Healthy participants (n = 15) were randomised to consume meals containing potato, pasta, or rice, either alone or combined with equal amounts of lamb or soy protein, and with lamb fat content equalised across meals. Blood glucose and insulin responses were measured over the postprandial period, and subjective appetite was assessed. Results: Both lamb and soy protein reduced postprandial glycaemic responses and increased insulin responses compared with the carbohydrate staples consumed alone. Soy protein induced a slightly lower glycaemic response than lamb protein (132 ± 18 vs. 144 ± 13; 103 ± 16 vs. 123 ± 19; 112 ± 12 vs. 130 ± 17 mmol/L.min, potato, pasta and rice, respectively), although differences were small and not consistently significant. Adding protein also reduced the differences in glycaemic responses between the carbohydrate staples, particularly diminishing the higher response to potato. Both protein sources suppressed appetite to a similar extent relative to the carbohydrate-only meals. Conclusions: Adding protein to mixed meals had a dominant effect on postprandial glycaemic, insulin, and appetite responses, whereas the specific protein source (lamb versus soy) had a minor influence. These findings indicate that, under the conditions of this study, both animal and plant proteins were effective in moderating postprandial glycaemia and enhancing satiety.