Why was this study done?

The proper macronutrient (protein, carbohydrate, fat) makeup of a meal is a hotly contested field of research. This study was designed to investigate the effects of a standard carbohydrate meal of approximately 58 grams, consumed along with varying amounts of protein 0, 15.8, 24.1, 33.6, and 49.9 grams to determine the effects of serum glucose and insulin.  

What did the study find?

Increasing protein loads produced a dose-dependent decreased glucose response, but higher insulin responses, when compared to the protein-free meal.

Effect of protein dose on serum glucose and insulin response to sugars
Authors: Spiller GA, Jensen CD, Pattison TS, Chuck CS, et al. Am J Clin Nutr. 1987 Sep;46(3):474-80.
PubMed Citation PMID: 3307373.
PDF Manuscript: Am J Clin Nutr, 1987.