Have there ever been conducted tests on how fast dogs digest various foods? That is, to determine the glycemic load of various foods with respect to dogs?
Yes, but it's complicated. The glycemic index of dog foods depends on various variables, like which carbohydrates are contained in it and how much fiber and which type(s) of fiber the food contains.
This study compared the glycemic indices of different dog foods with starches from different sources:
The GI [glycemic index] (±SE) [standard error] of the single starch sources and dog foods was: white bread: 47 ± 11, cooked white rice: 71 ± 14, cooked green lentils: 60 ± 20 (P = 0.569), traditional grain: 83 ± 17, whole grain: 56 ± 8, grain-free: 41 ± 6, and vegan: 65 ± 15 (P = 0.154). No statistical differences in glycemic response over time were observed between the single starch sources or the extruded diets tested (P = 0.1412; P = 0.2651). The insulinemic response elicited by the extruded diets was also not different (P = 0.079); however, the traditional grain diet did have the slowest time to peak for insulin (P = 0.0078).
This study also compared how fast different starches raised the blood sugar level:
Dogs’ immediate post‐prandial glucose and insulin responses (AUC ≤ 30 min) were greater for brewer’s rice, corn, and cassava flour diets (p < 0.05), and later meal responses (AUC ≥ 30 min) were greater for sorghum, lentil and pea diets (p < 0.05).
This study even compared the metabolism of carbohydrates in cats and dogs:
Two studies were designed to determine the research objectives:
• Determine species metabolic differences (dog versus cat) to both low and high glycemic index carbohydrate sources, and link these to postprandial cardiovascular and methylglyoxal results;
• Compare long-term health benefits of diets formulated with pulse starch to a modified cornstarch diet in dogs versus cats, including digestibility, glycemic, insulinemic, and cardiovascular responses, and postprandial methylglyoxal levels.
It found no significant differences in how cats and dogs metabolize carbohydrates.
Another study looked at different fiber contents of dog food and measured the effects on diabetic dogs.
Mean postprandial plasma glucose concentration measured every 2 hours for 24 hours, beginning at the time of the morning insulin injection, was significantly (P less than 0.05) lower at most blood sampling times in dogs fed IF [diet containing 15% insoluble-fiber] and SF [diet containing 15% soluble-fiber] diets, compared with dogs fed the LF [low-fiber] diet.