Background: Numerous online searches, textbooks and other sources seem to pin the average length of the human gut from mouth to anus (oroanal) between about 5-10m in length. To pick a reputable example, this page from Harvard suggests that the small intestine is typically 2.91m and the average large intestine is 1.90m. The paper they cite (Helander & Fändriks, 2014) suggests this makes up most of the ~5m of the average human gut. Helander & Fändriks (2014) performed a lit review to come to these numbers, so their average lengths are probably some of the best estimates we have. (Other sources can be seen here and here, and Raines et al. 2014 seem to suggest that intestines of living people are much longer).
Regardless, Helander & Fändriks (2014), similar to Raines et al. (2014) and other sources, comment that there is "considerable variation between individuals" throughout the literature they reviewed.
My question: is the length of a given human gut predictable by the diet of their ancestors? In other words, are gut lengths between individuals more similar between family members and their ancestors than for others whose ancestors lived elsewhere in the world (and presumably had an alternative diet)?
- For example: Are the guts of people of western European descent more similar to each other than the gut lengths of those of East Asian descent (or Sub-saharan descent, etc.).
Reasoning: Gut length (& complexity) of different animals varies based on digestive needs (which are a result of the typical diet of the animal). The general assumption (with support) is that diets high in food with indigestible material results in increased gut dimensions.
See Naya et al., 2009 for a general discussion about "gut flexibility", list of review citations, and results indicating correlation of population (diet) with gut length in toads.
Davis et al. (2013) found strong evidence that diet and intestinal complexity were linked in terapontid fishes -- specifically, more complex guts seemed to be associated with shifts away from carnivorous diets.
Gut plasticity does not seem to be immediate. For example Ferraris & Diamond (1993) found (unsurprisingly) that intestinal length was independent of diet in a week-long mouse study.
Discussion of variation in human diet can be found here.
I haven't been able to find any sources discussing this "gut flexibility" phenomenon in humans.