A new paper published in The American Journal of Clinical Nutrition looked at how much energy we eat ends up in the feces and urine, rather than being used for our bodies.
12 subjects consumed dye-labeled meal replacement bars to see how much ended up not being absorbed. Of the energy consumed, about 10.48% of energy intake ended up being excreted (not absorbed), with ~8% being in the feces and 2.5% being in the urine. As with nearly all things related to humans, there was significant variation between individuals, ranging from 6 to 15% of total energy intake, which is about a 200 Calorie difference from lowest to highest. That’s big!
For some background, previous studies have looked at fractional energy absorption (FEA), which is the proportion of energy consumed to energy absorbed. On average, healthy individuals absorb an average of 90-95% of the energy contained in food, with the rest being lost in the feces (~5%) and urine (~0.5%). Controlled feeding studies have shown that individuals’ energy absorption can range from 80-95%. Overfeeding seems to increase the fraction of energy lost in the feces and urine in absolute terms, although there is significant variability between individuals. These differences may partially contribute to the substantial differences in weight gain among individuals during overfeeding.
Less is known about variation in FEA during low energy availability. I would predict some variance (because humans), but preserving bodily function is more important for survival compared to preventing body fat gain. Between that and the relatively high efficiency already, I would suspect less variance at increasing absorption.
Anyway, just some interesting fodder. What do you think?
