A Bomb Calorimeter You Are Not! And That's Why Even the "Adjusted" Energy Values on Food Labels Are 5% Off

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A couple of you will probably remember the SuppVersity news from ~1 year ago in which I discussed the revelation that nuts (pistachios and almonds) effectively deliver much less energy (-25%) than the "label" or databases on the Internet will tell you. In other words, while these healthy snacks are still extremely calorie dense, our bodies are not exactly as well equipped as a bomb calorimeter to use that energy.... "bomb calorimeter"? That reminds me of a study I read about a week ago and remembered, today, when I was thinking about writing an article on a topic that is not the 10214124x study on how aerobic exercise is good for elderly obese diabetics, or how extract XYZ from whatever TCM medicine ameliorates the weight gain in obese rodents.

The study, I was thinking of was conducted at the Department of Nutrition and Dietetics, VU University Medical Centre in Amsterdam (Wierdsma. 2013) and with it investigating the practically highly relevant ability of our tummy to extract and digest (=make bioavailable) the energy from the foods, we eat, it is truely one of a kind. After all, corresponding ...
"[...] reference values for energy and macronutrient absorption are scarce, especially for adults in an outpatient ambulatory setting, which forms the usual circumstances for dietetic and nutritional interventions or therapy." (Wierdsma. 2013)
That may seem hilarious in view of the bazillions of money we are spending year by year to figure out "why we are fat", but believe it or not, due to the lack of data we (=science) simply estimate the ‘standard’ energy absorption to be at a level of 95%.

"We use 95% of the energy we eat" - says who?

What's the significance of the data? It goes without saying that measuring the energy that is not absorbed is only one of the things that will eventually be necessary to elucidate "how much calories the average dietary carbohydrate-, fat- and protein calorie effectively delivers. This information would yet still have been something we'd have to have before the dozens of studies on the thermic and other metabolically relevant effects of foods had been conducted. Why? Well, at least in those cases (a non-negligible part of the currently available research), where this effect was estimated by simply monitoring the weight gain of the subject over longer time periods, the result critically depends on the amount of energy from carbs, fats and protein that did even make it into the participants organism. If the latter is not 95%, but only 90% and differs from macronutrient to macronutrient, all previous results would be skewed.
This assumption is based on a single study from the 1970s, in which the amount of non-absorbed healthy adults is reported to be approximately 5% when digesting a standard diet (Southgate. 1970). A study like the one at hand, which was designed to
"[...] assess faecal energy, subdivided in its major contributors of fat, protein and carbohydrate losses, to quantify standard intestinal absorption capacity in healthy adults on a Western European diet in an ambulatory setting in The Netherlands by using a feasible and unique methodology of intestinal absorptiometry reflecting routine practice." (Wierdsma. 2013)
Put differently, the scientists wanted to find out (a) whether a calorie is a calorie or maybe just half a calorie and (b) whether this relation is different for the three main macronutrients, carbohydrates, fat and protein. What? Yeah, you'd think we'd know that all along, but aside from certain experts who know exactly "why we are fat", we obviously don't.

So what did the scientists do?

The Dutch researchers recruited 25 healthy subjects who had been deliberately picked from the staff of institutional healthcare workers at the facilities the authors are working at all had specific dietetic and healthcare knowledge and were thus skilled in adequately registering nutrient intake and meticulously collecting stools. Yep, you read me right: Over the course of 4 subsequent days, the study participants did not just have to track everything they ate and drink, they also had to collect specimen from what left their bodies "undigested" (=stool samples ;-).
"All faeces were collected during 72 h (day 2–4), as per the protocol, in specifically designed 5-L buckets. Faeces were weighed (faecal wet weight in g/day ), homogenised and immediately stored at <4°C until analysis. To measure faecal macronutrient content and to calculate intestinal absorption capacity of the healthy subjects, the faeces were analysed for energy, fat and nitrogen content." (Wierdsma. 2013)
Based on the nutrition data and the stool samples, which had been recorded and collected by all, but two male lazi-a**es who failed to comply to the rigorous requirements of the study protocol the scientists determined the intestinal absorption capacity and the faecal production and composition of male and female subjects separately:
Figure 1: Relative energy and nutrient absorption in male and female subjects (Wierdsma. 2013)
As you can see in my plot of the data in figure 1, the mean (SD) intestinal absorption capacity (as a percentage of nutritional intake) was different for fats, proteins and carbohydrates, so that the average subject digested only
  • 89.4% (3.8%) of the total energy their meals provided,
  • 92.5% (3.7%) of the "fat calories",
  • 86.9% (6.4%) of the energy from protein and
  • 87.3% (6.6%) of the calories from the carbohydrates
in their meals (the values in the brackets denote the corresponding standard deviations). Accordingly, fats are not only the most energetically dense macronutrient, they are also the one healthy individuals digest best an with the lowest inter-personal differences. Overall, ...
"[w]omen had a statistically significantly lower energy absorption capacity compared to men (88.0% versus 91.8%, respectively, P=0.02). A similar trend was seen for fat and carbohydrate absorption, although this was not statistically significant (P=0.19 and 0.06, respectively)" (Wierdsma. 2013)
The latter may be surprising, after all, it's usually the women who complain they'd only have to 'look at a piece of cake to gain weightÄ and how unfair it was that men 'can eat whatever they want and still stay relatively lean' (obviously, the latter is not only a function of how much you eat, but also of how much energy you expend and the symbolic piece of cake may well satify 50% of a woman's, but only 25-30% of a man's daily energy requirements).

What goes in must go out again ;-)

Figure 2: Daily faecal production (g/day; x-axis) negatively correlated with intestinal energy absorption capacity (% of the energy intake; y-axis) (n = 23) (Pearson’s r = 0.46, P < 0.05 for the total group, r = 0.65, P < 0.05 for women and r = 0.71, P = 0.05 for men; Wierdsma. 2013).
The mean daily stool production for both sexes was 141 (49) g (29% dry weight) and amounted to an energy loss of 891 (276) kJ, while the individual contribution of  fats, protein and carbohydrates was 5.2 (2.2) g, 10.0 (3.8) g and 29.7 (11.7) g, respectively. Accordingly, the average nutrient contribution to faecal energy content was
  • 23% (10%) for fat,
  • 20% (8%) for proteins and 
  • 57% (23%) for carbohydrates.
With the stools of the female study participants containing a lower percentage of water than those of men (P<0.05), the energy content per gram of wet faeces was higher in women than in men (P<0.05), while the daily faecal nutrient losses were not statistically significantly different between men and women. Moreover, the scientists observed that ...
"[...]"the daily faecal production was positively correlated with faecal energy loss in kcal (Pearson’s r =0.80, P<0.001) [and negatively] correlated with intestinal energy absorption capacity (%) (Pearson’s r = 0.46, P<0.05)." (Wierdsma. 2013)
- an observation that means nothing else, but "the more you eat, the more inclined your body will be to have some energy pass through undigested"... or, in other words: Once you starve yourself, your body will try it's best use each joule of energy it can get! 

Bottom line: With the main results of the study at hand being that the average calculated standard for energy absorption in healthy Dutch adults is 90% and thus 5% less than it was previously assumed, it should be obvious that the already skewed "energy in vs. energy out" calculations you may have been conducting (against my explicit recommendation!) based on food labels and co are even less reliable than previously assumed.

"Accept There is No Magic Macronutrient Ratio" (read more)
On the other hand, the 5% more or less energy won't make a difference anyway, after all a main characteristic of our bodies is that they are no overtly simplistic bomb calorimeters, and their energy demands, as well as uptake depend on the total amount and composition (macronutrients, vitamins, minerals and other co-factors) of what we eat. If you want to gain or lose weight you will therefore not be able to forebear doing a 2-week food-log. Record all the foods and energy containing drinks you consume, sit down and (a) re-evalute the food quality, (b) take stock of the macronutrient ratios and (c) add / subtract foods, not calories or "energy" to establish a 20% caloric defict or 10% caloric surplus as a starting point for your diet / bulk. It's easy, it's bullet proof, but it's less convenient than relying on unreliable formula and databases.

  • Southgate DA, Durnin JV. Calorie conversion factors. An experimental reassessment of the factors used in the calculation of the energy value of human diets. Br. J. Nutr. 1970; 24:517–535.
  • Van de Kamer JH, Ten Bokkel Huinink H,. Weyers HA. Rapid method for determination of fat in feces. J. Biol. Chem. 1949; 177: 347–355.
  • Wierdsma NJ, Peters JH, van Bokhorst-de van der Schueren MA, Mulder CJ, Metgod I, van Bodegraven AA. Bomb calorimetry, the gold standard for assessment of intestinal absorption capacity: normative values in healthy ambulant adults. J Hum Nutr Diet. 2013 May 6.
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