It occurs to me that I haven't seen this question addressed anywhere: Is everyone's digestive system, at all times and under all conditions, equally efficient at extracting energy from food? (Rhetorical question.)
To illustrate with an extreme example, lactose is a sugar and carries energy, but some people can digest it (and their bodies will get the full complement of calories from it), and some people can't (it'll get digested by bacteria and they'll get gas and feel unpleasant; I'm pretty sure they don't get calories from it). It is incorrect and stupid to say that an item of food with significant lactose component carries "N calories" without specifying whether the person who eats it can digest lactose. Likewise, with cellulose, I think cows get a lot more energy out of cellulose than humans can.
Also, it seems likely to me that people's digestive systems' performance varies with "outside" conditions in the body. For example, I've heard that being pumped full of adrenaline puts digestion on hold; and subjective experience suggests to me that if you eat a lot of food (over a period of, say, a day), your system will send it through pretty quickly, without digesting it fully.
I suspect there are a lot of other situations in which one's body doesn't get all N calories from an "N-calorie" item of food. I suppose that if that happens, then the remaining part probably will be digested by bacteria, and one may notice symptoms of... oh, hey, there's a name for this, "indigestion". (Though if the amount of left-over energy is really small, then the symptoms are probably too slight to notice. If 10 or 20 calories a day matters, then this may make things unpredictable.)
I suppose one could count the bacteria digesting the food that you didn't as "calories burned", but then that means you may be burning a bunch of calories without doing any physical activity.
Does someone more knowledgeable than me know a) whether these things are really significant and b) whether these researchers take them into account?
It is pretty clear that digestion varies dramatically from person to person, and from moment to moment for the same person. You hit upon two major things in your question; what flora are in your gut, and how the rest of your body is.
Anecdotally I've noticed I can change my stomach on purpose by what I eat, and rebalance the flora. For example I used to eat Tums all the time to deal with acid indigestion. Then I did a 3 day fast at the suggestion of my mother in law, and found I was free of indigestion for months after that. Now whenever I find myself getting heartburn more than a couple times I do a fast and it clears up for awhile.
There is increasing evidence the balance of flora also contributes to how you process food, and can contribute to greater development of fat when out of whack.
I also often wonder about the indigestion issue. A few years ago I moved house and found a fantastic Indian kebab shop a five minute walk away. Most evenings consisted of having a kebab and maybe a couple of cans of beer as well. The morning's indigestion was unpleasant but worth it for the tastiness. The real fun was that I was slowly losing weight on this fantastically unhealthy diet, something that the girls I was living with hated me for after seeing what went in :)
I think you're right about the concept of sharing energy with gut microbes. I calculate that a 160 pound man would lose about 250 calories to gut microbes each day if he were consuming sufficient protein, fat, and minerals to sustain efficient gut microbe multiplication activity. A diet poor in supportive nutrition would curtail gut microbe activity resulting in more efficient energy absorption into the bloodstream. That is, a higher proportion of the energy consumed would be available for metabolic needs and fat storage. In my own experience, if I consume extra calories, I just have more bowel movements and don't gain any weight unless the extra calories are carbohydrate.
One thing I've always done is if I slip up and eat a lot more than I wanted to during the course of a day (e.g., big lunch, big dinner) I'll go ahead and keep eating a whole lot the rest of the day.
I figure my body can only store so much fat in one day so any additional calories won't hurt. I've never seen this idea proven or disproven anywhere but logically it seems to make sense to me.
A high-carb meal would spike insulin which leads to fat storage. The cycle only lasts a few hours, so even if there is a limit per cycle, you wouldn't want to have 3 cycles instead of one.
"The human large intestine contains a microbiota, the components of which are generically complex and metabolically diverse. Its primary function is to salvage energy from carbohydrate not digested in the upper gut. This is achieved through fermentation and absorption of the major products, short chain fatty acids (SCFA), which represent 40–50% of the available energy of the carbohydrate. The principal SCFA, acetate, propionate and butyrate, are metabolized by the colonic epithelium (butyrate), liver (propionate) and muscle (acetate). Intestinal bacteria also have a role in the synthesis of vitamins B and K and the metabolism of bile acids, other sterols and xenobiotics. The colonic microflora are also responsive to diet. In the presence of fermentable carbohydrate substrates such as non-starch polysaccharides, resistant starch and oligosaccharides, bacteria grow and actively synthesize protein. The amount of protein synthesis and turnover within the large intestine is difficult to determine, but around 15 g biomass is excreted in faeces each day containing 1 g bacterial-N. Whether bacterially synthesized amino acids are ever absorbed from the colon remains unclear."
"Obesity results from alterations in the body's regulation of energy intake, expenditure, and storage. Recent evidence, primarily from investigations in animal models, suggests that the gut microbiota affects nutrient acquisition and energy regulation. Its composition has also been shown to differ in lean vs obese animals and humans. In this article, we review the published evidence supporting the potential role of the gut microbiota in the development of obesity and explore the role that modifying the gut microbiota may play in its future treatment. Evidence suggests that the metabolic activities of the gut microbiota facilitate the extraction of calories from ingested dietary substances and help to store these calories in host adipose tissue for later use. Furthermore, the gut bacterial flora of obese mice and humans include fewer Bacteroidetes and correspondingly more Firmicutes than that of their lean counterparts, suggesting that differences in caloric extraction of ingested food substances may be due to the composition of the gut microbiota. Bacterial lipopolysaccharide derived from the intestinal microbiota may act as a triggering factor linking inflammation to high-fat diet-induced metabolic syndrome. Interactions among microorganisms in the gut appear to have an important role in host energy homeostasis, with hydrogen-oxidizing methanogens enhancing the metabolism of fermentative bacteria. Existing evidence warrants further investigation of the microbial ecology of the human gut and points to modification of the gut microbiota as one means to treat people who are overweight or obese."
"A dramatic rise in obesity has occurred among humans within the last several decades. Little is known about whether similar increases in obesity have occurred in animals inhabiting human-influenced environments. We examined samples collectively consisting of over 20 000 animals from 24 populations (12 divided separately into males and females) of animals representing eight species living with or around humans in industrialized societies. In all populations, the estimated coefficient for the trend of body weight over time was positive (i.e. increasing). The probability of all trends being in the same direction by chance is 1.2 × 10−7. Surprisingly, we find that over the past several decades, average mid-life body weights have risen among primates and rodents living in research colonies, as well as among feral rodents and domestic dogs and cats. The consistency of these findings among animals living in varying environments, suggests the intriguing possibility that the aetiology of increasing body weight may involve several as-of-yet unidentified and/or poorly understood factors (e.g. viral pathogens, epigenetic factors). This finding may eventually enhance the discovery and fuller elucidation of other factors that have contributed to the recent rise in obesity rates."
My own guess is that some of the obesity epidemic can be explained by bacteria. This would either be a new strain of bacteria, or (more likely) an old one that has mutated due to the abundant supply of food that humans have been exposed to over the last 150 years. Likely this is a bacteria that has lived in the guts of mammals for a very long time, possibly millions of years.
It is interesting to consider that, while a good deal of research has been done into how mammals react to famine, no research that I know of has been done into how the bacteria in our gut responds to our experience of famine. Or prolonged abundance.
Of course, there are many novel factors in the environment today. Some chemical that effect our metabolism is also a possible explanation.
To illustrate with an extreme example, lactose is a sugar and carries energy, but some people can digest it (and their bodies will get the full complement of calories from it), and some people can't (it'll get digested by bacteria and they'll get gas and feel unpleasant; I'm pretty sure they don't get calories from it). It is incorrect and stupid to say that an item of food with significant lactose component carries "N calories" without specifying whether the person who eats it can digest lactose. Likewise, with cellulose, I think cows get a lot more energy out of cellulose than humans can.
Also, it seems likely to me that people's digestive systems' performance varies with "outside" conditions in the body. For example, I've heard that being pumped full of adrenaline puts digestion on hold; and subjective experience suggests to me that if you eat a lot of food (over a period of, say, a day), your system will send it through pretty quickly, without digesting it fully.
I suspect there are a lot of other situations in which one's body doesn't get all N calories from an "N-calorie" item of food. I suppose that if that happens, then the remaining part probably will be digested by bacteria, and one may notice symptoms of... oh, hey, there's a name for this, "indigestion". (Though if the amount of left-over energy is really small, then the symptoms are probably too slight to notice. If 10 or 20 calories a day matters, then this may make things unpredictable.)
I suppose one could count the bacteria digesting the food that you didn't as "calories burned", but then that means you may be burning a bunch of calories without doing any physical activity.
Does someone more knowledgeable than me know a) whether these things are really significant and b) whether these researchers take them into account?