In retrospect, I was fortunate that the first low-carb book I read was Richard Mackarness’s Eat Fat and Grow Slim, because it includes a section on the contribution of fibre to weight loss. Most low-carb plans routinely tell us to get some fibre, but Mackarness alone equates the eating of high-fat, high-protein foods with the eating of fibre. I notice that the Web bristles with debates between gurus, bloggers, and various other visionaries who espouse either a low-carb or a high-fibre diet. This is surely a false division. Both camps insist they follow man’s ancient diet but dispute whether that diet comprised mainly animal food or mainly plant food. For Mackarness, early man was an omnivore: his digestive system, which we inherit, was adapted to either type of food.

No one knows quite how the human digestive system evolved, but clearly it did so on unprocessed food. For that reason alone, I believe giving up processed food should help towards the maintenance of a natural body weight. This is an unoriginal notion, of course; but it’s intriguing to speculate how our digestive system evolved. The body had the ability to sift the nutrients it needed from all that roughage.

Long before we became apes–who eat much more roughage than us–we were simple life forms with anaerobic digestive systems; we probably fermented the mass of ingested roughage to free the nutrients we required and then shunted the residue out of our bodies. Many animals, such as ruminants, still digest much of their food through anaerobic bacterial fermentation in their guts. We, however, developed an upper gut–the small intestine–to digest the more nutrient-dense materials that humans came to eat. We retain, nevertheless, a primitive large intestine, which ferments a significant fraction of food that resists digestion in the small intestine.

Now here’s the interesting thing: it turns out that many of the calories we consume in certain fibre-rich foods–nuts, for example–are so entrapped in fibre that they pass through the small intestine without being digested there. Many of them pass clean out of the body without being digested. A percentage of the calories in such foods could therefore be discounted, letting us eat more of those foods than we thought safe.

Thinking about this phenomenon, I saw--as some people have suggested--that certain foods have fewer available calories in them than are marked on labels or listed in nutrition tables. We are used to deducting fibre from carbohydrate totals (many labels now do it for us); we do not take account, however, of the macronutrients trapped in or blocked by that fibre. Studies show that some fats, carbohydrates, and proteins remain sequestered in insoluble fibres such as cellulose; they also show that soluble fibre encloses many nutrients in a resistant gel, preventing many of them from digesting in the small intestine, and releasing them, if at all, only in the lower intestine, to be fermented into availability by anaerobic bacteria.

Does that mean our digestive system is chaotic, inefficient, random? On the contrary, I believe it is an infinitely nuanced regulatory system designed to ensure that we absorb no more nutrients (and calories) than we need. Unfortunately, the system wasn’t designed to cope with refined food, which, depleted of fibre, releases its nutrients (and calories) into the bloodstream unrestrained. Our digestive system may know how to keep us slim (yes, even us), if only we would let it.