Much research has been done on nuts. Most of the abstracts, which summarise findings, are available to read online; in many cases, whole papers appear online. A phrase in one of them raised my eyebrow:

“[…] there is some intriguing evidence that nuts can help to regulate body weight […]” (García-Lorda et al, Nut consumption, body weight and insulin resistance, 2003)

This drew my attention, because the idea that a food might regulate body weight was new to me. But the phrase lodged in my mind.

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A significant proportion of nut research addresses the fate of the “missing calories”. Many studies show that the daily addition of nuts to a diet does not lead to weight gain, despite the extra calories. Such a finding just winds scientists up. Where, they itch to know, do those calories go?

Most attempts to explain the lack of weight gain from nut consumption hinge on their satiating character, which may reduce consumption of other food. Satiety is a fascinating subject–I have read a lot about it–but I knew satiety was not a factor in my case, because my self-experiments were based on a fixed daily intake of nuts: 1450 calories, my usual limit on my reducing diet.

Another explanation advanced by analysts is that nuts stimulate extra energy expenditure; but my energy expenditure remained the same as usual during my self-experiments with nuts. (Self-experimentation may not be scientific or transferable, but it does have the advantage that individual factors are easier to isolate. Scientists cannot, for ethical reasons, make trial subjects eat only nuts--therefore their studies struggle for definite conclusions amidst confounding circumstances and possibilities.)

Two internal mechanisms for calorie waste are also documented: increased digestive metabolism after nut consumption, and excretion of a percentage of the calories in nuts. I believe both were at work in my case. Sometimes I felt hot after a meal of nuts, betraying heightened metabolism; but digestive thermogenesis surely couldn’t explain my weight loss, on a mono-diet of nuts, of four pounds a week. So I looked into the matter of calorie excretion.

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Among the reviews, studies, and abstracts I read, I often came across remarks such as the following:

“The absolute and relative increased excretion of stool fat on the almond diets may have resulted from lower levels of fat absorption due to the structure of lipid-storing granules in nuts or to various nut fibre components.” (Zemaitis and Sabaté, Effect of almond consumption on stool weight and stool fat, 2001)

“Fecal fat and energy loss is greater with consumption of whole peanuts compared to peanut butter, oil or flour. This may contribute to the less than predicted change of body weight observed with peanut consumption.” (Traoret et al, Peanut digestion and energy balance, 2008)

“Although the data do not permit accurate calculations of the energy loss due to limited bioaccessibility, a working estimate may be 10-15% of the energy from the nuts. Thus, direct metabolizable energy is a more accurate representation of actual energy value for nuts than package label or table value. This inefficiency stems from resistance of the parenchymal cell walls of nuts to microbial and enzymatic degradation. Thus, cell walls that are not ruptured during mastication may pass through the gastrointestinal tract without releasing the lipid they contain. This is supported by data demonstrating greater energy loss from whole nuts compared with nut butter; a higher energy requirement to maintain body weight during nut consumption; as well as microstructural analyses of fecal samples. Because lipid is the primary energy source in nuts, work on bioaccessibility has focused on this nutrient. However, the resistance of the cell walls of nuts to degradation in the gastrointestinal tract would also limit the bioaccessibility of other nutrients they contain, including protein, vitamins, minerals, and phytochemicals.” (Mattes and Dreher, Nuts and healthy body weight maintenance mechanisms, 2010)

At first, I was reassured, on reading such statements, to find corroborated my experience of weight loss on nuts. But then I scratched my head. The usual estimates for the extent of fat non-availability in nuts are 10% to 20%--surely these percentages were too low to explain the amount of weight I was losing.

Half way through my third nut experiment, I noticed something. It struck me that my healthy bowel movements on a diet of 1450 calories of nuts a day (not much over 200 grams of nuts) were remarkably ample, given the low weight of the nuts I had eaten. What if I were excreting not just a minor portion of the ingested nut content but nearly all of it? It seemed a staggering idea. Maybe once my body had extracted the nutrients it wanted from the nuts, it had passed the rest on as superfluous. After all, I was overweight; why would it want more fat? Was I staring down the pan at the holy grail?

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In some ways, it felt like a mystical moment. For days my digestive tract's only intake had been one of the most primitive foods on earth, one I’d come to hold in awe for its weight-loss magic. My diet was successful enough: on about 1450 calories a day, I’d usually lose a pound a week, sometimes two–the amount we're told is the maximum likely for a healthy body. Now here I was losing four pounds a week on the same calories. I had no dysfunction, such as diarrhoea or steatorrhoea; in fact, nuts have always agreed with me. Could it be they were performing a primeval function, that of helping to regulate my bodyweight, which was much too high? As hunches go, I didn't know if there was anything in it. I still don't--but I'm on the trail.