I’m on a Keto diet. She Who Must be Obeyed has decreed that my girth has grown excessive, and I’ve been on this protein- and fat-rich regime, denied carbs, for several weeks now. Given that I have perhaps not been the most compliant subject, still consuming a beer a day and occasionally breaking the rulers with a meal out, I’ve been rewarded with discernible, if not dramatic results. One of the key changes of dietary habit has been at breakfast – no more fruit, yoghurt and Meuesli; instead, eggs. Eggs and bacon, eggs and smoked salmon, eggs in omelettes, eggs fries, eggs scrambled.

When I last went to the doctor, I mentioned my diet, receiving a general nod of approval. The doctor asked me how I was finding it, and I said it was OK, but that I was getting a bit sick of all the eggs. Now if you’d told a doctor a couple of decades ago that you were eating eggs – plural – every day, they’d have told you you were insane. Consumption of an egg when there was not a Z in the month was tantamount to suicide, because eggs were concentrated balls of cholesterol, and cholesterol, we were told, was the big killer of the western world.

So I was pleasantly surprised to find that my doctor didn’t bat an eyelid at the news that I was now eating 14 eggs a week, often with bacon, that other villain of the late 20^th century chattering class diet.

And, come to think of it, when was the last time you heard the fatwa on butter? I can remember when butter was as much a killer as eggs, and we were assured that margarine was far better for you. Now that advice has been quietly dropped, since it was found that most margarines are significantly carcinogenic when fed to mice, and that in any case butter contains several components vital to our health. Furthermore, the saturated fats that gave butter its bad name have been determined to be far less deadly that we were solemnly assured.

The connection between obesity and poor health outcomes is at least well established. But how many times have you seen the rules for weight-loss diets flip-flop?

This all came to mind when I read an item by Will Jones in the weekend’s Daily Sceptic, concerning the tergiversations of the World Health Organisation, exemplified by its most recent pronouncement to the effect that artificial sweeteners were now determined to be ineffective as a means of achieving weight loss, and may indeed by harmful. Will quotes an article in the Daily Telegraph by Christopher Snowden, who points out that the history of the WHO’s dietary health advice is a litany of prescription and counter-prescription, all delivered with oracular certitude, and a straight face. And as Stanford University’s Prof John Ioannides has revealed, much, perhaps most of this advice has been based on half-baked science.

All this has prompted me to set down in writing a theory I have nursed for some time.

The human race has spread to most parts of the globe. Basically, when humans have found somewhere they could survive for a day, they’ve tended to set down roots and see if they could survive for a lifetime. Humans’ ability to clothe themselves, and to make fire, has meant that they have settled in a variety of environments which we tend to take for granted, but which ought to astonish us.

And wherever they settled, humans fed themselves on what was locally available. Individuals whom the local fodder suited would survive, thrive, and procreate. Those with whom it disagreed would tend to die earlier, and be in general less fertile. Over time, a race of people would develop which was genetically predisposed to thrive, or at least survive, upon the local tucker. There is, I suggest, some support for this idea in the celebrated longevity of highly autochthonous populations such as Japan and Georgia. And it would, I suggest, be comparatively easy to determine scientifically the nutritional elements that made for a healthy diet for members of such a population. Easy, but scarcely worthwhile, for a population which was already eating what it could, and was an evolutionary product of that diet.

However, that very adaptation might well mean that one could certainly not take for granted the health of individuals plucked from their autochthonous environment and forced to eat the diet available in some distant land. And of course, humans have continued to migrate. In ancient times this migration was generally fairly slow, and we might imagine that its pace did not outstrip the ability of a race to adapt to changing dietary conditions. Once they mastered long-distance navigation, however, they began settling in all sorts of places where the available diet was very different from what they were adapted to, and might or might not suit them. Long-distance navigation also allowed a vast explosion in dietary variety, and made the acquisition of exotic foods easy and routine.

Miscegenation among people from widely different dietary backgrounds further obscures the nutritional picture. The children, for instance of a father with Irish ancestry and a mother of Vietnamese origin may have inherited the nutritional predispositions of one parent, both, or perhaps neither. In Australia, we have a particularly highly miscegenated population, but all across the globe people are consuming diets that may have little or no resemblance to those for which they are congenitally adapted.

So, what does all this have to do with the WHO’s pathetically unsuccessful attempts to prescribe rules for healthy eating? Well, if these ideas have any merit, it would be fair to conclude that this is fool’s errand. So many of the people for whom these rules are intended are so far removed, genetically, from their autochthonous ancestors that there is little reason to suppose that there is any such set of rules is to be found. They should just hold their peace on the matter, observing the Hippocratic injunction to ‘first, do no harm’.

I have no doubt that the good folk at the WHO would retort, in the unlikely event that they were to read this post, that I have no credentials as a nutritional scientist.

But then on all the evidence, neither do they.