Here I’m going to use the word ‘complex’. The function of fat in the body is complex, and the means of successfully and eternally losing it can be complex as well — though not nearly as complex as you might think.
To be lean and healthy, you ideally need three strengths, and these are appetite control, hunger acceptance, and enthusiasm for exertion (which practically means: strong muscles). These are the mechanisms or factors that determine everything else: your microbiome (gut bacteria, mainly), your insulin response to food, your ability to fight infection and repair injuries, hormonal balance, and so on. So in a real sense, being lean isn’t something you have in addition to being healthy: it is actually, in people that have good nutrition and muscle mass (unlike, say, prison camp survivors) a sign and indicator, indeed guarantor, of that very health.
Let’s start with the importance of muscles, and the predominance of muscle tissue in lean, fit people such as Princess Kate (and…cough…me). Muscles are fat suppressors because of how they work. The benefits can be summed up as three: firstly, muscles are where the energy goes when you ingest food (in addition to your liver) — and the glucose so stored is termed glycogen. Guess what happens when the muscles and liver are full up with glycogen? That’s right, the excess energy gets stored away as fatty acids known as triglycerides (because they are packaged in bundles of three).* It stands to reason that the more muscle tissue you have, to act as immediate storage baskets for your fuel, the less your body is forced to partition that fuel as fat.
The second benefit of muscle tissue is that it requires more calories than fat does to be maintained: it is ‘metabolically expensive’, as they say. That means that the mere fact of having muscles, never mind the calorie-burn involved in using them, results in more calories burned overall.
Thirdly, when you use your muscles enough to encourage their development — which is to say, they become more efficient and bigger, through cellular repair — your body gets the message that those muscles are needed and therefore diverts fuel for their upkeep, which again prevents fuel from going straight into your fat cells. Working your muscles hard encourages muscle partitioning of food over and against near-complete fat partitioning of food.
I did say the subject was complex! But in a way, it’s remarkably simple. Muscle gives your food somewhere to go before it must be stored simply as fat (a little bit of fuel will always be stored as fat, but the question is: how much?). And muscle trains your body to ‘view’ muscle as something to preserve and build — re-directing it from its project of storing fat. And when this process in any individual has been honed and well-practised over time, what happens is that the muscle, in elbowing out the excess fat, becomes highly visible. The curves you see on that person tend to be muscle curves rather than fat curves. There is tautness, and there isn’t any jiggle (except for the bosom, of course).
How do you get those muscles in the first place? By using them in a way that stresses them, which prompts them to get bigger for next time. (Filling a vase with flowers doesn’t stress your muscles, but lifting your dog off the bed very well might!) What motivates you to use your muscles in such a challenging way? Well, having enthusiasm for activities that require exertion. Or to simplify, as we did at the beginning: having enthusiasm for exertion. And Princess Kate, who is clearly a sporty sort with a love of games, has had that enthusiasm from a very young age.
Now we reach back to the other two important factors in slenderness: appetite control and hunger acceptance. The first is a willingness to delay gratification: to eat later instead of now, to eat a smaller portion rather than fill right up, and to space out indulgences between more restrained and conscientious nutritional choices, rather than eating indulgently all the time. There is nothing in appetite control that need imply an indifference to good food or the incapacity for gourmet riches. It’s just that a lean person does not feast on energy-rich food constantly, six or three or even two times a day. And — this is important — the older the person gets, the less likely she is to indulge in energy-rich foods as a matter of routine (youth does have its advantages!). Some lean people can and do eat anything they want and find as well that they ‘can’t gain weight’. By definition, such people are in energy balance and they are not over-indulging. But age, and its concomitant hormonal changes, tends to mean that what was once normal eating, under the line of fat gain, has now crossed the line to become more than enough for fat gain. As with many other aspects of life, older adults must generally be more disciplined in their habits than younger ones. This is not because there are no health penalties of self-indulgence in youth, but it is still true that young people can get away with more — for now.
Finally we come to hunger acceptance. That is related to the ability to delay gratification, but I think it deserves its own category as a factor, as truly another resource employed by the very lean person. It is no good, we can all agree, delaying gratification if it simply means a compensatory binge that undoes all the restraint you showed earlier. Hunger acceptance is the willingness to receive the body’s signals about peckishness and ignore them. I don’t mean signals of true hunger, of faintness and hypoglycemia, of headaches and weakness. I mean the hormones, chiefly ghrelin, directing you to fill your stomach with something.