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21 August 2014

Slightly overweight people live longer – so is it time to rethink our dietary advice?

One of the most unfortunate consequences of this constant revising of recommendations is the belief that no one knows what a good diet is. But look at the bigger picture and the consensus holds steady over time.

By Julian Baggini

Glutton meets gourmet: our ideas about healthy eating are becoming ever more confused. Image: Valero Doval

I am the first to admit that I’m carrying a bit too much flab. Most people wouldn’t say that I was actually fat, however, and that, according to the Chief Medical Officer for England and Wales, Professor Sally Davies, is precisely the problem. In her annual report published in March she wrote: “I am increasingly concerned that society may be normalising being overweight.” In the land of the wobble-bottoms, the fat-thighed man is slim.

I didn’t get this shape for want of detailed information about how I should be. It has all been neatly quantified for me. Among the critical numbers I should use to measure the healthiness of my lifestyle are a body mass index (BMI) of between 20 and 25, as well as no more than 30 grams of saturated fat and at least five portions of fruit and vegetables per day.

But each of these has been called into question in recent months. This leaves the public confused and cynical about the validity of medical advice, which seems to change with the seasons. Some scepticism is indeed called for, but not over the specific guidelines.

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The problem is with the basic premises on which the guidance is created. We are being given targets, limits, minimums and maximums, but health just isn’t a simple numbers game.

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Take weight. It is said that a BMI of between 20 and 25 is ideal, but when you look at the evidence to support this the picture becomes much more complicated. A huge study of Canadians showed that if you plotted a graph of BMI against mortality rates, the line, as you might expect, forms a U shape, with the very underweight (with a BMI of less than 18.5) and the very overweight (BMIs in excess of 35) most likely to die prematurely.

Where it gets interesting is when you look for where the U bottoms out, indicating lowest mortality. You would expect that to be roughly 22-23, right in the middle of the 20-25 range. In fact, the authors of the study concluded, “Overweight (BMI 25 to 30) was associated with a significantly decreased risk of death.” In other words, the people most likely to live longest are officially a little overweight.

Nor is this finding a one-off. Other large-scale meta-analyses have reached the same conclusion. Another study was published in the Journal of the American Medical Association last year, and once again the reaction of many was disbelief.

Then there’s saturated fat. A Cambridge-led meta-analysis of 72 studies published in March in Annals of Internal Medicine said there was an absence of evidence to support the current advice to reduce intake of saturated fats and increase that of polyunsaturated fats. The results of the study were not just surprising but seemingly impossible, given that the association between higher-saturated fat consumption and raised blood cholesterol levels is indisputable and also that higher cholesterol is associated with higher incidence of heart disease. This has led to a chorus of criticism of the paper from public-health experts and nutritionists, with many left unclear as to whether this now means they can spread the butter as happily as margarine.

As for the five-a-day advice, this increasingly looks as though it won’t stand up. Worries about how the human body absorbs sugars have led many to recommend removing juices and smoothies from the list of permitted portions, while the exclusion of potatoes from the approved list appears to have been motivated more by a fear of encouraging people to think of crisps and chips as healthy than by any sound nutritional science.

In late March, a team from University College London published a study in the Journal of Epidemiology and Community Health which concluded that five portions of fruit or vegetables were not enough. “Our study shows that people following Australia’s ‘Go for 2 + 5’ advice will reap huge health benefits,” said the lead researcher, Dr Oyinlola Oyebode. Yet a report that appeared in the British Medical Journal in July found there were no discernible benefits from eating more than five portions of fruit and vegetables a day.

One of the most unfortunate consequences of this constant revising of recommendations is the belief that no one knows what a good diet is. The truth is more complicated. It is indeed the case that no one knows exactly what is best for us, and people do make mistakes identifying both supposedly healthy super-foods and nasty, dangerous ones. But look at the bigger picture and the consensus on what works holds steady over time.

In 1933 the British Medical Association advised that we should obtain 12 per cent of our calories from protein, 27 per cent from fat and 61 per cent from carbohydrates. The current official UK dietary advice is similar: we should obtain 10-15 per cent of our calories from protein, 33 per cent from fat and 50-55 per cent from carbohydrates, while the World Health Organisation recommends a more elastic 10-15 per cent, 15-30 per cent, 55-75 per cent split. The general advice to eat a wide range of foods, including plenty of plants and whole grains as well as modest quantities of refined carbohydrates, fatty meat and dairy products, has been consistent for several decades.

So why is it that the headlines always seem to concern about-turns by health experts? The root of the problem is in the science of health itself, which has been too reductionist in its approach, attempting to break down the ideal diet into discrete, measurable units. Healthy eating becomes not about food, but about calories, vitamins, minerals, good and bad fats. Similarly, a healthy body is defined by a series of measures: weight, blood pressure, blood cholesterol levels. These numbers are not entirely meaningless: they all correspond to things that matter. The problem is that, in isolation, they tell us very little about what we ought to do as individuals.

This is perhaps clearest in the case of weight. It is widely agreed that unless you are severely overweight or underweight, far more important than how much you tip the scales by is whether you are fit (and that you don’t smoke). It also seems likely that it is better to be overweight because you eat too much of a good diet than to be at your ideal weight by eating the right amount of the wrong stuff. So people who eat well and are physically active with a BMI of 27 are generally healthier than those who maintain a steady 22 by leading sedentary lives but consuming little more than Lean Cuisine frozen ready-meals and Diet Coke. Likewise, the thin smoker or heavy drinker is dicing with death far more than the slightly chubby, clean-living tennis player.

Similarly, if you look more closely at the saturated-fat study reported in March, you can see how both conventional wisdom and the paper’s conclusions contain important elements of truth. What the paper suggests is that if you examine the intake of the various fats and nothing else, only the overall level appears to affect health, not which fats you eat. Whether this turns out to be correct or not, we know that improving health outcomes is not merely a matter of more of this or less of that one nutrient. What also matters is everything else that is going on in your diet. Research from 2010 in the American Journal of Clinical Nutrition found that reducing saturated fat had no health benefit if the fat calories were replaced with carbohydrate. So only knowing that someone has cut down on the butter won’t enable you to know they are eating more healthily.

As for fruit and veg, this appears to be a classic example of the mess that is made when people try to translate complicated science into easy-to-understand health advice. Government agencies want clear, easy-to-follow guidance that does not lead to perverse outcomes or confusion. Five-a-day looked as if it met these criteria.

It set what was an achievable minimum, and excluding potatoes while allowing nothing to count for more than two would help people resist getting up to quota by eating only one or two things that would not be especially healthy because of the way they were prepared. But the result was that, for too many people, consumption of fruit and veg became a kind of medicinal inconvenience to be got out of the way. Five-a-day became a target, not a minimum, and portions just don’t tell the whole story anyway. Vegetable tempura served with white rice scores as many points as fresh tomatoes on wholemeal bread but it would be absurd to think they were equally good for you.

The core problem with the reductive approach is that it fails to capture the extent to which the body is a system in which the elements of diet work sometimes with and sometimes against each other to produce effects that are seldom attributable to one factor alone.

Take bone density. The reductionist approach suggests that what is needed to improve this, especially among older women, is more calcium – and, broadly speaking, that is correct. However, if you merely increase the amount of calcium ingested, by means of a supplement, you do not necessarily increase the calcium that gets fixed in the bones. You might even cause other problems. One is that the calcium may end up not in your bones, but inside your arteries. Prompted by evidence that this was indeed the case, a study published last year in the open-access journal Nutrients concluded: “Calcium intakes that consistently exceed the RDA [recommended dietary allowance] of older adults, especially when a substantial amount of the calcium is derived from bolus intakes of calcium supplements, may accelerate arterial calcification and raise the risk of cardiovascular events.”

So, how do you ensure that the calcium you ingest goes to the bones and not the arteries? One well-known factor is Vitamin D but another would appear to be fat: you need enough of it for absorption to be optimal. This finding was supported by a 2007 report in the American Journal of Clinical Nutrition. Yet, believing fat to be bad, many of us consume low-fat, high-calcium foods and fail to get the full benefits of either.

Saturated fat is another complex one. In the competition for unqualified food evil, it is right up there along with sugar and refined flours. But again, how bad such fat is for you seems to depend on exactly where it comes from and even what it is eaten with. Perhaps surprisingly, there is good evidence that cheese does not raise LDL cholesterol (the bad kind) in the blood. So whereas 50 grams of saturated fat in cheese might well be fine, the same quantity in butter might not.

Not only that, but it seems that how bad these fats are depends on what else there is on your plate, or in your glass. This is at the heart of the so-called French paradox: until recently at least, the French suffered lower levels of cardiovascular disease than their relatively high consumption of saturated fats would suggest. No solution to this paradox has yet been convincing enough for experts to reach a consensus about what explains it, but it seems it must involve something about how the fats fit into the wider diet. It could be that they are consumed along with wine, or that a great deal of French people’s fat comes from cheese, or even that the French eat more slowly. Whatever the answer, the French show that looking at levels of fat consumption by itself is not enough.

One further factor complicates health advice. Epidemiological studies are good at identifying the point at which a specified level of consumption begins to be associated with increased health problems across a population. If, for instance, you plot alcohol intake and mortality, you get a J-shaped graph showing that those who drink a lot or nothing at all are at greater risk of death than those who drink moderately. The lowest risk is believed to lie at one or two drinks per day. Above this the risk starts to rise. Health advice is based on this, recommending no more than 28 units a week for men and 21 for women. (That until recently the advice was 21 and 14, respectively, shows how hard it is to quantify such matters.)

But if you look for where the curve of the J starts to rise higher than teetotalism, you will find that point is reached as you pass four to five units a day, according to one comprehensive meta-analysis that fits other findings. Which shows that, as a population, even at that point, the risk to drinkers is only marginally higher than it is for those who don’t drink. Worse still, your personal risk can only be assessed accurately in conjunction with other factors, such as weight, blood pressure and so on. The idea that your alcohol intake is too high at this point is mere conjecture, based on statistical averages and a somewhat arbitrary decision about where on the rising line of the graph the warning light should be fixed.

All this runs counter to the increasingly popular ideal of the “quantified self” – measuring and keeping tabs on all your critical health data. What then is the health-conscious eater supposed to do?

The good news is that the answer is simple. In most cases there is no need to worry about what is unhealthy except at the extremes. Weight is a serious problem if you are obese, alcohol consumption worrying if you’re downing a bottle of wine most days, fat consumption too high if you’re living on full English breakfasts and ghee-soaked curries. If you have a mixed diet, based mainly on home-cooked whole foods, you need not worry about the fat in the cheese or the sugar in the cakes you make from time to time.

That leaves us with the now famous seven-word maxim of the writer Michael Pollan: “Eat food, not too much, mostly plants,” just as long as you understand “food” to be what you do or could make at home and not things that could only be products of industrial processes. It turns out that the more we understand how nutrition works and how complicated it is, the simpler the dietary advice we need to follow. I call this the Pollan paradox. In theory, any number of changes might make your diet healthier, but in practice diet is too complicated for us to be able to micromanage it. Too much is unknown about how the elements of diet interact for us to be able to change the variables to achieve the result we desire. I’ll drink – moderately – to that. 

Julian Baggini is a philosopher. His latest book is “The Virtues of the Table: How to Eat and Think” (Granta, £14.99)