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Joining Forces to Tackle Obesity, 21-22 January 2002

Transcript : Preventing Obesity Through Dietary Strategies

Speaker

Dr Susan Jebb

Head of Nutrition and Health Research, Medical Research Council

DR WILLIAM DIETZ: I’d like to get this afternoon’s session underway. The first topic for this afternoon is Preventing Obesity Through Dietary Strategies and the first speaker is Susan Jebb, who is Head of Nutrition and Health Research at the Medical Research Council. Susan.

SUSAN JEBB: Thank you. Can I have the first slide please. It’s been a great opportunity to be here over the last couple of days and hear, particularly yesterday, so much goodwill from so many interested parties who all recognise that they have a contribution to make in terms of tackling obesity. And I hope that in a few years’ time we’ll look back on this meeting as being a real turning point, when we moved from all the talk and really stepped forward into action.

What I’d like to do over the next 15 or 20 minutes is to talk about some of the dietary strategies that I think need to underpin the interventions which are going to go on in a whole variety of different places. And also the interventions which are going to look both at the prevention of obesity, and I’ll focus mostly on that, but also a few words about treatment.

It’s actually quite surprising that amidst all of the talk so far, although the concept of energy balance has underpinned everything we haven’t actually seen this slide and it’s here to remind us that everything we do in obesity is really founded around this very simple energy-balance equation. If energy in is greater than energy out, people will gain weight and vice versa. Now the reason I’ve put that up today is to re-emphasise that obesity is an issue about energy intake, diet, and about energy expenditure, primarily physical activity, because of course resting metabolic rate is largely determined by an individual’s own characteristics: height, weight, age and gender. I really refuse to get embroiled in an argument as to which is more important - diet or physical activity. Both are critical. And of course some interventions will focus on one rather more than another, but we should never forget that there are two sides to any discussion about obesity. Now because I’m talking about diet, I want to remind you that diet works on both sides of this equation. There are two sides to overeating. You may just be overeating per se and that leads to obesity, or you could be overeating relative to low energy needs and that second statement tends to put the emphasis on the low energy needs, low physical activity. But it reminds us that even if you are extremely sedentary you will not gain weight unless you fail to down-regulate your intake to match those low energy needs. And it’s the coupling between energy intake and energy expenditure, which is critical.

Diet is a very critical element in any consideration of body weight and that’s because there’s far greater flexibility in energy intake than we have in energy expenditure. If we look at the natural coefficient variation of energy intake on a day-to-day basis it’s about 25%. But, in fact, some people can eat nothing at all on a single day. They can completely starve, or on very low calorie diets, they may be down to perhaps as little as 500 calories a day, but on some days we are perfectly capable of eating an enormous number of calories, perhaps even in excess of 5,000 calories. However, if you look at energy expenditure, firstly the day to day coefficient variation is a lot smaller, around about 8% and actually the time in which most general average healthy individuals can manipulate their energy expenditure is relatively modest. If you stay in bed all day you may expend only a little more than your basal metabolic rate but since basal metabolic rate is the largest component of energy expenditure that doesn’t reduce your energy needs all that much. Conversely, if you’re extremely physically active you might expend twice as many calories as you would have done had you stayed in bed. But still the flexibility in intake is very much greater than in expenditure and so there is greater scope for manipulating intake to effect changes in energy balance.

One of the pieces of data, which is often used to undermine the fact that diet is important, is this data which we published as part of the BMJ paper back in 1995 and which Imogen Sharp showed earlier today. Energy intake has indeed declined. This is data taken predominantly from the National Food Survey with some adjustments of food eaten outside the home and with corrections for some of the notable omissions - confectionery, soft drinks and alcohol taken from food disappearance data. So even when one has made as many adjustments as we reasonably can, there is still an apparent decline in energy intake. People are often very taken aback by that, but in fact if you start calculating the decrease that there’s been in physical activity, it’s perfectly possible to accept that energy intakes may have declined by 500 calories a day or more yet we are still getting fatter. So we don’t have to dismiss the dietary data in order to believe that diet is important in the epidemiology of obesity.

Another important fact, which has been touched on is, that whilst energy intake has been declining there has been a profound change in the proportion of macronutrients in our typical UK diet. If we look back over the last 50 years or so we can see that there has been a very dramatic increase in the proportion of fat in the diet and that has largely occurred at the expense of carbohydrate, which has decreased. Protein and indeed alcohol make a relatively small contribution and have been rather stable, so they’re not shown on this graph.

Although fat intakes as a proportion of energy do appear to have declined a little in recent years, this decrease is small in comparison to the previous huge increase. We still eat a proportionally high fat diet. And there is now very good evidence that that is contributing to the huge problem we’re seeing in relation to obesity. I just want to spend a couple of moments to review the spectrum of evidence, which suggests that dietary fat is a critical component of the diet. This is data from an epidemiological study, the Leeds High Fat Study. They took two groups of women, those consuming a diet low in fat, less than 35% energy, and high in fat, greater than 45% and this is the distribution of their body mass index in the two groups. Now interestingly when you first look at this you see that the average BMI in the two groups is exactly the same. But when you look at it in more detail, you see that there’s a very pronounced shift towards higher BMIs in the high fat group. In the high fat group the data is not normally distributed and so you get a great excess of people who are clinically obese. In fact if you take the BMI cut off of 30, people on a high fat diet are 19 times more likely to be clinically obese than those on the low fat diet in this study.

Now why is that? There’s been a lot of work in highly controlled experimental studies to try to understand it, and this is data from a study done in Cambridge, which Phil James referred to in one of the opening presentations. If you just focus on these first three bars initially. This is a group of lean, young, healthy subjects who were offered a diet containing 20, 40 or 60% energy as fat, over a period of one week on each occasion and were allowed to eat as much or as little as they wished. What you can see very clearly is that as the fat content of the diet increased so their energy intake also increased. In the first section of the graph, this was conducted in a whole body calorimeter. So we had absolutely precise details of exactly what they were eating and at exactly how much energy they were expending, but in fact when the experiment was repeated in free living subject we saw the same effect. It seems that on high fat diets, it’s very easy for people to overeat. Why people gain weight on high fat diets is because they tend to eat more calories and that’s probably because fat is so energy dense. Fat contains nine calories per gram relative to just four calories per gram for protein or carbohydrate. So in the same portion of food if it’s high in fat you’ll get far more calories and that explains why fat is strongly associated with an increased risk of overeating and gaining weight.

But is it just fat? Well what the third section of this graph suggests is that it may actually be about the energy density of the diet. In the first two studies the energy density fluctuated as the fat content increased but in the third group of studies the energy density was equalised. It was a rather artificial situation. But nonetheless what you can see is that when the energy density of the high fat diet was made to equal that of the medium fat diet, in fact the overeating was abolished. So when we’re thinking about strategies to reduce obesity, yes focusing on fat is valuable but we need to think more broadly, we need to think about the overall energy density of the diet.

But here’s the data first of all relating just to fat. This is series of studies, which were collated by Arne Astrup, which looked at the impact of ad libitum low fat diet on weight loss. So this was a situation in which people were not recommended to actively lose weight, they were simply instructed to change the proportion of macronutrients in their diet. And what you can see in most situations there was some weight loss, and if you look at the overall effect you can see there was a small but significant reduction in body weight. People on a low fat diet do lose weight. The magnitude of this effect is quite modest and I’d be the first person to say that of course this alone is not sufficient to treat established obesity. But it’s an extremely effective strategy for preventing the very modest weight gain, which most people experience over many years. So even ad libitum low fat diets have a role to play in weight management. If you want people to actively lose weight over a prolonged period of time you probably have to add in other strategies and we’ll talk about that in a moment. I think it’s also important to remember that when you recommend a reduction in fat in a diet, you are trying to reduce energy density and there are other steps, which we often think of as part of the overall healthy diet, will also contribute to a reduction in energy density. So by increasing the proportion of complex carbohydrates, you can reduce the energy density simply by shifting the proportion of fat to carbohydrate, especially if you put the emphasis on complex carbohydrates and that’s particularly important in ad lib diets because you don’t want people to be feeling hungry, Increases in fruit and vegetables will also help to drop energy density.

The other issue is portion size and this is particularly important if one’s talking about inducing actual weight loss, because of course if you eat absolutely anything you want on any occasion and in any amount it’s perfectly possible to gain weight even on a low fat diet. So it’s important to consider portion size. I think this has become critically important in the world we live in today where increase in portion sizes are often used by manufacturers’ to increase sales of their product. It’s the same as with eating out where if you get larger portions actually the cost to the company is relatively small but it encourages greater sales. I was struck by the fact that a packet of crisps now is 55 grams. When I was small, it was only 25 grams so these bags of crisps have doubled in size. How often do you see people, particularly if they have a bag of crisps in their packed lunch, eat half and save half for tomorrow? Not very often. I am concerned that large portion sizes may be encouraging people to eat more calories than they actually need. We know that portion size is strongly habituated. People get used to eating certain amounts of food and I think there is a concern that if you’re constantly exposed to large portion sizes, you begin to see that as normal. There’s been relatively little work done in this area. A small amount in children, but almost nothing in adolescence and adults and I think this is something we’ve really got to look at.

Here’s one piece of data, which has looked at portion size, which emphasises a very important interaction with energy density. This is an analysis of some dietary data collected by a group of lean and overweight subjects. The investigators divided the foods they consumed by energy density - low, medium and high - and they looked at the typical portion size that those individuals consumed. What you see is that lean individuals choose larger portions of low energy density foods compared to their overweight friends. But if you look at high energy density food, predominantly high fat, the obese subjects choose a portion, which is almost twice the size of their lean counterparts. So when we’re talking about tackling obesity we not only need to consider the type of foods people are eating, but also the quantity.

Now another issue in relation to diet which has cropped up a number of times, but which I’m interested that nobody’s really talked about in detail, is the question of snacking or grazing as it’s sometimes been described. The difficulty we have here is that it’s very hard to obtain data, which is really relevant to the real life situation. Quite clearly there has been an increase in snacking behaviours and a decline in meal eating. And you could simplistically say, "And there’s been a rise in obesity too, the two must be related". But of course associations do not imply causality. If we look at the epidemiology, in fact the evidence is rather mixed. Some studies show that eating more often keeps you thin and some studies suggest that eating more often makes you fat. It’s very hard to work out what’s going on. Also, these studies are confounded by two important effects: The first is the tendency towards under-reporting, which has often been shown to be much more common and of much greater magnitude in the obese and may include under-reporting. The second is the problem of post hoc effects; obese people may have fewer meals because they skip meals as a weight control strategy. So the epidemiology is extremely difficult to untangle. If we look at experimental studies in highly controlled situations where you give people the same number of calories as two meals or ten meals what you find is no difference in terms of energy expenditure or body weight. Reinforcing the suggestion that calories are king. Calories at the end of the day are what count. If you eat the same number of calories it makes no odds how often you eat them, in terms of frequency during the day.

Experimental studies where people have tried to impose snacking regimes have tended to suggest that if you impose snacking, people compensate for the calories in those snacks at mealtimes and that the net effect is no significant change in body weight. But, we have to remember that’s it’s extremely difficult to mimic real life snacking behaviour because snacking depends on availability, on cost and it also depends on the social and the cultural environment that you’re in when you start snacking. These factors are very difficult to mimic in experimental situations. So we need to look more closely at the effect of snacking on weight but what the data seems to be saying is there’s nothing about snacking per se which causes obesity but inevitably, the effect for individuals will depend on what you snack on, how often you snack and how much.

So I think we do have a reasonably clear dietary strategy for weight management. We need to reduce the proportion of dietary fat, increase the proportion of carbohydrate especially from complex wholegrain sources, increase fruit and veg, ensure that snacks do not contribute to excess energy intake, and consider portion size especially in terms of inducing actual weight loss.

But tackling obesity is about much more than just weight control. If all you’re talking about is gaining or losing weight then all you really need to think about are calories and the energy balance equation. But actually when we want to treat obesity we want not only to make people slimmer, we want to improve their health and that I suspect is what motivates 99.9% of the audience here today. If people were fat and healthy we would probably all find another job. It’s because people are fat and sick that we worry about it. So when we develop a dietary strategy to tackle obesity we also have to find one, which helps to reduce or certainly not increase the risk of co-morbid disease.

What I want to very quickly demonstrate to you is that all the features I’ve spoken of in relation to a dietary strategy for obesity fit absolutely with strategies for tackling other chronic diseases. Here we see the impact of low fat diets, this is the effect of the American Step One diet on plasma lipids - significant reductions in plasma lipids, and reductions in cardiovascular risk.

Here we see that the type of carbohydrate is incredibly important in determining your risk of developing Type 2 diabetes. With diets, which are low in fibre and with a high glycaemic index actually increasing the risk of developing diabetes in a prospective study. And finally the data from the DASH study, a high fruit and veg diet, which showed significant impacts reductions in blood pressure.

What we’ve talked about in most of this conference is actually the knowledge people need to control their weight and I’ve outlined for you the strategy in relation to fat and carbohydrate and fruit and vegetables. But how many times have we said it while obesity continue to rise? Knowledge is not enough to effect dietary change. What we hope we can achieve over these two days is to actually fill in some of the gaps in two other areas. Firstly giving people the practical skills they need. Simply telling people to change their diet actually is not very helpful. They need to know how to change their shopping habits, their cooking habits and how to cope when they’re eating out. People like eating out, they’re not going to stop eating out, therefore we need to help them to eat out in a way, which doesn’t undermine their weight control practices. And we need to give them support. Perhaps that hasn’t been emphasised enough. But the one thing we know about helping people, certainly to lose weight, is that ongoing support really matters and that can come from a whole variety of places - friends and family, health professionals, commercial weight loss organisations or other private groups - but people need support. And these are the areas we’ve really got to fill in and where we’ve got to make great strides if we’re going to effect sustained changes in behaviour.

So whose job is it to tackle obesity? I’m really encouraged that there are people here from so many different disciplines because I hope that means that we recognise that we all have a role to play in tackling obesity. As the Audit Office report said, this is not just about the Department of Health, in fact it’s not just about government. Of course there are very, very important things that they can do but it’s about what’s going on in the NHS, it’s about the food industry, it’s about finding a role for commercial organisations. What about employers? What about occupational health schemes? What attention are they paying to the health of their workforce in helping them to manage their weight? We’ve talked quite a bit about schools, what about parents? What about individuals? What we’ve seen is that no one group can do all that needs to be done on their own because if they could they would probably have done it by now. We’ve all got to work together. In terms of diet I think there is now a dietary strategy to prevent obesity at a population level,which is very well established and has a very broad consensus of support. Most importantly the advice is totally consistent with all the dietary strategies for the prevention of other chronic diseases and so fits perfectly with our efforts in relation to coronary heart disease, diabetes and cancer. The question now is not really what should the dietary strategy be, but how are we going to implement it? And I’m preaching to the converted here today I think when I say that what we need is concerted and co-ordinated action and hopefully this meeting will be the first real focus for that in relation to obesity. Thank you.