Why We Eat (Too Much) Read online

  The sensor also works the other way around, so that if you have drunk too much water and the blood is over-hydrated, it will turn off the first signal to the brain and you will not want to drink any more. It also flicks on switch number 2 in the kidney, leading to lots of dilute urine being produced. Less water in and more water out – over-hydration corrected.

  Counting Calorie Intake? We Never Count Water Intake!

  This negative feedback system works constantly to regulate the amount of water that we have in our bodies. It works subconsciously. In a whole year, we drink over 550 litres of fluid. That’s the equivalent of five full bathtubs of water passing through our bodies every year. But we never have to measure that water to make sure that we are drinking the correct amount. Doctors don’t have to warn us that if we take in 6 litres of water more than we excrete we could die of over-hydration – they know it is powerfully regulated without having to think about it. We don’t have a ‘water in – water out = water stored’ equation in our minds. This is because we know that our water balance is controlled by our biological negative feedback mechanism. And the mechanism is exquisitely accurate. Of the 550 litres consumed per year an identical amount is lost from our bodies, all without a conscious thought.

  People do occasionally die from drinking too much water (6 litres in a short period of time), but they are consciously over-hydrating. Rare examples are: inexperienced runners in a marathon, who fear dehydration and therefore force themselves to drink too much, or young students playing drinking games. Either can be rapidly fatal.

  Just like the hydration systems, energy metabolism (i.e. the amount of energy taken in, the amount used and the amount stored) is critical for the survival of any species. All species go through times of feast and famine; the ones that survive and thrive are the ones that can predict exactly how much energy may be needed and should be stored for the future.

  Six Big Macs … with Six Sides of Fries … and Six Cokes

  Let’s go back to Metabology Rule 1. This is the rule that most people use to understand obesity: (Energy In) – (Energy Out) = Energy Stored. Scientists have calculated that to store 1kg (2lb) of fat you need to take in 7,000 extra kcal.1 That’s the equivalent of six Big Macs, six sides of fries and six Cokes – on top of the usual calories that are needed every day. So, fit in a Big Mac meal on top of your normal meals for a week (excluding Sunday) and you will gain 1kg or about 2lb.

  The traditional explanation for the overwhelming rise in obesity in the last thirty years is that we have been consuming too many delicious Western-type foods, too many Big Mac meals. On top of this, we have more cars, dishwashers, video games etc. and therefore do not move around as much as we used to. Basically, the conventional wisdom is that we have created a society in which it is easy to become too greedy and too lazy and this has led to us getting fat. It’s our fault. If we just take Metabology Rule 1 to explain obesity, then this conclusion must be correct.

  Figure 1.2 7,000kcal translate to 1kg of weight gain

  Why aren’t All Americans over 300kg?

  If we look at the data, it seems that this conclusion is correct. The rise in the rates of obesity started in the early 1980s and this seemed to coincide with the rise in the consumption of calories by the population. In fact, if you look at the statistics from the US, the rise in the calories in the food supply exactly corresponded to the rise in obesity rates.fn2 2 In 1980, the average American man consumed 2,200kcal per day. By 2000 he was consuming 2,700kcal per day.3 In 1990, he weighed 82kg (12 stone 12lb) and twelve years later the average American man weighed in at 88kg (13 stone 12lb). The data seems to back up the traditional theory of obesity – that it is a simple energy-in/energy-out equation. But there’s more to the story.

  So, at first glance it seems to be clear: calories cause obesity. But hang on, if we look at the figures more carefully, they don’t add up. The average American man is eating 500kcal more per day during this period. What’s that per year? 500 × 365 = 182,500kcal extra. How much weight should the average American man gain per year if we use Metabology Rule 1?

  Figure 1.3 Obesity rates started to take off in 1980, coinciding with the rise in calories consumed Source: C. L. Ogden and M. D. Carroll (2008). Prevalence of Overweight, Obesity, and Extreme Obesity Among Adults: United States, Trends 1960–1962 Through 2007–2008. National Health and Nutrition Examination Survey (NHANES), June. National Center for Health Statistics.

  If we assume that the amount of physical activity wasn’t increased, and there is certainly no evidence of this, then applying our rule over a year leads us to the following conclusion:

  500kcal per day over one year:

  extra energy in – extra energy out = energy stored

  182,500kcal – 0kcal = 182,500kcal

  1kg fat = 7,000 extra kcal

  Expected weight gain over one year = 182,500 / 7,000

  = 26kg (4 stone)

  A predicted 26kg weight gain in one year. In twelve years, the weight gain of the average American man would be 312kg (49 stone)! But the actual figures say that in this period the average American man gained 6kg (13lb) in total (or 0.5kg per year, not 26kg per year). What has happened to Metabology Rule 1?

  This takes me back to my first few visits to the US, usually for conferences or to teach surgery. When you first visit, everything seems bigger, including the people. I observed the portion size and the type of foods that Americans eat. I went to their gas stations and supermarkets and saw how everything had been supersized, with tremendous amounts of sugar and fat added to their foods. My thought at the time was, ‘Why aren’t Americans even larger?’ Looking at the figures now – 182,500 extra calories per year – I wonder again why all Americans do not weigh 300kg.

  The actual weight gain for Americans who, as a population, were consuming 500kcal extra per day was only 0.5kg (1lb) per year. This equates to 3,500kcal extra, stored as fat, over the year, or just 11kcal per day, the equivalent of one potato crisp, per day, over the calorie limit. Not one packet of crisps, but one crisp. This means that although the average American is consuming so much more than necessary, they regulate their energy balance to within 0.4 per cent of perfection. A separate validation study that more accurately measured energy usage over a year and weight gain found the system to be even more accurate, with only 0.2 per cent of calories ingested being stored as fat.4

  What happened to the ‘missing energy’ of 489kcal per day? To answer this, we need to go back to the rule that is often ignored when explaining obesity: negative feedback.

  Hoarding Energy

  Remember that the negative feedback rule is designed to protect the body against unhealthy changes – by activating processes that will oppose those changes. We know that there are many of these types of mechanisms at work in the body, helping to preserve a healthy state. The regulation of our temperature and hydration are just two of these systems. We know that energy regulation and storage is a critical part of survival in animals. You need to store energy for times of need, but you cannot hoard energy indefinitely, because if you do, as with any hoarding behaviour, things get messy and there is no room to move. So, we should not be surprised if the amount of energy stored within our bodies (just like the amount of water) is also controlled by a negative feedback mechanism. This would explain why, in the presence of so much over-consumption of food, American men’s weight edged up by much less than predicted.

  But how could a negative feedback system work to stop massive weight gain? We know the energy has gone into the body, but the body has not stored it. Therefore it must have been used up somehow. But where? Let’s recap energy expenditure:

  Energy expenditure = Active energy expenditure (gym)

  + Passive energy expenditure (walking/moving)

  + Basal metabolic rate (breathing/heartbeat/temperature control)

  How is the extra energy used up? Do people sense they need to exercise when they over-eat? Most people give it a few seconds’ thought but don’t act on it, so
we can discount active energy expenditure as the most likely scenario. Some scientists say that people might fidget more when they over-eat and this uses up the extra energy in the form of passive energy expenditure.5 But to use up nearly 500kcal per day just twitching your legs is a lot of twitching considering that walking for a mile uses less than 100kcal. I don’t think we fidget our way through that much energy. What about the basal metabolic rate? Does the body ratchet this up in order to stop us storing too much energy?

  The Vermont Prison Feast

  To start to answer this question we must go back fifty years to an extraordinary experiment.6 A team of American scientists, led by Ethan Sims, set up their lab in Vermont State Prison in Burlington, Vermont. They were studying obesity and wanted to observe and analyse what happened when a group of men deliberately over-ate to increase their body weight by 25 per cent over a three-month period. Over-eating takes time and needs to be supervised. The scientists had started the study using students, but aborted it as the students did not have enough time, between their studies, for supervised over-eating. Prisoners were much more suitable for the study. They had nothing else to do and their activity could be monitored (they were to be barred from physical exercise). The scientists negotiated the promise of early releases for the prisoners who were able to gain enough weight to meet their target.

  The scientists employed a dedicated chef for the prisoners and upgraded their plates from tin to china. Breakfast was a full American: eggs, hash browns, bacon and toast. Lunch was unlimited sandwiches. Evening meal was steak or chicken with potatoes and veg. Before bed they sneaked in another full American breakfast. The men started out by increasing their calorie consumption from 2,200kcal to 4,000kcal per day. The scientists observed a steady rise in the weight of the prisoners, but then a strange thing happened that puzzled the scientists. Despite eating 4,000kcal/day, the prisoners stopped gaining weight. They could not put on any more and were still a long way short of their target of a 25 per cent weight increase.

  2,200 to 4,000 … to 10,000kcal

  So the scientists ratcheted up the calories. Most men had to eat 8,000–10,000 calories per day to keep putting on weight, four times what the scientists had calculated would be required. Astonishingly, a few of the prisoners seemed resistant to further weight gain, even at 10,000kcal. Why could they not put on any more weight? The answer came when the scientists measured the metabolic rates of the over-fed, and now overweight, prisoners. In all cases their metabolism had increased considerably. The men seemed to be adapting to the over-eating environment by burning off more energy in order to protect themselves from runaway weight gain. Does this sound familiar? It may explain why our average American male put on 6kg rather than the 200kg-plus that we had calculated from the increased consumption of processed foods in the 1980s and 1990s.

  In 1995, a research group from Rockefeller University Hospital in New York investigated the effects of a 10 per cent weight gain on two groups of patients.7 One group started with a normal weight and the other group were obese. Interestingly, the obese group had a higher than predicted resting metabolic rate than the non-obese group at the onset of the study, before any weight gain. A high-calorie drink made up of protein, fat and carbs was used to drive weight up. This helped the scientists calculate more precisely how much energy was being taken in. What happened to their energy expenditure when the two groups made the 10 per cent weight-gain target? As with the Vermont Prison study, the basal metabolic rates of all the subjects in the Rockefeller study increased – in the non-obese group by over 600kcal/day and in the obese group by even more, over 800kcal/day.

  In a later study in 2006, researchers at the Mayo Clinic in Rochester, Minnesota, analysed twenty-one previous over-feeding experiments, including their own.8 They confirmed that on average the basal metabolic rate did indeed rise by an average of 10 per cent in response to the over-feeding. The more energy that was taken in by over-feeding, the more the body tried to burn up those extra calories to stave off weight gain.

  More Firewood – More Fire

  These over-feeding studies suggest that, yes, there is indeed a negative feedback mechanism controlling our weight and stopping us gaining too much weight too fast. Imagine that you have a log fire at home. Every winter’s day you have one log of wood delivered and every evening you relax by the fire and burn that piece of wood. Imagine, now, that you receive three logs of wood each day. What would you naturally do? You may not have much space to store the wood, so you would probably burn the excess, keep warm, have more energy and avoid the chill.

  The scientific evidence that we compensate after over-eating by burning off more calories is compelling – and it fits in with our epidemiological evidence: we don’t gain 26kg (4 stone) per year, just 0.5kg (1lb). But, if you ask most dieticians or doctors if they are aware of this mechanism – metabolic adaptation to over-eating – they will say no. This is not covered in their training. Why not? You would expect that something so fundamentally important would be understood by the medical profession, and should be accepted public knowledge.

  Some scientists still argue that the increased energy expenditure that we see when we put on weight is because the body has become physically larger. A larger body burns more energy. However, when we analyse the figures this theory doesn’t add up. Most people who gain weight, especially in over-feeding experiments, but also in everyday life, put on the excess weight as fat and not muscle. Fat expends a minimal amount of energy; compared to muscle it is a very efficient organ. In the Vermont study the prisoners had to consume 50 per cent more calories than expected just to maintain their increased body weight. Because their metabolisms were so ‘hot’, they all lost their extra weight as early as twelve weeks after the experiment ended and they were able to resume normal eating. None of them needed any type of diet to get back to their normal pre-study weight.

  A study from Arizona, looking at fourteen men who over-ate 100 per cent more calories than normal, found that within the first forty-eight hours of over-eating (i.e. before any significant weight gain had occurred) their BMR had increased by an average of 350kcal per day.9 The conclusion? Over-eating leads to the burning of energy through an increasing metabolic rate. When we compare how most of our organ systems are kept in check by negative feedback, then it should come as no surprise that there is some sort of negative feedback to protect us from storing too many calories.

  Are our bodies trying to protect us from ourselves by burning more energy when we take in too much food, in a similar way that our kidneys rid us of excess fluid when we drink too much? This would explain why some people seem to be resistant to excessive weight gain despite eating far too many calories.

  But here is an important issue raised by Metabology Rule 2. If the negative feedback mechanism is working to stop some people gaining as much weight as predicted, then it should also be working to stop people losing weight when they go on a diet. Could this explain why diets often fail?

  ‘I Can Lose Weight, but I Can’t Keep It Off!’

  I hear this statement in every clinic that I have worked in. At least one patient will have said it in every clinic, every week, every month over the fifteen years that I have been seeing patients struggling with their weight control. Sometimes I tell the medical students who sit in on the clinic that my next patient will tell us this, and almost all patients prove me correct. Here is a typical example:

  I have been dieting since my teenage years. I have tried all the diets out there. Weight Watchers, Slimming World, LighterLife, the red and green diet, the cabbage soup diet. I have tried them all.fn3 I can lose weight but I can’t keep it off. I can lose 5 or 10kg (1–1½ stone) on a diet but then after two or three or four weeks the weight loss stops. I’m still on the diet, I’m still counting my calories and starving hungry and tired and irritable, but after a while the diet doesn’t seem to work any more. When I go and see my doctor and tell him that the diet is no longer working, he tells me that it is impossible and that I
must be sneaking extra food in. He basically doesn’t believe me. So, I stop the diet and the weight piles back on … fast. Usually I regain all of the weight that I had lost and then gain even more!

  This is the classic story that I have heard many, many times in my clinic. But it does not correspond with the simple ‘calorie in and calorie out’ rule. It’s difficult to understand why someone can restrict calories, sometimes to 1,200 calories a day, and after a while they stop losing weight.

  Let’s see what happens if we apply the same type of system that maintains our bodies’ hydration – our negative feedback system – to weight control and our energy storage, i.e. our fat. Let’s apply Metabology Rule 2. If the system mirrors our hydration system – and we know all biological systems work in a similar way, so this is likely – then there will be one sensor and two switches.

  The sensor will detect the amount of energy stored in the body as fat. Once it senses a change in the amount of fat stored, whether it goes up or down, it secretes a hormone which leads to a message being sent to the two switches. The two switches control:

  The amount of energy we take in – by controlling our appetite

  The amount of energy we use up – by controlling our basal metabolic rate.