Why We Eat (Too Much) Page 9

  Controlled experiments asking people to recurrently diet are impossible to conduct properly – we saw in chapter 1 that in order to have a scientifically supervised diet subjects have had to be confined (i.e. in prison) – so to do this properly over years is not practical. Therefore animal studies are more suited to monitoring the effect of weight-cycling on metabolism and obesity. An interesting study from the University of Bergen in Norway compared mice fed in three different ways.3 The first group had a regular low-fat diet, the second group were fed a high-calorie diet and the third group had a diet alternating between high-calorie (for ten days) and 70 per cent of their previous energy intake, i.e. a diet (for four days). There was a total of four diet cycles in eighty days. The typical cycle of weight loss during the diet, followed by weight regain when normal feeding, was resumed – but with an overshoot. More weight was put on after every weight-regain episode. If you asked a patient who had been dieting for many years to draw a graph of their weight loss, subsequent weight regain and weight-regain overshoot with each diet they undertook, it would look exactly the same as in the experiment with the dieting mice (see Figure 3.2): yo-yo weight fluctuations, plus long-term weight gain every time.

  At the end of the study the mice that had gone through intermittent calorie-restriction diets ended up weighing more than the mice that had been eating a high-calorie diet throughout their lives. Dieting seemed to be counter-productive to weight regulation.

  A striking aspect of this study was that the total calories consumed by the dieting mice and those on the high-calorie diet were exactly the same. The dieting mice had somehow developed improved feeding efficiency and a thriftier metabolism, and yet their weight set-point had been nudged upwards by the experience of repeated food restriction.

  Why does this happen after diets? Why do we regain the weight that we had lost, then invariably put on even more? I think that each time we diet we are adding to the data that the brain is using to calculate our weight set-point. The brain cannot tell the difference between a diet that we go on from our own free will and a food shortage caused by an environmental catastrophe like a famine. To the brain it is the same thing – both diets and famines equate to calorie restriction and negative energy balance. These incidents are added to the database when calculating how much energy (fat) it would be desirable to store. The more famines/diets the body has had to endure in the past, the higher the subconscious brain will want your weight set-point to be – it wants that insurance just in case the next diet/famine to come along is critical. This fits in with the research out there, plus, more importantly, with the actual experiences of patients who are struggling with their weight – weight loss, weight regain and then, as the set-point is notched upwards, the weight settling at a higher level than when they started the diet. Recurrent dieting is a great way to train your body to become obese.

  Figure 3.2 Weight gain in mice on a weight-cycling diet compared to those on a high-calorie diet Source: S. Dankel et al. (2014). Weight-cycling promotes fat gain and altered clock gene expression in adipose tissue in C57BL/6J mice. Am J Physiol Endocrinol Metab, 306(2), January, E210–24.

  Metabolic Variation

  In medical school, we were taught that our patients’ basal metabolic rates could be calculated if we knew their height, weight, sex and age. Using a complicated equation called the Harris–Benedict formula we would be able to advise patients exactly how much energy they were using and therefore help them to estimate how many calories they needed to consume per day to maintain, or lose, weight. This formulafn3 is the one now used in many smartphone apps to inform people how much basal energy they are burning. These apps are designed to empower the planning of calorie intake in response to metabolic output. Using our first law (energy stored = energy in – energy out), the users can plan their weight-loss strategy. However, there is a fundamental problem with this equation and therefore all apps that use it. The equation calculates the average metabolic rate expected for that person, but it fails to consider the wide variability in metabolisms between people of the same size, shape, age and sex. In other words, the equation ignores our inherent metabolic variability.

  CASE STUDY – CHANGING METABOLISM

  Two friends sit down together for dinner at their favourite Italian restaurant. The women had been flatmates for years, cooking and eating together while at university a decade before. Now it was time to catch up. They are strikingly similar in appearance: same height, weight and build, and a bystander would be forgiven for thinking that they are related, but they are not. They are both overweight, but not obese, maybe dress size 12–14.

  One of the friends is fretting over the menu: she is starving but can’t find a low-calorie option suitable for her; the other friend is not as hungry and is not concerned with calories. As their conversation turns to diets, the starving friend admits to really struggling to hold her weight down. But her old friend reminds her that when they had lived together ten years earlier, they ate together and exercised together and had identical metabolisms.

  If we could have checked their metabolic rates ten years earlier we would have confirmed that, yes, they were indeed identical. However, now the starving friend who is looking at the low-calorie option has a metabolism much lower than her friend, probably 200 or 300 calories less per day. The reason? She has been fighting to get from size 14 to size 10 for the last decade – unsuccessfully. This has resulted in her weight set-point being raised to the equivalent of a size 16. The subconscious brain wants size 16, just in case the next diet/famine to come along is more severe. It must protect the body’s capacity for survival. Our recurrent dieter is fighting against this losing battle by conscious calorie-counting and denying her appetite, while her body is responding with a lower metabolic rate. We can guess who the winner is going to be.

  10km Run or Three-Course Meal?

  If you take a group of ten people who are the same sex and age and size, the Harris–Benedict formula will give you an accurate calculation of the whole group’s average resting metabolic rate. If they all had sedentary jobs and didn’t go to the gym, then you would expect that they would all use up a similar total amount of energy per day. Let’s say in this example the app told us that they used 1,500kcal/day as basal metabolism. However, when you measure each member of the group’s actual metabolic rate you would see that there is a striking variability between individuals. The lowest metabolic rate of the group of ten people would be 1,075kcal per day whereas the highest metabolic rate would be 1,790kcal per day.4 Just as with The Biggest Loser contestants after their weight loss, this difference of 715kcal/day is equivalent to the low metabolizer having to go on a 10km run every day in order to have the same energy expenditure as the high metabolizer, or the high metabolizer being able to eat the equivalent of a three-course meal extra every day compared to the low metabolizer!

  The difference in metabolism in people of the same size is governed by whether their current weight is above, below or the same as the subconscious brain’s desired weight, i.e. their weight set-point. If you are heavier than your brain’s desired weight for you, then your metabolism will speed up; if your weight is below your set-point, as it is when you are a few weeks or months into a diet, your metabolism will slow down.

  Figure 3.3 The difference between the highest and the lowest metabolizers in a group of equally sized people Source: J. Speakerman et al. (2004). The functional significance of individual variation in basal metabolic rate. Physiol Biochem Zool, 77(6), November–December, 900–915.

  The Dimmer Switch

  The average male energy intake is about 2,500kcal per day. This is the same as 10.5 million joules of energy per day. Each day contains 86,400 seconds. From this we can calculate the rate (or power) of energy that an average male uses. The power needed to run a human body is about 120 watts – that’s the same power that is required for a light bulb. However, as we have seen, this is just an average. The amount of energy used can range from 60 watts to over 240 watts. Imagin
e the variability in people’s metabolism as being like a dimmer switch attached to a light bulb – it can be set to shine brightly or glow dimly – or anywhere in between.

  How Does Metabolism Change?

  The dynamic, changing metabolism that I have described is a key feature of our energy-regulating system, but scientists are still uncertain as to exactly how these metabolic changes occur. If they could find the answer to this, then they could target a drug or therapy to halt metabolic changes and make it easier to lose weight on a diet. Having observed hundreds of obese patients, and studied the literature on metabolic variability, I think that the two most likely mechanisms are:

  The level of metabolic stress in our bodies, set by something called the autonomic nervous system.

  The amount of heat we produce from chemical energy, a process called thermogenesis.

  When we match what patients experience, while either dieting or over-eating, with the research that is out there, these are compelling explanations of how metabolic changes occur within us.

  Run or Fight?

  Let’s start with the autonomic nervous system. This is called autonomic because it is autonomous or automatic. We have no conscious control over this system. It is also commonly known as the fight or flight response. The subconscious brain will determine whether we are in a safe environment or whether we are in danger and will adjust the autonomic nervous system (or ANS) accordingly.

  Want to know what the fight or flight response looks like in real life? A couple of years ago I was walking in the countryside with my pet spaniel, Maxwell, through a large grass field. As we neared the centre, I noticed that some of the cows in the field, a herd of about ten of them, were going to be blocking our route to the exit gate. I had never had any issue with cows before and would normally have walked straight through them, but something told me to skirt around them this time. In normal circumstances, you would expect the cows to ignore you and go on grazing, but this particular time they had their ears up and then I noticed that they were not actually heifers but adolescent bullocks. As they started their charge towards us, I let go of Maxwell’s lead and, for the first time ever, ran like a professional sprinter to the field’s 5-ft barbed-wire fence – one I would not normally be able to climb (I’m not very athletic) – but I vaulted over it, landing in nettles with numerous cuts that I could not even feel. As I looked back, I saw the herd chasing after poor Maxwell, ears flapping, who was having a similar autonomic nervous system response. If it hadn’t been for our ANS response, both man and dog could have been trampled by these angry young bulls.

  Once danger is sensed, we have an innate ability to switch on the afterburners – this is the fight or flight response. Either way, whether we are running away from danger or are cornered and have to fight, we become strong and fast and have more acute vision and clearer thinking. The medical term for this is the sympathetic nervous system (SNS) response. Here is what the SNS or fight or flight response does to your body:

  Increased heart rate and blood pressure to pump blood to the musclesfn4 for running or fighting

  Sweating to cool the body down during the anticipated exertion

  Constriction of the blood vessels to the skin so that blood can be preferentially pumped to the heart and brain, leading to a pale appearance

  Increased blood glucose level to feed the muscles and brain

  Faster breathing to increase oxygen to the blood

  Increased oxygen and glucose-rich blood to the brain, increasing speed of thought

  Dilation of the pupils (for better vision)

  Release of natural opiates or morphine-like painkillers (called endorphins) in anticipation of injury.

  The SNS’s fight or flight response is triggered by the hormone adrenaline, which pumps through the bloodstream and activates the SNS, a series of nerves located in the core of our bodies, along our spinal column. It would probably have been an evolutionary survival advantage to have these superhuman traits all the time, but this cannot be the case for one simple reason: energy. The fight or flight response uses up a lot more energy compared to times when we are going about our normal activities. Our SNS reserve kicks in at times of mortal danger.

  Time to Relax

  The opposite of the SNS, adrenaline-fuelled survival response is the relaxation response. This is due to activation of a parallel system called the parasympathetic nervous system (PNS). When this system is more active, our bodies relax into a more energy-conserving state. We are in a safe environment, so it is all right to reduce heart rate and blood pressure; we have more even and relaxed breathing; we reduce blood flow to the brain and just … chill.

  The traditional way of thinking about the autonomic nervous system is as a way of the body adjusting to different levels of danger – this is what doctors are taught in medical school. But what if this system also had another function? And what if that function were to alter our energy expenditure to offset food excess or deficit? If this were the case, how would our bodies react to energy excess, to over-feeding? We could hypothesize that by activating the SNS, energy expenditure could be increased; just like changing to a lower gear while driving, you don’t drive faster but you know you are using up more fuel.

  What Happens If You Over-Eat?

  How would SNS activation manifest itself in response to over-eating? How would it make us feel if this were really the way that we adapted metabolically to food excess? We would be more likely to have a high resting pulse rate and to suffer with high blood pressure. We would sweat more than normal; we would have a high blood glucose, which would then stimulate an insulin response (I’ll explain this later) and make us crave sweet foods. Our muscles would feel strong. Our brains would be rich in glucose and oxygen and we would feel clear-headed and alive. We would feel good psychologically as a result of the trickle of endorphin painkillers that the SNS provides. Does this feeling sound familiar? Holiday time!

  What would happen if this system were also protecting us against weight loss when we went on a diet? In this case, the PNS, the relaxation system, would dominate in order to try and reduce energy expenditure and limit weight loss. Our heart would use up less mechanical energy by reducing our pulse rate (the speed it pumps) and reducing blood pressure (the force it pumps). There would be less blood pumping to our muscles and for this reason they might start to feel fatigued more easily. There would be less blood flow to our brains compared to when we are well nourished, and we would perhaps start to notice that it was more difficult to concentrate; we might even get confused and agitated more easily. We might miss that trickle of wonderful endorphins that we had become accustomed to and therefore feel depressed and empty. Does this sound familiar to anyone out there who has dieted? I firmly believe that what patients describe and what these ANS responses lead to fit very neatly together.

  For those of us that do not happen to be on a diet (i.e. most of the population living in our calorie-rich environment, and consuming more calories than we need), what would happen? What effect would metabolic adaptation, in the form of an SNS-type response, have on a population which over-eats? We know from chapter 1 that we are consuming 500kcal more per day than thirty years ago. We also know that most of this excess energy, all but 0.2 per cent of it, is somehow burned off without effort, otherwise we would all weigh over 300kg. A population adapting to over-eating by over-activating their SNS would develop two major health problems: high blood pressure and chronically high blood glucose levels leading to Type 2 diabetes – which are exactly the health problems we see in industrialized cities. In addition, the population would find it difficult to wean themselves off the natural opiates, and the feeling of wellbeing, that the metabolic response to over-eating gave them. The food industry might try and profit from this feeling.

  SNS Increase – Metabolic Rate Up

  There is evidence to back up this theory from Rudy Leibel’s research at Rockefeller University.5 When they performed their research on metabolic changes after 10 per cent weight ga
in and 10 per cent weight loss they also measured the activity of the subject’s autonomic nervous system.

  After a 10 per cent weight gain the subject’s metabolism was hot – they were burning many more calories, seamlessly and without effort. The researchers noticed that during this time the subject’s SNS (fight or flight) activity was increased and PNS (chilling) activity was suppressed. This fitted in with the increase in metabolic rate of 600kcal/day that they measured after the weight gain. The increased SNS activity seemed to be the cause for the high metabolic rate.

  When they measured ANS activity after 10 per cent weight loss, mimicking a conventional diet, they found a much more relaxed state, conserving energy by activating the PNS. I would expect that if they had asked the subjects how they felt after the 10 per cent weight loss, muscle fatigue and dulling of thought processes would have been prominent symptoms.

  Further studies have confirmed that when humans chronically over-eat their SNS activity is raised, and when they are starved their PNS acts to conserve energy.6 Curiously, the metabolic adaptation that is explained by this process seems to have been missed by most doctors and scientists. Most labs are just not looking in this direction for their obesity treatment.

  So there is compelling evidence that metabolic adaptation takes place and that it is driven by changes in the autonomic nervous system. But there is evidence that there is another way in which our bodies match our metabolism with our food intake and shift the body towards its ideal weight set-point. This second method is called thermogenesis. This theory suggests that extra energy is burned off – literally as heat.