Why We Eat (Too Much) Page 4
If the energy storage system in our bodies is really like our hydration system, then it will direct more energy intake than we really need. Remember, we can just about survive on 700cc of water/liquid per day but our hydration system wants us to drink 1,500cc.
The insurance mechanism built into our bodies tells us to drink double the amount of water than the minimum required for survival. Biological systems like to be on the safe side, so they habituate us to drink much more water than needed. In the same way, maybe our energy-regulating system directs us to consume more calories than we need and then burns off the excess. This would also mean that, when we calorie-restrict, it is all too easy for the body to cope with this. It would be similar, in the hydration system, to consuming 1 litre of fluid per day and not the recommended 1.5 or 2 litres. You would be able to survive indefinitely on 1 litre of water per day, but your biological feedback system would be screaming for more fluid by giving you a raging thirst and reducing the amount of urine excreted to a minimum. You would survive, but feel pretty terrible. Does a similar thing happen with our energy regulation when we go on a diet?
Let’s look at the evidence that our bodies adapt to calorie-restricted diets in a similar way they do to fluid restriction.
The Minnesota Starvation Experiment
In 1944 researchers at Minnesota University, led by Ancel Keys, an up-and-coming young scientist in nutrition, set up a study to look at what happens to people’s metabolism during starvation.10 The Second World War was coming to an end and the US recognized that millions of Europeans could be facing famine. They wanted to know the best diet to keep them alive. The Minnesota Starvation Experiment, as the study became known, recruited thirty-six male volunteers, who were conscientious objectors but wanted to help the war and subsequent peace efforts. They signed up to be confined to an allocated living area within the university football stadium, from which they were observed for one year.
The scientists first monitored them for twelve weeks while they ate a normal diet (the 3,200kcal/day stated in the study seems excessive, but the subjects were doing manual work). They were then put on a calorie-restricted diet of about 1,500kcal per day for twenty-four weeks while still doing manual work, and their weight, mood and metabolic rates were measured. After the diet period they were then observed for a further twenty-four weeks on an unrestricted diet.
As planned, during the twenty-four-week diet the subjects lost about 25 per cent of their weight. However, the scientists noted that their metabolism had plummeted by more than would be explained by the reduction in the size of their bodies. Their BMRs decreased by on average a massive 50 per cent of the starting value. Half of this value, 25 per cent, was unexplained by the change in the size of the subjects (smaller people have lower BMRs than larger people). It was as if their bodies were trying to adapt to the starvation environment they found themselves in – by shutting down energy expenditure to the absolute bare minimum. Their heartbeat and breathing were slow and their body temperature was low.
When the group resumed eating normally, their weights increased much more rapidly than would have been expected for their size. The scientists attributed the fast weight gain to the sluggish metabolism that the enforced diet had produced. In all the subjects the weight regain exceeded their initial weight at the start of the study. All the subjects ended up heavier than when they started the study. The distribution of their weight had also changed: they had lost some muscle mass and not regained it. All the weight regain was in the form of fat deposits (these outcomes may seem familiar to readers who have tried extreme diets).
Of interest to anyone who has tried dieting was the report of the psychological changes that the enforced diet brought to the subjects. They suffered with depression and anxiety, and had difficulty concentrating. They became hypochondriacs, worried about their health and wellbeing. They fantasized throughout the day and night about high-calorie foods. They lost their libido. One of the subjects became so depressed that he was reported to have chopped off three of his fingers with an axe. Many recurrent dieters will sympathize with the psychological craziness brought on by dieting. The Minnesota Starvation Experiment was the first study to prove that when you restrict a person’s calories, they will respond, or adapt, by reducing their BMR. Less energy in leads to less energy out.
More recent studies have confirmed these phenomena.11 Professor Rudy Leibel and his team at the Institute of Human Nutrition, Columbia University (and formerly Rockefeller University, New York), have been researching the changes in metabolic rate with dieting and over-eating since the mid-1980s. One of his lab’s seminal studies recruited students to live within the hospital for periods ranging from three months to two years (I hope they got good grades after this). He studied, in great detail and using new techniques to measure metabolism accurately, what happened to the metabolic rate if a person over-ate and gained 10 per cent of their weight; or dieted and lost 10 per cent of their weight; or dieted for longer until they lost 20 per cent of their weight. Each test of metabolism cost the lab $500, so the experiment was expensive to run and therefore has not been repeated by other labs. Leibel found that when a person over-ate and gained 10 per cent of their weight their metabolism increased by 500kcal/day, just as in the Vermont Prison over-eating experiments. When his students lost this weight, and continued to lose weight until they were 10 per cent lighter than their initial body weight, he found that their BMR decreased by 15 per cent (or about 250kcal/day), more than could be explained by weight loss alone. This suggested that the body’s reaction to calorie restriction is to decrease the amount of energy expended, just as in the Minnesota Starvation Experiment and as we would predict if energy regulation has a natural negative feedback mechanism to stop runaway weight gain or weight loss. When Leibel measured metabolism after a 20 per cent weight loss, he saw only a modest further decrease to 300kcal/day. It was as if the body’s protective mechanism, the negative feedback switch, was activated at 10 per cent weight loss.
Figure 1.4 Unpredicted changes in metabolism after weight gain and weight loss Source: R. Leibel et al. (1995). Changes in energy expenditure resulting from altered body weight. N Eng J Med, 332(10), March, 621–8.
All the studies on over-eating and under-eating had to be conducted in an enclosed environment, and were difficult to complete because the volunteers had to give up their normal lives for long periods of time. It was therefore difficult to recruit enough subjects and for this reason these types of studies are few and far between – and rarely quoted.
There are many studies observing the short-term effects of dieting on metabolism, but they are not relevant in our quest to explain the experience of real dieters. The long-term studies go part of the way to validating, and explaining scientifically, what dieters are experiencing. But only half the way, because so far we have only talked about one of the two switches in the negative feedback mechanism of weight regulation: the metabolism switch.
The nature of the second switch has meant that, until now, only prisoners, conscientious objectors and desperate research students would agree to participate in such studies. The problem is that this switch is too powerful to control and therefore people need to be confined, or virtually imprisoned, to prevent them from acting on it.
The Hunger Switch
One of the most striking elements of the Minnesota Starvation Experiment was the psychological changes the volunteers experienced as they lost weight and hunger overcame them. The subjects lost interest in their passions and surroundings. They would obsess over food and constantly stare at cookbooks, fantasizing over them like some sort of alternative pornography. The men would grow anxious and irritated if their small ration of food arrived late. One of the subjects started dreaming of cannibalism and, when confronted by the lead scientist about sneakily buying food during an authorized excursion, threatened to kill him. He was immediately released from the study and transferred to the psychiatric ward, but made a rapid recovery from his breakdown when he was fed
normally for a couple of days.
Hunger is probably an even more powerful switch than metabolism when our bodies try and protect us from weight loss. We now know that the hunger switch is in the part of the brain controlling our body weight. It is a small pea-sized area at the base of the brain, just behind our eyes, called the hypothalamus. But its small size should not be misleading – it contains the switches of powerful basic needs, including the ability to produce a desperate thirst and a voracious appetite. The power of these two switches should not be underestimated. They will drive a human to extremes of dangerous behaviour to secure their goals of water or energy for the body. Most people who live in the developed world only experience hunger when they go on a voluntary diet. I am amazed by the self-control of some of my patients who can at times starve themselves for weeks on end. The strength of the hunger signal can be the same as that of a parching thirst for someone who has lost a considerable amount of weight. Its psychological effects can control your life. If you are in an environment where images and adverts and smells of delicious high-calorie foods surround you, there is only going to be one winner in the diet game – your hunger.
CASE STUDY – HUNGER WILL ALWAYS PREVAIL
Several years ago, a unit at a teaching hospital treated two teenage patients with exactly the same condition. They had tumours of the pituitary gland, the pea-sized gland that controls our hunger and thirst. Both of the patients, who were in their late teens, had undergone brain surgery to remove the tumour before it grew so big that it would put pressure on the optic nerve and cause blindness. The teenagers were of normal weight before their brain surgery, but following it the pituitary gland didn’t work properly. It was unable to switch the hunger signal off. No matter how much they ate, they still felt ravenously hungry. They gained weight rapidly until eventually they found themselves at the doors of a metabolic surgery unit. Weighing 180kg and 200kg (28 and just over 31 stone), they were both deemed sensible enough, motivated enough and fit enough to have an operation (called the sleeve gastrectomy) to make their stomach smaller. The procedure was to make the stomach into the shape of a tube or sleeve and reduce its size drastically. They both had successful operations and lost considerable amounts of weight within the first year of surgery. However, their pituitary switch remained untreated – their hunger drive had not been changed by the surgery, only the size of their stomachs. The boy who had weighed 200kg had confessed before surgery to sometimes eating a box of crisps (a forty-packet variety box). Unfortunately, after one year, when less supervised by his parents, his appetite had driven him back to his box-a-day habit. Both of our patients who had lost weight regained all of it within two years, despite having a very small stomach capacity. The hunger drive really can overcome anything.
The major studies that I have described, looking at weight gain and weight loss and their effect on metabolism, are rare. It’s difficult to recruit subjects for such arduous and long-lasting studies when ravenous hunger, or nauseous satiety, must be faced, and that’s why these studies of human volunteers are so few and far between. However, there are many animal studies that confirm the presence of metabolic adaptation to over- and under-eating. Negative feedback operates in many species to protect against extremes of weight gain or weight loss.
In 1990, a breakthrough in our understanding of metabolic adaptation came when scientists discovered a hormone that was produced by the fat cells and seemed to work on the hypothalamus to switch hunger and metabolism on and off. Finally, we had the last piece in the jigsaw that would prove the existence of negative metabolic feedback. The hormone is called leptin.
The Fat Controller
Leptin is released by fat cells – not in response to any signal, it is just released. That means that the more fat you have, the more leptin there is in your blood. Leptin is the signal to tell the hypothalamus how much fat we are carrying; it’s like the petrol gauge in your car telling you how much further you can go, how much energy there is in the tank.
With the discovery that fat produces the messenger hormone leptin, we now have a negative feedback system for energy expenditure that, as predicted, looks remarkably like the negative feedback mechanism for hydration. The signal comes from fat, via leptin, and the two switches for hunger and metabolism that control energy in and energy out are in the hypothalamus.
Leptin works like this. After a period of over-eating, the volume of fat increases. Leptin is produced within the fat cells and goes directly into the bloodstream. The hypothalamus (weight-control centre in the brain) reads the leptin message and realizes that there are adequate energy stores and that no more is needed. It then acts by decreasing appetite and increasing both the feeling of satiety (therefore decreasing the amount of energy taken in) and the body’s metabolic rate (increasing the amount of energy burned). These factors act to keep weight within a predetermined range (see the description of the weight set-point below).
Leptin also acts powerfully to stop weight loss. When weight is lost after a diet (or famine/sickness), the amount of fat available is reduced. This means that leptin levels in the bloodstream decrease. The hypothalamus senses this and acts to stop further loss of energy by increasing appetite, decreasing satiety (increasing energy in) and reducing resting metabolism (decreasing energy out). These actions slow down or stop further weight loss. When food becomes freely available once more, weight will be gained. This system explains how many people seem to be able to seamlessly control their weight for years and decades, without dieting or counting calories.
But there is one problem with this system. It does not explain why some people become obese. If the system worked perfectly, then obesity would not be such a problem. We have accepted that the system works almost to perfection: there is a 0.2 per cent error. Yes, 0.2 per cent of the excess calories that we consume, on average over a whole population, are stored and not used up. But if the negative feedback system is so powerful that it can alter metabolism up or down by 25 per cent and seriously affect the amount of food we take in by altering our hunger drives, then why is it not 100 per cent effective? Why does it differ in this respect from the hydration system which is always accurate to 100 per cent and gives perfect control of water balance within our body for a lifetime? There must be a biological explanation for this.
Fat Storage is Calculated
Let’s think laterally about this, because it doesn’t make sense that a biological system almost works. Let’s assume that it does work to 100 per cent efficiency, but that the brain has made the decision to store more fat. The brain has sensed, from the environment, that it would be in its best interest to carry more fat. To clarify, the negative feedback system for energy (fat) storage is working to perfection, but the brain has calculated, based on the incoming data from the environment, that it needs to increase the stored energy reserves. We would expect that it has made this decision using information from the past as well as the present to predict future energy needs – it may even be using genetic data passed from previous generations.
Preparation for a Famine
Why would our brain calculate that it would be safer if we carried more energy? Why does it want a bigger fuel tank? The most obvious explanation is that it senses that food may become scarce in the future; it senses a famine or a long harsh winter is on the way. Maybe it has received signals in the past of major food shortages (historically a famine, but in the present day more likely a low-calorie diet). It logs these experiences and calculates that, to be on the safe side, we may need a little more fat just in case the next food shortage is worse. Or maybe it has sensed that the quality of the food in the environment is similar to that found in the autumn and it’s time to tell the body to store more calories for the winter – just like a brown bear which will automatically develop a voracious appetite and increase its body weight by 30 per cent in a few short weeks before hibernating, in response to cues from its environment.12
Our energy storage is too important to be left to free will. Although it
appears that the amount we eat is under conscious control, in actual fact, it is our subconscious brain that controls our underlying hunger and eating behaviour. If the brain wants more energy on board, it will signal more hunger and less metabolic waste, and our weight will go up.
Suggesting that energy storage is under conscious control because we can deliberately stop eating for a period of time is like suggesting breathing is under conscious control because we have an ability to hold our breath. We don’t have to remember to breathe – our subconscious brain does that for us. If we change environments and live on a mountain with thin air, we don’t have to tell our brain that we need to breathe more quickly or deeply – the subconscious brain will sense the environmental change and breathe more deeply for us. In the same way, I think that for some of us certain environmental signals (such as an impending famine or a long winter, as we will see later) lead the brain to want to store more fat.
The Weight Set-Point
The level of energy (fat) storage that our brain calculates is necessary for our survival is called our weight set-point.13 This is like the thermostat that controls the temperature in our house. It will reach, and then maintain, the level that it is set at, by using negative feedback systems.
The weight set-point is the king of Metabology Rules 1 and 2 – it drives them. If your weight is less than your weight set-point (maybe you have been sick or on a diet), then Metabology Rule 2 (negative feedback) will kick in and you will be directed to eat more and your metabolism will shut down. Metabology Rule 1 will then act to shift the weight back up (more energy in + less energy out = energy storage). Likewise, if our weight is above the set-point (maybe we have been over-eating on holiday), then Rule 2 will direct us to eat less and at the same time our metabolism will increase – Metabology Rule 1: less energy in + more energy out = weight loss – until the weight set-point is achieved.