Why We Eat (Too Much): The New Science of Appetite

by Andrew Jenkinson

‘metabolism’, the chemical processes in cells related to energy,

But the other rule is less widely understood. It states that our bodies try and maintain a healthy internal environment by a process called negative feedback. This is the way the body works to stop you losing (or gaining) weight too fast.

The specific function of fat is energy regulation. As we will see, fat not only stores energy but also controls how much we use.

Each fat cell has the unique ability to store energy for times when it may be needed. The more energy it stores, the more bloated it becomes and the more the fat cell expands in size. In the initial process of becoming fatter, you do not grow more fat cells. The number of cells stays the same, but each one becomes swollen with its stored energy, growing to six times its original volume. When there is no more room within the cells, the number of fat cells in the body increases – from an average of 40 billion to over 100 billion in some cases. Unfortunately, if you suck the fat cells out with liposuction (a common, short-term fix performed by plastic surgeons), more and more fat cells are produced to compensate.

You might therefore think that there would be a major evolutionary advantage to having a large energy store. However, it isn’t in your interest to be carrying an oil-tanker’s worth of energy around as this is going to limit your ability to go about your normal survival activities like hunting or running away from hungry predators. So there must be a mechanism to control the size of these fat tanks: fat is very clever, and very efficient, at self-regulation.

If you want to simplify things, then just think of a human as a box. This box transforms chemical energy from food into heat, movement and thought. The rest is stored.

If we were to lie in bed all day and all night we would still use up to 70 per cent of the energy that we normally do – through breathing, heartbeat, temperature control and all our cells’ chemical reactions. The amount of energy that we use to perform these subconscious tasks is called our basal metabolic rate (or BMR).

FACT BOX

Initial weight loss misconseption

Any excess energy is stored first in the liver (as a type of sugar) and then in fat cells (as fat). The liver can only hold a couple of days’ worth of energy; it is generally full to capacity, so in practice excess energy is usually stored in fat.

In the organ systems we explored in medical school we saw many examples of biological negative feedback. These are protective mechanisms that keep us on an even keel (in medical language this is called homeostasis). It means that harmful changes are sensed and automatically counteracted – the reason for negative feedback is to maintain order and health.

Let’s look at the hydration system. This negative feedback system has one sensor connected to two switches.

The kidneys need to purify the blood of waste (urea) by producing urine. They can do this by producing just 700cc per day.fn1

We don’t need to drink 1.5 litres – we could survive on about 700ml per day – but as an insurance mechanism our thirst switch is ratcheted up so that we have plenty of essential water going through our system.

If we go without anything to drink for a few hours, then the kidney senses this. It sends a signal to turn on the switch located in the brain that controls thirst – the water-in switch.

Dehydration process

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

Scientists have calculated that to store 1kg (2lb) of fat you need to take in 7,000 extra kcal.1

considering that walking for a mile uses less than 100kcal.

the basal metabolic rate did indeed rise by an average of 10 per cent in response to the over-feeding.

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.

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.

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.

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.

Read the next section to understand how leptin acts in under and over-eating conditions

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.

Negative feedback system error - how much excess energy stored

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

Interesting fact about brains working principles for calculating fat amount to be stored

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.

Hunger is under 100 percent brains control just like breathing

The level of energy (fat) storage that our brain calculates is necessary for our survival is called our weight set-point.

Weight set point definition

Figure 1.5 The weight set-point

How weight set point works

It becomes a struggle of wills between your conscious desire to be a particular weight and your brain’s subconscious power to regain its desired weight set-point. Invariably – but unfortunately for all dieters – biology always wins.

Summary The secret of successful and sustainable weight loss is to understand how our bodies regulate our body weight set-point. It is not as simple as energy in and energy out. Now that we know that the set-point is the master controller of our weight, we need to find out how our brains calculate where it is set. Various factors in our environment, our history and our family background determine our individual set-point – whether it will be set as slim, obese or somewhere in between. Later chapters will discuss how to identify the signals (in the type of foods we eat and the way we live) that control our weight set-point. Once we understand these signals, our weight set-point, and therefore our weight, will finally be under our control.

This method of manipulating the gene pool of the herd to make them more likely to exhibit characteristics that are valuable to the farmer is known as selective breeding.

However, the food that humans were accustomed to eating started to change around 20,000 years ago with the advent of agriculture, so we need to go back even further to the time when our ancestors ate only what they hunted and collected.

You will not be surprised to learn that the Hadza tribe do not suffer with an obesity problem. They consume meat, berries, fruits, tubers (like sweet potato), and their favourite food is natural honey straight from the hive. These are the foods that they have been consuming for 150,000 years and they see no reason to change their lifestyle.

This suggests that high-calorie availability will not affect weight, as long as those calories come from natural foods.

BMI is calculated by dividing a person’s weight in kilograms by their height in metres squared. BMI = kg/m2. The normal healthy range of BMI is 18–25kg/m2. Someone is underweight if their BMI is less than 18 kg/m2; overweight if their BMI is 25–30kg/m2

Therefore if a child becomes obese it is 75 per cent because of their genes and 10 per cent influenced by their parental upbringing and home environment.

Brave Polynesians

The story of natural selection in pacific islands. How did they become tend to be obese

As I write this, I turn to the globe on my desk, kept there so that I can daydream about the world and what’s happening in it.

You can imagine the hardship and risk involved for those people to safely reach the Pacific Islands. Quite often only those people who were ‘strong enough’ to withstand the starvation of a long trip survived to live in these islands. There was therefore automatically a huge selection bias for anyone settled there. Those people who had enough fat reserves before the journey, or those with metabolisms that could shut down in the face of starvation, had a much better chance of surviving the long journey. The sailors and passengers who did not have this insurance perished and did not have the opportunity to pass their genes on to the next generation.

The thrifty gene hypothesis is based on the theory that people who have an efficient metabolism, or excess fat reserves, can survive periods of famine better than those without.

A sad irony for African Americans is that they were enslaved to increase the agricultural workforce, with many working on sugar plantations. The increased availability of sugar as a commodity and its falling price was a by-product of these plantations. Now the new generation of African Americans, still holding the legacy of their metabolically efficient and strong ancestors – genetically primed with a thrifty gene – are again engaged in a fight, this time against obesity and diabetes, brought about by the legacy of the sugar trade.

Previously we had assumed that the genes we inherited from our mother and father were set in stone. It was thought that they could not be altered. This perception is now changing and it has been proved that selected genes can be switched off (in medical terms this is called methylation because a methyl molecule covers the gene).

Betting on a Hungry Future

How epigenetic prepare the organism for the environment before birth

While these questions are still being investigated, there is a suggestion that some epigenetic traits from previous generations do survive for up to four subsequent generations.21

They have worked out how long it would take animals and humans to evolve using Darwin’s theory and the numbers don’t add up. There is not enough time for us to have evolved by simple natural selection or rare genetic mutations. This is where epigenetics may offer us a fascinating alternative theory of evolution that was discredited many years ago. We know from epigenetic studies that changes to genes, in response to the environment (let’s call them ‘epi-mutations’), occur 100,000 times more frequently than the simple, old-fashioned genetic mutations of Darwin.

The Original Theory of Evolution

Lamarck & Darwin

We have learned that if your mother was starving or undernourished while she was carrying you, then your genes will be turbocharged (by epigenetic changes) to give you a survival edge.

Why would over-nutrition, or obesity, during pregnancy lead to the development of traits that predispose the offspring to gain further weight? This seems to be counter-intuitive. We can understand why obesity traits are found in the genes of those babies who are predicted to grow in a harsh environment, but what is the advantage in terms of survival of having these traits if the food environment is predicted to be plentiful? The answer may come from the lack of micronutrients in the Western diet. Despite a mother being over-weight, she may have vitamin, mineral or other deficiencies because the processed food she is consuming doesn’t contain the same nutrition as old-fashioned fresh foods (we discuss this further in chapter 8). The genes in the foetus sense the deficiencies in the mother and the genetic expression of those genes is thus altered to ensure that enough foods are consumed in the future environment to avoid a similar deficiency. Scientists at Duke University have confirmed that administration of vitamin supplements to mice during pregnancy can profoundly alter the way the offspring look.27

Figure 2.5 Over-nutrition and under-nutrition of pregnant mothers can lead to a higher risk of offspring developing obesity in adulthood.

The other strategy – for drug companies and scientists – would be to target the obesity genes with their own tailor-made epi-mutations in order to reverse the effect of the gene.

There are many different genes that can contribute to people being either obese or lean. One of the first to be identified was the FTO gene. We know people with this gene are on average 3kg (half a stone) heavier than those without the gene.