Alarm Bells

It is customary when talking about weight to begin with a series of alarming statistics, and so we’ll proceed accordingly. According to the World Health Organisation in 2022 one in eight people globally were obese - including 160 million children and adolescents - and around 43% of adults globally were overweight. Worldwide, adult obesity has doubled since 1990 and adolescent obesity has quadrupled. At this point we usually talk about an impending ‘obesity epidemic’ set to ruin civilisation. We ask who is responsible and all too often point the finger at the individual, caught frozen with a glazed donut in hand. We then ask what we can do about it, and whatever we propose - injections, more exercise, food labels… -it is hard to escape a slightly defeatist tone to the whole issue. As if we were doomed by our own insatiable appetites, and the best we can hope for is to suppress them for a while.

Weight remains a topic fraught with misunderstanding - despite so much media noise - and when we don’t understand something we tend to give up on it, or wait in hope for someone to fix it. We also develop assumptions which aren’t true. For instance, when analysing the causes of obesity we will see that it makes no sense to blame people for being overweight, and if we take a closer look at the ’obesity epidemic’ we see that actually obesity is not really the problem at all, and by fixating on it we are losing sight of a far bigger problem. But perhaps most importantly, losing weight and getting healthy doesn’t have to be a struggle, or worse, a penance. In fact getting healthy simply requires a switch from trying to satisfy our appetites to satisfying our hunger, in ways that are enjoyable.

Before taking a dive into this, it is useful to go back to the very basics and ask some rudimentary questions like, what exactly makes us put on weight in the first place.

Liberating Carbohydrates

The history of agriculture could be seen as a series of advances which have allowed for higher yields of calories relative to the effort put in. There has always been a pay-off between the amount of energy spent getting and digesting food and the amount of calories ultimately used or stored. For around 2 million years our paleolithic ancestors invested a lot of time and energy in hunting, gathering, preparing, chewing and digesting food. They lived off meat, fish, nuts, vegetables, mushrooms and fruits. Starch - carbohydrates - such as found in roots or cereals was locked within fibre and took effort to release. Then came the development of agriculture around 10,000 years ago which eventually saw carbohydrates become the main staple of our diet. Starch has now been refined to such a degree that, with the exception of wholemeal bread etc., we have easy access to essentially pure carbohydrates, that is glucose, with very little investment needed to digest it. Put simply, we no longer have to work for our calories. It would be tempting to see liberation of carbohydrates as the main reason why nearly half of all adults globally are now overweight…and yet only a small fraction of the glucose we ingest actually gets stored as fat. Something else must be doing it.

Around 300 years ago we began refining sugar, mainly from cane or beet and more recently from maize in the form of high fructose corn syrup. Sugar is a pairing of a glucose and a fructose molecule, around half and half. Once ingested, the glucose and fructose are separated very quickly in the gut. The glucose is used quickly for energy or stored as glycogen. All of our cells require glucose to function. It is vital to life. On the other hand fructose makes sugar taste sweet but beyond this has little nutritional value other than it’s conversion into fat, which was useful when we faced periods of scarcity, such as winter. Our Stone Age ancestors probably ingested around 15g of fructose a day, plus a lot of fibre in the fruit they got it from, and for the first few hundred years of refined sugar’s iniquitous history, it was not consumed in great quantities, being too expensive for the ordinary person. But by 1977 our diet contained around 37g per day and by the 2020s we ingest on average around 75g of fructose a day.

Hiding In The Open

Why is there so much sugar in our diet? It is tempting to say that we eat so much sugar because we like it, but like many of our common sense assumptions, while not altogether wrong, it misses the mark. Sugar by itself isn’t that nice. If offered half a bowl of pure sugar, most people would struggle to eat it, but mixed into ice cream and the struggle disappears. Few people could drink eight teaspoons of sugar in a glass of water, but mixed with citric or malic acid, as with orange or apple juice, or with salt and other flavours as in cola, and we’ll easily gulp it down.

The primary role of sugar in our modern diets is as an additive, and this is the real reason we now ingest so much. As an additive sugar is extremely profitable. It makes us more likely to buy that particular food or drink. It also acts as a cheap preservative, making food easier to transport and store. Conversely fibre is less palatable and more perishable, so has gradually found its way off the supermarket shelves. It is this combination of low-fibre and high-sugar that is characteristic of ultra-processed foods. Most of us eat some form of ultra processed food daily, some more than others. Over the months and years this exposure to ‘hidden’ sugar leads to weight gain, or metabolic disease, or both.

To see how this happens we need to unpack a little of what sugar does in the body.

Glucose Metabolism

The human body has been described by the writer Gary Taubes as ‘a busy chemical plant transforming and re-directing energy’. Every process necessary to life - from movement to thinking, to cell division to digestion - requires a transfer of energy, and the main molecule responsible for this is called adenosine-triphosphate (ATP) which in turn gets ‘refreshed’ by glucose. Most of the glucose we eat gets sent to the organs, muscles and tissues to be used in this way, but around a fifth goes to the liver where it is stored as glycogen (muscles also store glycogen). The important thing to note here is that glycogen is non-toxic. You can carb-load as much as you like but the glycogen won’t do any damage - it simply sits there waiting to be broken back down to glucose when needed. The second point is that only a very small amount of the glucose is converted by the mitochondria (the power houses of the cell) to triglycerides and thence to fat, so one would need to eat a lot of glucose to actually put on weight. With the exception of sugar, carbohydrates do not in themselves make us fat. However they do make it hard to lose weight once it’s on, because they are such a calorie-dense source of energy, and too many carbs can blunt our sensitivity to insulin. So while glucose by itself is okay, when twinned with fructose it sets up processes that make us gain fat.

Unlike glucose, all of the fructose we ingest is metabolised by the liver. It doesn’t provide energy for tissues. (Sweet tasting energy gels often used by runners often use dextrose or maltose which are chains of glucose molecules.) The mitochondria of the liver then has to metabolise the fructose. The small amounts of fructose ingested by our paleolithic ancestors weren’t an issue for the liver, and in any case the plentiful fibre slowed its absorption. However the modern way of ingesting large boluses of fructose with little or no fibre quickly overwhelms the mitochondria, leading to high triglyceride and fat production. Moreover this type of fat tends to be deposited around the internal organs, so called visceral fat, which is more harmful than the more spread out subcutaneous fat.

Alcohol is (Regrettably) Sugar

It is worth noting that another common sugar that is metabolised in the same way as fructose is alcohol, which is why seasoned drinkers normally develop a beer belly. In fact a standard can of beer carries roughly the same sugar hit as a can of soda. A few differences are that the brain is actually able to use alcohol as a source of energy. Also in terms of general use, alcohol is self-limiting; you can only drink so much before you have to stop, whereas with sugary drinks one can potentially keep drinking all day. Also soda often contains salt and caffeine which boost their addictive quality and with some justification soda has been called ‘alcohol for children’.

At this point we can mention the growing condition of non-alcoholic fatty liver disease (NAFLD), also known as metabolic dysfunction-associated steatotic liver disease (MDASLD) or ‘sugar belly’, depending on how diplomatic you want to be. Historically a fatty liver was the result of many years of alcohol misuse. but we are now seeing more NAFLD which can progress to non-alcoholic steatohepatosis (NASH), a precursor for cirrhosis. The culprit is almost certainly high sugar consumption. Since 1980 cases of NAFLD and NASH have doubled alongside fructose consumption. In the United States an estimated 6 million have NASH and it is the third leading cause of liver transplants. Around 31% adults and 13% children are estimated to have NAFLD in the US. Depressing though these figures are, the good news is that NAFLD is to a significant extent reversible with the right measures, i.e. cutting sugar from the diet.

The Insulin-Leptin Double Act

Hunger is one of our fundamental drives and we have evolved feedback signals to both motivate us to find food and tell our bodies when we have had enough. These feedback loops are mediated by our nervous system - such as our ‘rest and digest’ parasympathetic system - and hormones. Two of the most important hormones are insulin and leptin. Insulin is secreted by the pancreas after we eat. It plays the crucial role of enabling glucose to enter cells. Another role of insulin is to tell the adipose cells to store energy in the form of fat. The general rule with regards to good metabolic health is; the less insulin needed, the better. If the cells are sensitive to insulin then less will be needed and - because insulin tells adipose cells to store energy - less fat will be laid down.

*Insulin allows glucose into cells and tells fat cells to store energy*

However, with chronically high glucose intake - for instance if we are snacking - insulin remains elevated to try to mop up the excess glucose. Furthermore with refined carbs like sugar we get spikes of insulin, and the subsequent crash leads to lethargy, brain fog, and the craving for more. Over time raised insulin leads to reduced sensitivity to it as our body ‘down-regulates’ the insulin receptors, leading to insulin resistance. The combination of high blood glucose and insulin resistance is characteristic of metabolic syndrome and type 2 diabetes.

It is worth mentioning in passing two exceptions when evolution ‘says’ it’s better to gain weight; puberty and pregnancy. In both cases our body is trying to gain weight and hormonal shifts occur that make us insulin resistant. This may account for why adolescents are more at risk of obesity. On the other hand, it is important that teenagers know that a bit of extra weight at that age is to be expected, and will most likely come off again so long as they avoid a lot of sugar. This is also why some women develop gestational diabetes which normally reverses after the baby is born, though is also a time of risk where many mums put on weight which they find a challenge to lose later (especially when coupled with the demands of parenthood).

*Leptin, released from fat cells, tells our brains we are full.*

Leptin is the second part of the act. When fat cells start to fill up they quickly release leptin. The role of leptin is essentially to make sure that we don’t eat too much, as being continually full is not a useful evolutionary strategy, we need to go out and get things done. Leptin travels via the bloodstream to the brain where it acts on the hypothalamus. It is a satiety signal which says we’re not hungry anymore. It also tells the brain that we can now engage in metabolically expensive activity. In other words, we can burn energy. This leads to activation of the sympathetic ‘fight-flight’ nervous system, which manifests as fidgeting ‘nervous’ activity. This whole insulin-leptin feedback loop is known as ‘anorexigenesis’ - that is, eating provokes a cascade of hormonal responses which then reduce our appetite. Now we’ll see what happens with chronic sugar exposure:

*Chronically high insulin blocks the satiety signal, so we feel hungry despite being full.*

The Sting in the Tail

As we’ve seen, with a chronically high intake of glucose insulin becomes less effective at unlocking the cells, so we also wind up with more circulating insulin. The increase in fat storage also leads to more leptin being released. But here is the sting in the tail - insulin blocks the action of leptin on the brain. This leads to the paradoxical effect where, even though we have too much fuel, the brain is not getting the satiety signal - it ‘thinks’ that we are hungry and don’t have energy to burn. This manifests as an increased appetite and reduced drive for physical activity. This is ‘orixogenesis’ and is the flip-side of the normal process previously described. Sugar therefore presents a triple-whammy to our bodies: 1. The pure glucose numbs our insulin sensitivity, leading to higher insulin in the bloodstream, leading to higher fat storage. 2. Fructose gets converted to triglycerides that get stored as fat and 3. Insulin blocks leptin, so we feel hungry, and tired, despite having surplus fuel.

Changing Our Minds About Weight

As the months and years progress, the above process leads to the build up of fat, be it in the liver or around the waist. Why certain people are more prone to putting on weight is down to many reasons. Undoubtedly some people are more prone to putting on weight due to genetic factors, then there are hormonal changes of adolescence and pregnancy; many people simply aren’t in the habit of eating salads, or don’t have the time, and there is a strong statistical association between obesity and ‘lower’ socioeconomic status. Poorer neighbourhoods tend to have more shops selling ultra-processed foods and less branches of Whole Foods. Whatever the reason, it is more accurate to regard metabolic disease, and obesity, as a result of chronic exposure to sugar. When we re-frame obesity as an exposure issue we can locate the real cause; not as some mysterious quality like willpower that an individual is supposed to possess but in the reality of supermarket shelves groaning with refined carbs and sugar-rich products (and governments unwilling or unable to regulate the food industry).

An article in the prestigious medical journal The Lancet summed it up in 2011: “The simultaneous increases in obesity in almost all countries seem to be driven mainly by changes in the global food system, which is producing more processed, affordable, and effectively marketed food than ever before. This passive over-consumption of energy leading to obesity is a predictable outcome of market economies predicated on consumption-based growth.”

Now that we have a clearer idea of what actually causes us to put on weight, we can start looking at how to lose it and stop it coming back on. But before this, we need to unpack a few other common myths:

Eating Fat Makes You Fat - This is an especially stubborn myth. It gained credibility around the 1960s when increasing numbers of men in affluent countries were succumbing to coronary artery disease. Studies at the time showed a correlation between high fat diets and increased cholesterol in the blood. This was convincing enough that by the 1980s the US and UK governments issued dietary guidelines and doctors began promoting low-fat diets. Yet rates of cardiovascular disease continued to climb and it was far from proven that fat in the diet led to fat in the bloodstream. We know that the liver synthesises cholesterol. In fact, if you eat more healthy fats like omega-3, the liver responds by producing less cholesterol. Healthy fats are also too buoyant to stick to the lining of blood vessels. Also, how to explain significant outliers like France where a fat rich diet accompanies low rates of heart disease? Fat is calorific: it contains around twice as much energy per gram of carbohydrate.Yet unlike sugar fat promotes early satiety and is non addictive so we tend to eat less.
Eventually the fat hypothesis was debunked. In 2008 studies by both Oxford University and the UN Food & Agricultural Organisation found ‘no probable or convincing evidence’ that high dietary fat causes heart disease or cancer. The alternative hypothesis, that perhaps it was sugar causing heart and disease, was raised in 1980 by the UK doctor and professor John Yudkin, which he outlined in his classic book ‘Pure, White and Deadly’. Unfortunately not only was he ahead of his time, but he was also going against the prevailing medical orthodoxy, not to mention the sugar industry. He was smeared in publication and denied work. Only recently has he been vindicated.
Fat is Just Storage Tissue - Adipose tissue is far more than a storage sink for excess energy. It is also active endocrine tissue, continually secreting hormones such as leptin and oestrogen. It also is pro-inflammatory, which likely plays a role in chronic pain. Again, losing adipose tissue can go a long way to remedying conditions like chronic inflammation or polycystic ovarian syndrome.
A Calorie Is A Calorie - An authority on sugar is the paediatric endocrinologist Robert Lustig, whose lecture ‘Sugar: The Bitter Truth’ now has a few million views on YouTube. One of his central points is that calories are not the same. The body has to invest more energy in digesting certain foods. Protein takes around twice as much energy to metabolise compared to carbohydrate. The speed at which we digest calories is also important. Fibre plays a crucial role in slowing the release of carbohydrates, which is why fruit and vegetables are okay to eat (a note of caution - certain fruits like grapes and bananas actually release sugar very quickly). An almond contains around 160 calories, but only 130 get absorbed due to the high fibre. Fibre is very much the antidote to refined carbs and sugar, the more the better.
Obesity Is The Problem - Obesity is in fact a result of the problem but not the cause. The real issue is metabolic dysfunction, which leads to conditions like type 2 diabetes, chronic kidney disease, cardiovascular disease, fatty liver etc. Around 20% of people with obesity have healthy metabolisms. Even though they are at greater risk of metabolic disease there are still far more people of normal weight who are at risk and who will eventually strain our health and social systems.
Getting Sick From Sugar is a ‘Behaviour’ - While it is tempting to blame the individual for ‘getting fat’, this misses the point. People don’t choose to become sick or overweight. They become so due to metabolic processes which have been deranged by years of covert exposure to refined carbs. What may seem like small amounts of daily sugar - a breakfast orange juice or bowl of granola, a bun or few biscuits in the morning, a sweet latte or lunchtime smoothie with a meal deal, etc. - over the months and years have a cumulative effect. Many of us can’t understand why we gradually put on weight despite what seems to be a healthy diet.

The Limits of Lifestyle

There is a view that we ought to stop talking about obesity altogether due to its powerful stigmatising effect, and that we need to do what we can to accommodate larger people and even stop preaching the lifestyle mantra of ‘eat less, exercise more’. The findings of an analysis recently published in the British Medical Journal (BMJ) stated: ‘Rigorous evidence has indicated that lifestyle interventions, which involve a change in behaviour with a focus on restriction of energy intake and increased physical activity, have little to no effect on sustained weight loss.’ (BMJ July, 2025). The overall conclusion seems to be that simply telling people to eat less and exercise more is not effective.

That said, ‘lifestyle intervention’ can mean a lot of things. It’s not surprising that people don’t respond to advice alone, but there’s more to lifestyle medicine than just telling people what they already know. There are many community and city-wide measures that can be taken, from Park Runs, to healthy school meal campaigns, to ‘active transport’ initiatives, from increased cycle lanes and cycle-to-work schemes, to the legendary Ciclovias in South America, that offer a different future to business as usual, an alternative to the view that the best we can hope for is the mass roll out of medications to deal with obesity and metabolic disease.

What is the role of exercise? While exercise does not usually lead to weight loss directly - the body is just too efficient - exercise will increase good weight, that is muscle and bone density, as opposed to bad weight like visceral fat. Also, the more dense the muscle, the more mitochondria, which means that it can burn fuel a lot more effectively, even while at rest. Plus it increases sensitivity to insulin, which counteracts high blood insulin levels and the ‘numbing’ effect it has on leptin. Exercise increases our perception of how much we can exercise; the more you do the more you feel you can do.

Do Diets Work?

In the same edition of the BMJ mentioned above, another analysis of 99 randomised control trial showed that intermittent fasting (in particular alternate day fasting) produced ‘significant weight reduction compared with ad libitum diets’ (i.e. eating when you want). It also showed that ‘continuous energy restriction’ of 30% calories also produced weight loss though the effect was generally quite small 2-4kg, and that adherence waned over a year to around 9.5% restriction. And while intermittent fasting is a very effective weight loss tool again adherence seems to wane with around 22% still following the regime after a year.

If there is a common theme in these findings, it is that while diet or fasting does lead to weight loss, after a period of time in most people the weight comes back on. There are two important points to be made with regards to this. The first is that, as we have seen, there is more to health than just losing weight. You may not necessarily shed a lot of kilos by fasting and dieting, but most people report feeling a lot better in any case, they may have greater muscle density, and it improves metabolic health. Secondly, it is inaccurate to conclude that calorie restriction doesn’t work. The evidence shows that it does, but just that it is difficult to sustain.

A sustainable way to achieve a calorie deficit is to cut back on calorie-dense carbohydrates. However carbohydrates are part of a healthy diet and do not in themselves make you put on weight. Certain populations like the legendary Okinawans eat a lot of rice but don’t generally get overweight. The difference is that these cultures are physically active and also eat a lot of protein and fibre-rich vegetables. Remember that fibre effectively slows down the absorption of glucose so carbohydrates with fibre are far better, so aim for carbohydrates with a low glycaemic index, such as wholemeal. Over the months this sets up a positive feedback loop:

Regarding the low-fat vs low carb debate, numerous studies now show that low-carb diets are as, and in many cases more, effective than low-fat diets in losing weight. Overall the evidence tells us that a sustainable diet to maintain a healthy weight and metabolism amounts to: more protein, more (good) fat, more fibre, less simple carbs. Some diets - such as keto - go so far as advocating cutting simple carbohydrates completely. While no doubt effective there are many specialists who are sceptical about removing such a vital energy source from the diet and in any case for most of us a zero carb diet is not sustainable.

Turning Down the Food Noise: Appetite Vs Hunger

There has been a lot of interest recently in the phenomenon of ‘food noise’, that nagging persistent craving for something to snack on. This is likely a combination of psychological and organic factors, such as the blunting of the aforementioned insulin-leptin pathway. Sugar is also weakly addictive as it leads to a dopamine spike - before even the sugar enters the bloodstream - and anything that triggers a rapid release of dopamine quickly becomes addictive.

Another way of understanding food noise was set out by John Yudkin in Pure, White and Deadly in which he points out the difference between hunger and appetite. Hunger is a physiological need for calories. On the other hand appetite is what makes us eat even though we don’t need to. Hunger can be satisfied because of our organic feedback mechanisms, whereas appetite is the beast that gets hungrier the more we feed it. We could simplify this further: Healthy food satisfies our hunger, we seek rubbish food to satisfy our appetite. When we crave food, it is rarely a carrot or handful of peanuts we reach for (and which would easily keep our hunger at bay). The food industry is geared towards provoking our appetite.

In order to curb our food noise the first step should be pretty obvious - radically cut down sugar. The World Health Organisation recommends less than 50g a day. A general rule of thumb would be less than 10 tsp a day for adults and less than 5 tsp for children. Some will find this more difficult than others, and to them I would say, don’t despair: Recall that around half of sugar in our modern diets is effectively ‘hidden’; for instance the average breakfast cereal in the US contains or exceeds already the daily sugar limit for children. In eliminating these hidden sugars half the battle is already won. It can be a bit laborious counting the calories and percentages on packaging. A simpler way of avoiding hidden sugars is to prepare your own (or family’s) meals, from as basic ingredients as possible.

The next one-third of sugar comes in soda or fruit juice, including smoothies, so again by replacing this another significant step has been taken. Really children ought to be drinking only water or milk. What about desserts and baked goodies? Traditionally desserts and baked goods are eaten as treats or after dinner and eaten in this way they are far less likely to cause insulin spikes. It is only in the past few decades that sweet snacks have become a mainstay of our diet, and it goes without saying they are loaded with sugar. (For what it’s worth, the only advice I can offer is don’t have them close to hand. If they are in the kitchen, on your desk etc, they will be eaten.) It’s worth recalling that the odd dessert or treat only makes up one-sixth of the sugar we consume. Most of the sugar we now eat is added to what we eat or drink. It isn’t the guilty pleasures ruining our health, it’s the mass produced stuff, and by cutting this out you can reduce your intake by a clear margin.

On a personal note, when I cut my sugar intake the first week or so could be a bit challenging, especially the craving for something sweet after dinner, but the benefits soon far outweighed the odd sugar pang. Within a matter of days my afternoon brain fog lifted and mid morning hunger pangs subsided (I keep a back of nuts and dry fruit in my drawer if I really do need a snack). After a few weeks, fruit began tasting sweet and enticing again and I found it much easier to drink water as it no longer tasted so flat. I also felt far less jittery and reactive. It felt as if I were depriving my brain of the quick and dirty fuel of sugar. I got into the habit of eating a bag of salad from the supermarket for lunch each day, though of course with a nice rich dressing, protein and a crunchy topping. My belly retreated (a bit) and my sleep improved. We’ll see what happens when the days get shorter and the chill winds of October have me eyeing up comfort foods, but for now losing the weight of middle age was nowhere near as difficult as I had anticipated, and I’m in no hurry to go back to the sickly lethargy of sugar just yet.

Resources:

https://phcuk.org/product/real-food-real-budget-flyer-x-100-5-5p-per-booklet/

Truth about calories

https://robertlustig.com/wp-content/uploads/2017/07/Infographic.pdf

The Sugar Conspiracy

https://www.theguardian.com/society/2016/apr/07/the-sugar-conspiracy-robert-lustig-john-yudkin

Sugar Science University of California

https://sugarscience.ucsf.edu/the-toxic-truth/

BBC BMI and obesity calculator

https://www.bbc.com/news/health-43697948

Dr Unwin’s Infographics / Glycaemic Index

https://lowcarbfreshwell.com/what-is-a-low-carb-lifestyle/dr-unwins-sugar-infographics/

Low Carbohydrate Diet

https://phcuk.org/wp-content/uploads/2016/05/Eat-Fat-Cut-The-Carbs-and-Avoid-Snacking-To-Reverse-Obesity-and-Type-2-Diabetes-National-Obesity-Forum-Public-Health-Collaboration.pdf

Sugar: The real reason we put on weight, and how we can get back control of our health