How many spoonsful of sugar do you eat without even knowing?

Some experts think 'invisible' sugar is behind our soaring obesity rates. Samuel Muston investigates

We have a major epidemic of obese six-year-olds. They don't make bad choices. Something else is going on

January is a grey month, a burnt-out shell between streamingly festive December and "back in the pink" February, when we all feel just about OK and our bank balances skip back into the black. It is a time of navel gazing – quite literally – when the ghost of Christmas dinner past comes back to visit. A time when we look down at our much-grown waistlines in despair and think: "I'd better do something about this."

But what to do? Some of us pledge to take exercise. Others promise to give up red meat, butter and cake. Most of us will do what we have always done when we want to drop some weight. We'll follow the guidelines drilled into us by successive health tsars, chief medical officers and ministers of Health down the years: we'll cut our intake of fat. It is the standard course – but it simply isn't working.

According to the International Federation of the Red Cross, there are now more obese people (1.5 billion) than hungry people (925 million). The figures for diabetes are equally arresting: 5 per cent of the world's population now have the disease. In 1980, the figure was 2.5 per cent. Little wonder the Secretary-General of the United Nations recently said that the biggest threat to the world is not from communicable diseases such as HIV or malaria but the non-communicable: obesity, heart disease, cancer, Alzheimer's and diabetes. These conditions alone kill 35 million people each year.

As no one can claim with any degree of realism that they are unaware of the dangers of unhealthy eating, this raises a question. Do these figures represent an almost universal collapse of will? Are they down to bad choices on the part of the individual?

"That is the easy and pejorative answer," says Dr Robert Lustig, professor of clinical paediatrics at the University of California and a noted endocrinologist. "But that does not explain the epidemic of toddler obesity. We have an epidemic of obese six-year-olds. They don't make bad choices. Something else is going on.

"Our research shows that these behaviours are the result of biochemical alterations which are the result of changes in our environment. Biochemistry drives behaviour, not the other way around."

So what is the explanation? According to Dr Lustig, whose new book, Fat Chance: The Bitter Truth about Sugar, is out this week, it comes down to a change in diets in the 1970s. The Seventies saw the development of foods with manipulated low-fat contents. And low-fat food, according to Dr Lustig, is making us fat.

"When you remove fat from food, it tastes like cardboard. The food industry knows that. So when they took fat out, they had to add the carbohydrate in; and in particular fructose sugar," Dr Lustig says. In America fructose intake has increased 100-fold since 1970.

In the UK, the quantity of stand-alone bags of sugar sold has decreased. Yet from 1990 to 2000, consumption of sugar went up by around a third – and a significant quantity of that is sucrose, which is 50 per cent made up of fructose. This is what researchers mean when they refer to the rise of "invisible sugar" – few of us notice it but it very much exists.

Take a Volvic Touch of Fruit Lemon and Lime (1.5 litres) – how much sugar would you guess was in that? You'd be hard-pressed to guess: 16 sugar cubes. Or barbecue-flavour Pringles? They have 1.5 cubes. A bagel? One cube.

How have we become accustomed to sugar in just about everything that passes our lips in such a relatively short period of time? According to Charles Spence, professor of experimental psychology at Oxford University, we are used to it because we eat more – and we eat more because we need more. "Think of it like eating chillies – the more you eat, the more you need to eat to feel the level of heat."

This goes some way to explaining what the writer Felicity Lawrence found when she studied fruit-and-vegetable production in 2007. She discovered that farmers are increasingly concentrating on producing super-sweet varieties of fruit that hitherto had been thought sweet enough. Why? Because our whole diet has an ambient quantity of sugar in it – so a sweet apple suddenly seems bland.

Although sugar – as we know it – is made up of glucose, which is essential for life, it also has a fructose component. It is this that causes the problems, Dr Lustig says: "The mitochondria [the energy-burning factories in our cells] in our liver have a fixed capacity for burning the fructose. When you overload them with extra fructose, they will burn some, but they have no choice but to turn the excess into liver fat. That starts the cascade of insulin resistance, which then promotes chronic metabolic disease, including diabetes and heart disease."

Dr Lustig also contends that there is a further co-occurring effect for obese people. When fat cells have sufficient energy, they release a hormone called leptin, which is supposed to give feedback to the brain to tell it you have enough fuel on board and to eat less now and move more to keep a stable weight. Dr Lustig suggests that this cycle has broken down.

"Leptin levels are high in obese people, but it's not shutting down their appetites, and they continue to gain weight anyway. Clearly, leptin is there, but it's not working. That's leptin resistance. Obesity is leptin resistance. The question is: why is this happening? Because if we could fix leptin resistance, we could solve obesity.

"Our research [along with others'] has shown that insulin is a cause [though not the only one] of leptin resistance. So high insulin levels cause you to store energy in fat cells [obesity], and then prevent leptin from signalling [starvation] so that you eat more [gluttony] and burn less [sloth]. And what's the fastest way to high insulin levels? Sugar," Dr Lustig says.

If his thesis is correct, not only does it mean that obesity can no longer be seen as a product of recklessness on the part of the person, it also means as a society we have an urgent problem.

So what do we do? Curb availability. Go for the environment, rather than the behaviour. But is it likely that we'll see the imposition of a soft-drinks-and-biscuit tax any time soon? Probably not. And so are we likely to see even more people dying of those non-communicable diseases? Sadly, it's something of a certainty.

Fat Chance: The Bitter Truth About Sugar by Dr Robert Lustig, priced £13.99, is published by Fourth Estate