The main aim of the UK’s new tax on sugary soft drinks is to reduce obesity in children. But, apart from causing child – and adult – obesity, too much sugar also increases the risk of many serious diseases, from cancer to heart disease. And sugar’s calories provide only part of the explanation.

Just as important is insulin. When glucose levels in the blood rise, the pancreas produces insulin, the key that opens doors on cells to allow the glucose in. But too many sugary snacks can keep blood glucose levels high, and so more insulin is also produced. In response to the continual bombardment with insulin, cells change their locks so the insulin key no longer works. With cells desensitised to insulin, blood glucose levels rise even more and the pancreas responds by producing even more insulin. This dangerous state of high blood glucose and insulin can persist undiagnosed for years and is a driving force behind many diseases, even in those of normal body weight.

Why are elevated glucose and insulin so dangerous? High blood glucose is a well-established risk factor for type 2 diabetes. It also leads to free radicals being produced that damage blood vessels. The tradition of three meals a day allows time between meals for antioxidants to repair the damage. With snacking on sugary foods there may be less reprieve. The result is an increased likelihood of a heart attack.

What’s more, insulin – glucose’s partner in crime – is a cell “fertiliser” promoting the growth of cells, and making it more likely that a normal cell will cross the threshold into cancer. Raised insulin levels are linked to many cancers and may be an important risk factor for breast cancer in postmenopausal women. So the double whammy of high glucose and high insulin is an insidious driver of many diseases.

Prediabetes

The health significance of elevated blood glucose is sufficient for it to have acquired a new medical term: prediabetes. According to one report, a staggering one in three adults in the UK now has this condition – a figure that has tripled since 2003. But most don’t even know they are living with this pre-disease state and so take no remedial action. Being obese increases the risk of prediabetes, but a quarter of prediabetic people in the UK are of normal weight.

Each year, about one in 20 people with prediabetes cross the threshold into type 2 diabetes. And a recent review of a large number of studies also found that being prediabetic was associated with an increased risk, albeit small, for many different cancers. So detecting and treating this condition has huge implications for public health. In the UK, the NHS Health Check (a prevention programme for 40-74 year olds) will detect prediabetes, enabling patients to reverse it by adopting a healthier lifestyle. But far better to prevent it occurring by adopting that healthy lifestyle now.

Cutting down on sugary food and drinks is an obvious way to help prevent the dangerous cocktail of high blood glucose and insulin. Sugar added to processed foods is particularly harmful. But natural sources like fruit, though high in sugars, contain fibre, and fibre reduces glucose spikes in the blood by slowing the emptying of the stomach. It also gives a feeling of fullness, preventing over-consumption, whereas there is no such regulator in sugary drinks. Fruit also has the redeeming benefits of vitamins and other nutrients, while sugary drinks provide only empty calories devoid of nutrients. And studies show that fruit (but not fruit juice) is linked to a decreased risk of type 2 diabetes.

Another way to reduce spikes in blood glucose is by having sweet foods only after eating foods rich in fibre such as vegetables, beans or cereals. Some plant foods also contain natural chemicals that help further by blocking glucose uptake from the gut. Apples are a good example – and my research shows that onions also contain chemicals that can reduce spikes in blood glucose. This ability of various plant foods to reduce blood glucose may be one reason why the plant-based Mediterranean diet, even though it includes some sweet foods, is very effective at preventing and managing diabetes.

Makers of sugary soft drinks now complain that their products are being victimised by the new tax. Yes, many other sources of added sugar are also contributing to the epidemic of sugar-related diseases. But this is an opportunity for food manufacturers to do more to reformulate their sugary food products, not less.