Mexico has been a much-vaunted testing ground for a nationwide tax on unhealthy sugar-sweetened drinks. Its consumption of these is among the highest in the world, so in 2014 the government decided to act and introduced a 10 per cent levy.

Health campaigners around the world have championed this experiment. In the UK, for example, calls to follow suit , backed by supporters such as celebrity chef Jamie Oliver. Now the first robust evidence on the impact of Mexico’s tax is in, and the gains look very slender.

A soda tax sounds like a good idea. Soft drinks are the main contributor to high intakes of free sugars, which include those added to food plus sugars found naturally in fruit juice, honey and syrup. A 330-millilitre can of full-sugar fizzy drink contains the equivalent of 9 sugar cubes.

We also know the replacement of sugar-sweetened drinks with water or non-caloric alternatives reduces weight gain, especially in children. High consumers of sugar-sweetened beverages are at increased risk of developing diabetes and cardiovascular disease.

In October, , acting on advice from the UK government’s Scientific Advisory Committee on Nutrition, recommended free sugars contribute no more than 5 per cent of daily energy intake (equivalent to six or seven cubes). Current daily intakes in UK adults and teenagers are equivalent to 15 and 18 cubes respectively.

Then in November, the UK parliamentary Health Select Committee for a tax on sugary drinks following campaigning by 60 health organisations as well as Oliver.

At the time, evidence that this would reduce consumption and make a meaningful contribution to calorie reduction was scant. Modelling studies for Public Health England based on a 10 per cent price increase suggested a fall of 7.5 ml per day, which supplies less than 1 gram of sugar or about 3 kilocalories. Yet the select committee was told that the tax had been a success in Mexico.

However, evidence of its impact there, just published in the , is underwhelming. Purchases of taxed sugar-containing beverages fell by 12 ml per head per day and those of untaxed beverages (including fruit juice) increased by 36 ml/head/day.

The fall in taxed beverages was greater in the poorest households, at 35 ml/head/day, but even this is only equivalent to about 1 sugar cube (16 kcal) – a drop in the caloric ocean.

In addition, a sugar tax is likely to have a greater impact in Mexico than countries such as the UK, because Mexicans spend a greater proportion of their income on food and drink so a levy should bite more deeply.

Another problem is that while the soda tax debate has increased awareness of the hazards of consuming a lot of sugary drinks, it has detracted from nudging people to eat less and select a balanced diet.

The dental epidemiologist used the term “sweet nothing” to describe government inaction on cutting sugar intake. He was talking about tooth decay, a problem largely solvable with good oral hygiene and fluoride.

The UK government may well decide to do sweet nothing about sugary drinks, but the problem of reining in consumption of sugar as part of a wider strategy to curb obesity and diabetes remains urgent and unsolved.

Tom Sanders is emeritus professor of nutrition and dietetics at King’s College London

(Image: Edgard Garrido/Corbis)

Many distinguished academics have come unstuck when they use their position
to pontificate about nutrition. Professor Richard Lacey, the medical microbiologist
who became famous for publicising bovine spongiform encephalopathy (BSE)
and listeriosis in chilled food, has fallen into the same trap. So, in reviewing
this book, I was reminded of some graffiti I saw scrawled on the wall of
the Students’ Union ‘Beware of the pedagogic gerontocracy’. In the preface,
Lacey confesses to being a lifelong ecologist with leanings towards vegetarianism
and this bias is reflected throughout the book. No doubt this will appeal
to the demi-vegetarian North American market at which this book is aimed.

Lacey outlines a brief history of food from hunter-gatherer to early
agrarian civilisations. He is critical of modern agricultural techniques
but fails to acknowledge that they have taken the risk out of farming and
have enabled a stable food supply. He argues that the refining of foods
such as wheat and rice debases its nutritional qualities. Yet he forgets
to take into account the fact that food like brown rice severely inhibit
mineral absorption owing to its high phytate content. He neglects to mention
risks posed by the naturally occurring toxicants in foods such as solanine
in potatoes.

He has some strong words to say about intensive farming especially that
of poultry. I was frankly surprised that as the person touted by the media
as the expert on BSE, he fails at times to have a grasp of the facts. For
example, he wrongly states that the offending animal feed concentrates are
still permitted to be fed to pigs and poultry, a practice that was banned
in 1990.

He attacks fish farming, saying – quite wrongly – that farmed salmon
is low in n-3 fatty acids. As farmed salmon are fed on fish scrap, they
differ little in fatty acids composition from wild fish. He also shows his
ignorance about vitamin A. He makes a song and dance about the carotenoid
pigments used to colour salmon and then suggests that these may cause birth
defects: carotenoids have not been shown to be teratogenic. He also misquotes
the 1992 Helsinki Heart Study on pre-vention of heart disease, which was
a drug not a dietary trial, as evidence that dietary intervention increased
risk of heart disease. In fact, the published meta-analyses of cholesterol
lowering trials whether by diet or drugs show that lowering blood cholesterol
leads to decreased risk of heart disease. I was disappointed with this sloppiness
with facts. In parts of the book I found it difficult to distinguish between
fact and repeated opinion. The repetition of allegations means that suspicion
becomes fact, lending credence to myths. The book is littered with gobbets
of information that imply that danger is lurking in every mouthful. Yet
this is never put into perspective effectively.

The book cover claims that food can be fun. But after reading the book,
I can’t say I would cherish a meal with Professor Lacey. He recommends a
high fibre, low sugar, low fat diet with plenty of vegetables but I didn’t
need to read the book to be told this. And this conclusion is followed
by his mother’s recommended recipes many of which on analysis I found were
high in fat (saturated fat). The book certainly lived up to its title Hard
to Swallow.

Tom Sanders is currently Reader in Nutrition at King’s College but has
been appointed to the Chair of Nutrition and Dietetics in the University
of London from October 1994.