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A history of how our taste buds became adapted to liking sweet-tasting foods

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Human infants as omnivorous primates learn to use smell, taste and later touch to distinguish between those substances that are acceptable and those that are unacceptable to swallow. Their first neurological inputs come from molecular receptors for specific molecules in milk. A baby’s expression of taste is therefore a perceptual experience of nutrients and other chemicals. Taste receptors line the gut, sampling fluids and food around the mouth, checking on the processes of digestion and assessing the microbiome in the colon. Digestion of foods begins in the mouth, too, with salivary enzymes. We are unique among mammals in having a multiple copy polymorphism of the salivary amylase gene. It has been proposed this is an evolutionary adaptation that permits more effective early digestion of starches, resulting in some starchy foods appearing sweeter to the taste buds in our tongue and throat.

So early in life we are all likely to prefer sweet foods in the expectation they deliver energy. In the newborn, sweet flavours can help reduce pain levels if presented on a pacifier.  Children prefer milk feeds that are higher in sugars and glutamate — as are found in breast milk.

Unlike learning how to tie one’s laces, the process of developing taste preferences does not start on a level playing field. The evolution of humans as foragers has developed in almost all of us a taste preferences for sweet sugars. Most of us select sweet flavours as a favourite. One explanation for this is that sweet natural foods for a scavenging primate species, such as are found in fruits, some grasses and flowers, will be high in sugars that provide energy. As our next most favoured choices we are likely to select foods that are salty, fatty, starchy, acidic or with umami flavours, because each of these is likely to be accompanied by an intake of useful nutrients and micronutrients. More recently it has been found that children’s taste preferences can be strongly influenced by context and play. The association, for instance, of a well known cartoon character with a certain type of cereal or drink will reinforce their enthusiasm for that specific item. Regrettably such associations have proved more powerful for breakfast foods with added sugars than they ever did for Popeye and spinach.

The drive to find and consume sugar is a strong one. For instance, in the 18th and 19th centuries those seaports importing sugar cane into England, Glasgow, Liverpool and Bristol in particular had the highest rates of dental caries. In the century since then this great quest for sugar has been fulfilled by the delivery of manufactured and processed foods on a dramatic scale, sweetened and in large portions. The availability of sugar (once scarce and a luxury, but now available across the planet) and its provision in food and drink has overtaken the need for this energy source in the majority of those living in a developed country.

Today in the United Kingdom the most common reason for elective surgery in children is admission for the removal of decayed primary teeth: a legacy of our inbuilt, highly evolved pursuit of sweet taste.

Problems associated with high sugar intake were recognised in the 1980s to be an issue of global importance. Analysis of the consumption of different food groups by populations in the UK, Ireland, France, the United States and Canada over 30 years were consistent with those from some developing countries, including Mexico. These showed a shortfall in the consumption of nutrients, including vitamins A, C, D, E and folic acid, potassium and fibre. The key food groups containing these nutrients, including vegetables, fruits and whole grains, seafood, legumes and nuts, were not found as common elements of most diets. By contrast refined grains, solid fat, high sodium and added sugars were excessive in diets and drinks, in homes, schools, workplaces and restaurants of many types. It was observed that many processed foods, including weaning foods for infants, are consistently manufactured with added sugar and salt.

Of all the nutrients the disparity relating to sugar intakes is particularly striking. The World Health Organization and the American Heart Organization have recommended that 10% of daily calories be derived from sugars. This figure is derived from dietetic work carried out over a century ago, when Gowland Hopkins first measured the metabolic importance of sugars in animal diets. Further, these agencies set an upper limit of daily sugar consumption at 25g. Recently the US Food and Drug Administration has recommended a maximum of 50g.  This higher level is based on what might be practicable given current intakes. 

Recommended sugar intakes are based on what is required for good health. But how can they be best implemented? They seem difficult to achieve given that the average can of fizzy drink often contains over 60g of sugar. Adolescents interviewed for the national study in Ireland showed that many derive the majority of their calories from drinks with added sugar, as well as sugary snacks.

Given the long evolution of our enthusiasm for sweet tastes, it is predictable there are will be problems reducing sugar intakes. Can we expect a hardwired desire for sweet tastes to disappear? Strategies for tackling the problem have included the application of more effective labelling, the slow reduction in sugar contents of food and the use of alternative sweeteners that have low calorific value. The design of a tax on drinks with higher sugar content has been debated to a considerable depth in the British parliament recently and appears to be the most effective of the various approaches. This is based on the fact that this type of tax has worked at a national level in Mexico. The funding raised by the tax provides a valuable input to delivering the other public health initiatives required to help the obese, and fund preventative interventions in pregnancy and small children.

The application of labels, such as Jamie Oliver’s suggestion of having a teaspoon count for sugar contents on drinks, has been attempted in a variety of fashions but has challenges when dealing with schools, workplaces and restaurants. Several Canadian and American initiatives now require fast food outlets to display calorie counts on their menus. Their effects are being monitored.

If one slowly reduces the amount of salt in a diet, an adult can readily adapt to a significantly lower salt intake. This works. In the UK it has been estimated that awareness of the issues together with a national drive to reduce salt added to manufactured products such as bread and cheeses has already resulted in a significant reduction our average daily intake of salt.

Alternative sweeteners, such as amino acid substitutes and stevia, derived from a species of chrysanthemum, are interesting in that our taste buds readily ascertain they are not sugar. To most they do not have as satisfying a taste, although they clearly are ‘sweet’. This issue has been one known to pharmacists for generations in their striving to improve our compliance with unpalatable medicines.

A combination of methods will be needed to reduce sugar intakes across the world. Taxation is appealing at a national level, education is critical for any long-term success. We may all have to learn, or re-learn to shop, prepare and cook our food. More international approaches to issues of advertising and safer food manufacturing are needed, too, in order to make healthy choices the easy ones. Each of us could start today by reducing how much sugar we consume. Evolution calls.

 

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