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— Photo: K.N. Muralidharan

Genes and juices: The preference for fruit juices appears to be genetically controlled.

This difference between identical and fraternal twins affords biologists a unique opportunity to pick out the effects of heritability (genes, or ‘nature’) from those of environment, lifestyle and habits (or ‘nurture’). It was the maverick scientist Francis Galton, a cousin of Charles Darwin and the founder of eugenics, who realised this possibility first and coined the phrase ‘nature or nurture.’

In order that we take advantage of this nature-given case-control comparison between identical and fraternal twins, we need a registry, across the nation, of these people and record their traits and habits, health and sickness.

This needs a long-term follow-up of their lives, and keeping a detailed recording, so that appropriate analysis becomes possible.

Several countries in Europe, and U.K. and the U.S. have done so, thus providing precious scientific material for study and insight.

The Twin UK Registry is one such, which has diligently done so over the decades, and the results that scientists in the U.K. have been able to tease out of the extensive data are insightful.

Tosin Sulaiman, whose twin is a medical student at King’s College, London (and thus a participant in such twin studies), writes thus:

“There are any number of studies to chose from. From freckles in a person’s face to how people vote in an election, twin studies have helped scientists to discover how much influence genes have on disease and behaviour.

By comparing identical twins with fraternal twins when studying illnesses or traits, researchers can determine the role played by genes as opposed to environment. If you and your twin would like to take part in a twin study, visit www.twinsuk.ac.uk”.

The nature-nurture interplay becomes particularly important in understanding the role of diet and food habits in influencing the risk of diseases such as diabetes, obesity, heart attacks and cancer. Much of recent focus on this issue has been on nurture — what to eat and what not, how and when to exercise and so on.

Praise has been showered on Mediterranean diet (fish, olive oil, greens, red wine) as the one ideally suited. The so called French Paradox concerns what is in the food that they eat, different from the British or American, that keeps them fitter than the latter.

But is it all nurture? Are there no genetic factors behind these? A recent U.S. study, using twins, has provided some indication that genetic factors may “account for up to 50 per cent of the total variance in dietary behaviours in U.S. population.”

In plainer English this means that dietary preference may go with the genes that one inherits. African-Americans prefer certain foods, spices and sweets, while the Latinos another set and the Yankees yet another.

While such an analysis is suggestive and needs to be replicated and ‘rigourised’, it has implications on what to recommend as diet and health choices.

One prescription might not fit all. What makes one prefer, choose or stick to one kind of diet? Is it just habit or are there genetic predispositions? For example, as I travel across the world, I enjoy Italian, French, Middle Eastern, Ethiopian, Mexican, Chinese and Thai food, but there lingers the occasional hungering for thayir sadam (curds and rice). Is it due to the way I have been brought up, or my genetics, or both?

Why do many Bengalis miss fish in mustard oil, Punjabis rajma, and the Andhras chillies?

Teasing out the factors governing choice from those governing genetics would help doctors and nutritionists in their campaigns on health food and prescription to individuals.

This issue has been addressed in a recent paper by a British group led by Dr. Alex MacGregor of the University of East Anglia School of Medicine, Health Policy & Practice. He collaborated with the twin research expert Dr. Tim Spector of King’s College London, and issued a detailed questionnaire to over 3,200 female twins (from the Twin Registry UK) on their preference on five different types of diets.

These were: fruits and vegetables, high alcohol, traditional English cuisine, diet food, and low meat meals. The answers given by about 500 identical twin pairs were compared with those of 1133 fraternal twin pairs. These in turn were compared with date gathered from ‘singletons’.

Preferences shown by identical twins reared together reflect genetic effects. Differences between these and the preferences of fraternals suggest environmental or nurture effects.

And comparison of these with those of singletons adds grater rigour and differentials.

Such an analysis suggested that genetics contributes over 40 per cent to the food preference of all five-diet types.

More tellingly, the preference for ‘savoury tastes,’ fruit juices and refined grains appears genetically controlled.

How could genetics control such preferences? The authors point out that some people are unable to taste bitter substances, and this inability is genetically determined.

Genes responsible for taste receptor molecules are expressed less in some people than in others. Indeed, the phenomenon is not very different from the finding that some people metabolise certain drugs and medicines differently from others.

Personalised medicine is being talked about, now is the time to talk about personalised diet! As the authors state: “The relatively high heritability of specific dietary components implicates taste perception as a possible target for future genetic studies”.

India is a country rich in people of diverse communities, tribes and stocks. There must be millions of twins, identical and fraternal, yet we do not have a composite Twin Registry here.

This valuable research resource can be put together so that studies on the nature-nurture dialectic can be conducted in our national context. Now that the Union Health Ministry has installed the Department of Health Research (DHR), may I request the DHR that such a Twin Registry be started and maintained?

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