While training to be a cardiologist in the late 1980s, Anil Kumar rarely saw a patient with heart disease younger than 40. Now, as the senior cardiologist at Aster Medcity in Kochi, he routinely treats heart patients in their 30s and 20s, the youngest so far being 21 years old. He believes that people in his native state of Kerala are prone to heart disease, but knows that genetics is not the entire story. Changing lifestyle habits are almost certainly a cause for rising heart disease rates, but it isn’t always easy to pinpoint a clear link between what people eat and their propensity to heart disease.A look at recent statistics in meat consumption in Kerala might provide a clue. Old statistics are not available, but Keralites have been eating a lot of meat in recent times, with the production of meat increasing rapidly from 1995. Common sense suggests that there might be a link between heart disease and a meat-based diet with a high fat content.A decade ago nutritionists may not have hesitated to say that a meat-based diet causes heart disease, but now they are not so keen to make a connection, as science paints an increasingly complex picture of food and disease. "There is no clear link between diet and heart disease," says Anura Kurpad, professor and head of the department of nutrition at St John’s Medical College in Bengaluru.For several decades, saturated fat had been implicated as a major cause for heart disease, the next worst thing to eat after trans fat. Now it is no longer considered bad in moderation, and its former role has been taken by added sugar in the diet. Ten years ago, most cardiologists would have looked at cholesterol and lipid profile as an indicator of heart disease risk. Now they look at blood sugar levels as well, and some cardiologists think that high blood sugar levels are a better predictor of heart disease risk than moderately elevated cholesterol."The biggest change in recent times has been in the role of carbohydrates in disease," says V Mohan, chairman of Dr Mohan’s Diabetes Specialities Centre in Chennai. "Till then it has always been fat, fat and fat." Recent research has upturned not just the role of fat and sugar in heart disease. It is questioning the very assumption of a link between a single nutrient and disease, with enormous implications to how we eat healthy.Now nutritionists think that sugar is bad even in small doses, but not necessarily saturated fat. Traditional cooking oils may be all right, especially if these are being used for a long time in a region. Food supplements, especially vitamin supplements, are a waste of money in most cases, and could even do harm in the long run.Omega-3 supplements may be good for pregnant women , but their role is not clear in preventing heart disease. Red wine has been considered good for the heart for long, but recent research has not shown a clear link between wine drinking and a reduced risk of heart disease. "We have gone from nutrients to foods and from foods to dietary habits," says Srinath Reddy, former cardiologist at the All India Institute of Medical Sciences, Delhi, and now president of the Public Health Foundation of India.Dietary habits have taken a central role these days because scientists have realised that nutrients can balance each other out in a varied diet, with some nutrients often cancelling the harmful effects of other nutrients. Some nutrients can prevent others from doing their jobs or being absorbed well. Nutrients also act in combination, like B12 and folate, or calcium and vitamin D. Focusing on one nutrient from a diet distorts the big picture.Which is also why there is no need to seek an ideal combination or level of nutrients to eat. "Seeking an optimum level of nutrients is like asking for optimum level of happiness," says Anura Kurpad. Because nutrients act in combination, scientists find it hard to figure out how a specific diet is linked to chronic diseases. As all nutrition scientists have found out, it is very hard to do rigorous studies on diets and outcomes, and especially on single nutrients and outcomes. It is hard to control what people eat, and harder to separate the effects of one nutrient from another. Geography and genetics complicate matters even further, as do vested interests of the funders of many studies. So studies on nutrition often come with contradictory results.Even when studies are rigorous, what is relevant to a particular set of people may not be relevant to another. Like in the long and complicated history of the role of dietary fats in health and disease.The first indication of the role of fats in disease came early in the 20th century, but it was only in the 1950s that it started to be part of mainstream medical opinion. That saturated fats cause heart disease became more or less the unanimous opinion of many medical scientists, till a paper two years ago in the Annals of Medicine found no correlation between saturated fat intake and heart disease.In 2013, a paper published in the American Journal of Cardiovascular Disease found a correlation between consuming polyunsaturated fatty acids and heart disease. Many nutritionists and cardiologists have criticised the saturated fat study as being poorly done. And say that replacement of saturated fat with unsaturated fat decreases the risk, while replacement with sugar or refined carbohydrates increases the risk.This remains the mainstream opinion, but there is plenty of evidence to show that different people react differently to fats, and that there may be many people who can consume saturated fat without increase in risk. The Inuits of Greenland are a well-known example.They have a diet with 50 per cent fat content and almost no heart disease.Scientists now know that the Inuits have a genetic mutation that lets them eat high amounts of fat without leading to disease.The Inuit diet provides enough hints about generalisation from some studies. Most diet-disease studies have been done in the West, and scientists believe that the Indian situation is different if not more complex. By and large, the Indian body type differs from the western type. "Indians do not need as many calories as a westerner does," says B Sasikeran, former head of the National Institute of Nutrition and president of the Nutritional Society of India. The difference is not just in the amount of calories Indians need. We may also process nutrients slightly differently.The history of the Indian diet may provide clues about what is good for people in the subcontinent. India took to agriculture long ago, as there was sunshine the year around. In Europe, however, winters made agriculture difficult for at least a few months. So Europeans ate more meat, while Indians ate more cereals. This would have led to different genetic responses to nutrients in a diet. It has implications for the saturated fat controversy."The Asian population does not metabolise fat very well," says Anil Kumar of Aster Medcity.This could mean that smaller amounts of fat — compared with the western diet — may lead to comparatively higher levels of blood cholesterol, and even lower levels of cholesterol may lead to higher incidence of heart disease. Clinical evidence tends to support this idea, but research has revealed other complexities as well. Just like the Inuits, vegetarians in India seem to have picked up genetic changes that are important to fat metabolism.They have a mutation that allows them to make polyunsaturated fatty acids from plant precursors. This is fine in traditional diets, but recent changes have introduced an element of risk.If the vegetarian diet does not have the right balance of omega-6 and omega-3 acids, the body makes too much of arachidonic acid, a known cause of inflammation, heart disease and cancer.As people switched to oils — like sunflower and safflower oil — with high amounts of omega-6 fatty acids, they would have unknowingly increased their risk of heart disease and not reduced it. "There is too much omega-6 in Indian diet," says Kumar Kothapalli, research scientist at Cornell University, who found the mutation in Indian vegetarians two years ago.Since the Indian population ate a predominantly vegetarian diet, discovery of this mutation would be relevant to a substantial part of the country. It also brings back to an idea increasingly finding favour with nutritionists and medical scientists; traditional diets are superior to modern diets. Kothapalli believes that this is especially true of the oils we use in India.The oil story is a good indication of the complexities of nutrition, and the misconceptions in the general population. "Most people are confused about what is good and what is bad in oils," says Ryan Fernando, founder of the Bengaluru-based company Qua Nutrition. Popular opinion buttressed by advertisements favour sunflower and safflower oils, but nutritionists think otherwise. For example, virgin coconut oil may be better than sunflower oil that is being used now, especially to those who are used to it for generations.In a study published in the British Journal of Nutrition in 2014, scientists at the University of Kerala showed that use of coconut oil resulted in lower cholesterol levels than sunflower oil. Scientists may not quickly generalise from one study, except to conclude that there may be no need to change from traditional diets. However, very few in India use virgin oils any more. The refining process may remove nutrients that counteract the harmful effects of some nutrients that are known to be bad.Nutritionists also think that mustard oil, traditionally used in most of north India, is very good for Indian diets. "Mustard oil is a very good source of omega-3 acids," says Shweta Khandelwal, research scientist and professor at the Public Health Foundation of India. Using mustard oil would provide a better omega-3 and omega-6 balance than sunflower oil, and probably avoid the production of too much arachidonic acid.Khandelwal has been researching the effect of omega-3 fatty acids on the Indian population, and says that the benefits of supplementing this nutrient to pregnant women are very clear. However, benefits of omega-3 supplements to adults are less clear. "Supplementing omega-3 acids for reducing heart disease risk is questionable," says Khandelwal.Only a few years ago, the evidence for omega-3 supplements as a protector against heart disease seemed to be incontrovertible. The fish oil supplement market, however, continues to grow quickly and is set to touch $22 billion by 2022, according to the market research firm Grand View Research.In the last few years, research on the microbiome has introduced new complexities into our understanding of food and disease. The microbiome is the set of all microorganisms that live in our bodies. We now know that the weight of all microorganisms in our bodies exceeds the weight of our own cells, and that they are very important in health and disease. Get the wrong bacteria in your gut, and you may be predisposed to cancer or diabetes.Earlier this year, a paper in the journal Nature showed that imbalance of gut bacteria can lead to a higher risk for diabetes. Several factors affect the bacterial composition of the gut, one of them being diet.Gut bacteria also differ from individual to individual. Scientists are still learning about this extraordinary phenomenon, but what we know already provides enough clues about their influence on health.Changing diets will change the bacterial composition of the gut, and thus predispose or protect against disease. It is an entirely new way of looking at the link between diet and disease, and scientists have only scratched the surface.