Dietary studies: inconclusive, confusing, useless

Last Updated 24 October 2018, 17:57 IST

Recently, Professor Karin Michels of Harvard T H Chan School of Public Health claimed that coconut oil is “pure poison”. It was not a study, but only a lecture. She did not authenticate references. But the YouTube lecture got a million hits. Not many know that YouTube has since retracted the lecture. Dietary studies are controversial. The roles of saturated fats, carbohydrates, salt, sugar, vitamins, etc., are all under the scanner. These studies are mostly inconclusive, and confusing even for the specialists.

In September, The Lancet online published a global observational study, which found that “dairy consumption was associated with lower risk of mortality and major cardiovascular disease events in a diverse multinational cohort”.

The Prospective Urban Rural Epidemiological (PURE) study of 1,36,384 individuals in the age group 35-70 years, carried out in 21 countries from Argentina to Zimbabwe, recorded their dietary intake at the start of the study using country-specific validated food questionnaires. Researchers followed the participants for an average of 9.1 years.

This study found that a total intake of two or more servings of full-fat dairy products every day was associated with 22%, 34% and 23% lower risks for heart disease, stroke, and for death from cardiovascular disease, respectively. Until now, though, specialists had considered that saturated fat was bad for the heart.

Another study (The Lancet, August 11, 2018) concluded, “Sodium intake was associated with cardiovascular disease and strokes only in communities where mean intake was greater than 5 g/day. A strategy of sodium reduction in these communities and countries but not in others might be appropriate.”

That contradicts the advice of the World Health Organisation (fewer than 2 grams per day) and the American Heart Association (no more than 2.3 grams per day, with an ideal limit of 1.5 grams). Five grams of salt contains two grams of sodium. Many specialists argued that estimating daily intake of sodium from a single urine sample collected in the morning is not appropriate. In hot countries, sodium is lost through sweat as well.

A third study in The Lancet, published in August 2017, found that “carbohydrate intake was associated with higher risk of total mortality, whereas total fat and individual types of fat were related to lower total mortality. Total fat and types of fat were not associated with cardiovascular disease, myocardial infarction, or cardiovascular disease mortality, whereas saturated fat had an inverse association with stroke”.

“Our findings support that consumption of dairy products might be beneficial for mortality and cardiovascular disease, especially in low-income and middle-income countries where dairy consumption is much lower than in North America or Europe,” the lead author of the first study, Dr Mahshid Dehghan, McMaster University, Canada, asserted.

The study has some limitations; for instance, self-reporting of diets are notoriously erroneous. Based on consumption data collected only once at the beginning, it is incorrect to assume that the food habits remained the same over 10 years

In a linked comment in the same journal, Jimmy Chun Yu Louie (University of Hong Kong), and Anna M Rangan (University of Sydney) concluded that the dietary guidelines for dairy products do not need to change in light of this study. “Although a trend towards a protective effect is generally observed, the association between dairy consumption and cardiovascular disease is still inconclusive,” they argued.

Researchers of The Lancet study agreed that there is a need to undertake more research to discover the reasons for the findings. They cautioned that it is not advisable to eat more than moderate amounts of dairy products. If you eat more, obesity and more serious health consequences may follow.

Researchers from Harvard T H Chan School of Public Health criticised the study on key methodological problems. The study oversimplified “total carbohydrates” (different types of carbohydrates have different effects on health). High carbohydrate intake may indicate a “poverty diet”. This diet is high in sodium and low in animal products and vegetable oils.

“It is extremely challenging if not impossible to separate the effects of diet from poverty and under-nutrition”, Harvard scientists argued. They found that the study does break down total fat intake into saturated, mono-unsaturated, and poly-unsaturated fats but did not assess trans-fat (known to be high in South Asia) which may have confounded the associations for other types of fat.

Harvard researchers questioned the reliability of dietary data in The Lancet survey, which estimated that the average total fat intake in Chinese population (a third of the study population) is 17.7%. Other surveys have found this to be 30%.

All the three papers became controversial, as their findings were contradictory to prevailing knowledge. There was no critical appraisal of the studies in the mainstream media. Mostly, they copied the press releases from the journal. A few saved the day by quoting many experts!

The etiology of cardiovascular diseases is very complex; so is the biochemical interaction of various components of foodstuffs in the gastrointestinal tract. A part of them follow the simple laws of physics and chemistry. Our knowledge derived from dietary research is very partial. These research activities are so capricious that observational studies may not unravel their mysteries.

The PURE (Prospective Urban Rural Epidemiology) studies face such a conundrum. At times, their study secures misleading headlines. The PURE researchers question conventionally-held positions and get traction. Discerning readers may find them incomprehensible and unimpressive, if not useless. One need not lose sleep over these issues. The take-home message is, eat all locally available foodstuffs, but in moderation!

(The writer is a former Secretary, Atomic Energy Regulatory Board)

(Published 23 October 2018, 17:59 IST)

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