Everything in moderation, or so the old adage goes.

A varied diet has long been thought to be the gold standard when it comes to losing weight.

But, a new study quashes that theory, suggesting a diverse diet may, increase the risk of weight gain and diabetes.

Dr Marcia de Oliveria Otto, first author from the University of Texas Health School of Public Health, said: 'An unexpected finding was that participants with greater diversity in their diets, as measured by dissimilarity, actually had worse diet quality.

'They were eating less healthy foods, such as fruits and vegetables, and more unheathly foods, such as processed meats, desserts and soda.

'This may help explain the relationship between greater food dissimilarity and increased waist circumference.' 

Dr Otto added: 'Eat everything in moderation' has been a long-standing dietary recommendation, but without much empiric supporting evidence in populations.

'We wanted to characterise new metrics of diet diversity and evaluate their association with metabolic health.'

People with poor metabolic health are more susceptible to metabolic syndrome, an umbrella term for a range of illnesses including high blood pressure, high blood sugar, high cholesterol and an accumulation of abdominal fat.

Combined together, these risk factors double a person's risk of heart disease, which increases the chance of suffering a stroke or heart attack.

And people with metabolic syndrome are five times more likely to be diagnosed with type 2 diabetes.

Researchers used data from 6,814 participants in the Multi-Ethnic Study of Atherosclerosis, a study of whites, blacks, Hispanic-Americans and Chinese-Americans in the US.

They measured diet diversity via a number of different measures.

These included the total count,  the number of different foods eaten in a week, evenness, defined as the distribution of calories across different foods consumed, and dissimilarity, or, the differences in food attributes relevant to metabolic health - such as fibre, sodium or trans-fat content.

The researchers then evaluated how diet diversity was linked to a change in waist circumference five years after the beginning of the study, and with onset of type 2 diabetes 10 years later.  

Waist circumference is an important indicator of central fat and metabolic health.

When evaluating both food count and evenness, no associations were seen with either increase in waist circumference or the number of cases of diabetes.

In other words, more diversity in the diet was not linked to better outcomes.

Those people taking part who had the most varied diet were actually found to experience more central weight gain, with a 120 per cent greater increase in waist circumference than those with the most similar diets. 

To compare with the results seen for diet diversity, the researchers also examined how diet quality relates to metabolic health. 

The study is published in the journal PLOS One.