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INTRODUCTION

Let food be thy medicine and medicine be thy food

Hippocrates (460–370 BCE)

A healthy diet is one that optimises health by providing all essential nutrition while also maintaining an appropriate weight. The composition of such a diet, however, is subject to vigorous debate among both experts and lay people. Even in the 21st century, folklore and conventional wisdom continue to shape mainstream nutrition beliefs in the absence of convincing evidence. This is in part due to the difficulties and expense of performing suitable interventional research, with nutrition science historically based on less robust epidemiological research. Findings from these observational studies are often demonstrated to be erroneous when tested in randomised controlled trials.13

In recent years there has been growing interest in nutrition research thanks to increased funding and improved research methodologies. Combined with advancements in molecular biology and biochemistry, this has resulted in an improved understanding of healthy diets and underlying physiological mechanisms. These new understandings however, often contrast with existing orthodoxy and are not universally accepted.

Optimal dietary macronutrient composition is perhaps most contentious. Sugar has received much attention, many arguing it has increased potential for causing metabolic harms compared to other carbohydrates. The importance of sodium restriction, benefits of dietary fibre and required water intake are other longstanding beliefs that are being increasingly questioned. The gut microbiome is an exciting new area of nutrition science; however, many claims regarding prebiotics and probiotics are being made in advance of definitive evidence.

A significant amount of conventional thinking about nutrition is not well supported by evidence. In this chapter, we will examine the evidence base for current nutrition recommendations by exploring the current understanding of macronutrients, micronutrients, fluids and different dietary patterns.

DIETARY GUIDELINES

Dietary goals for the American people were first recommended by the US Senate in 1977,4 followed by the first edition of the Dietary Guidelines for Americans in 1980.5 These were in turn used as a template for the Australian dietary guidelines that were introduced in 1980.6 However, a recent systematic review and meta-analysis concluded that when introduced, the US guidelines were not consistent with best available scientific evidence.7

Since their inception, both the Australian and US dietary guidelines have evolved through regular revisions. Recent changes include removal of the recommended upper limit for individual sodium consumption from the Australian Dietary Guidelines8 in September 2017, and the removal of the recommendation to limit both dietary cholesterol and total fat in the most recent edition of the Dietary Guidelines for Americans (8th edition, 2015–2020).9 Many features of current dietary guidelines remain identical or very similar to their initial versions. The current Dietary Guidelines for Americans continues to advise that saturated fats should represent no more than 10% of total energy intake. Similarly, the current Australian Dietary Guidelines advise ...

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