• What is diet?
• Energy intake
• Nutrients
• Foods
• Dietary patterns
• Dietary behaviours
• References

Diet broadly consists of the selection and consumption of foods and drinks, from which nutrients are ingested and used for the body’s needs. A balanced diet is crucial for good health, but diet varies day by day. The variability makes it challenging to assess. Therefore, to conduct dietary research, accurate assessments of dietary consumption and behaviour are highly important.

As diet is complex, many methods and tools have been used to assess dietary consumption and behaviour, and it is important to take a systematic approach. Diet can be considered at a number of different levels depending upon the research question. There may be interest in analysing any or all of the following components of diet:

1. Total energy intake (also known as calorie intake)
2. Macronutrient and/or micronutrient intakes
3. Consumption of individual foods or food groups
4. Dietary patterns
5. Dietary behaviours such as the timing of food consumption

Research questions in dietary research can include, but are not limited to, the following:

1. Establishing links between intake of certain foods or nutrients or dietary behaviours and health outcomes
2. Assessing nutrient adequacy in individuals
3. Assessing nutrient adequacy in different populations
4. Detecting changes in diet over time (with or without a behavioural intervention)
5. Exploring socioeconomic or behavioural determinants of diet
6. Exploring genetic or biological determinants of diet

Consumption and expenditure of energy, along with any change in the body’s energy stores (e.g. fat, muscle, glycogen) is summarised by the energy balance equation:

1. The parts of the equation are often expressed as kilocalories (kcal), kilojoules (kJ) or megajoules (MJ), and usually expressed per unit of time, for example, kcal per day.
2. 1 kcal = 4.184 kJ, 1 kJ=0.239 kcal
3. The kJ and MJ are preferred as defined in the International System of Units; however, kcal (a measure of heat) is also commonly used.

As can be seen from the equation above, energy requirements will depend upon energy expended. This will be determined by factors that affect basal metabolic rate, for example, body size, age, and sex as well as levels of physical activity. Estimated average requirements (EARs) for energy intake in the United Kingdom (2011), for example, were estimated as shown in Table D.1.1.

Table D.1.1 Estimated Average Requirement (EAR) values for energy intake for British adults.

Source: [1].
* Median physical activity level (=1.63) was used. 1 kcal = 4.184 kJ, 1 kJ=0.239 kcal.

The word nutrient generally means any substance derived through food that supports growth, development, or homeostasis. Nutrients can be distinguished from drugs or other beneficial non-nutrient compounds by their incorporation into functional or structural molecules in the body. Nutrients are either essential or non-essential.

Essential nutrients cannot be synthesised by the body from other substances and therefore must be consumed in the diet. These include essential fatty acids, essential amino acids, vitamins, and certain dietary minerals. One example is vitamin C, which cannot be obtained from any other source than the diet, and must be consumed in sufficient amounts to prevent deficiencies. Prolonged under-consumption, malabsorption, or excessive loss of essential nutrients impairs essential metabolic processes, leading eventually to deficiency disorders.

A nutritionally-complete diet contains each essential nutrient in sufficient amounts to maintain normal growth, development, and homeostasis without inducing either deficiency or toxicity.

Non-essential nutrients are those which can be synthesised by the body. Many fatty acids and amino acids are not essential to obtain from the diet and can be produced from basic precursor molecules or modified according to metabolic requirements.

Macronutrients

The substances which provide the majority of energy can be classified into three main macronutrients, based on their chemical composition:

• Protein
• Fat
• Carbohydrate

Alcohol (ethanol) is also a contributor to total energy intake, although it is not a nutrient required to maintain body functions.

Not all energy within food is available for human metabolism because of losses during food utilisation. Therefore, we consider the energy available to humans, defined as ‘metabolisable energy’. The macronutrients and alcohol contain different amounts of metabolisable energy as shown in Table D.1.2.

Table D.1.2 Metabolisable energy conversion factors for nutrients and alcohol.*

Source: [2]
* Values of energy per mass vary by food matrix. They are often rounded to 9 for fat, 4 for carbohydrates available for the metabolism, 4 for protein, and 7 for alcohol. "Available carbohydrates" refers to carbohydrates summed up from free sugars (glucose, fructose, galactose, sucrose, maltose, lactose, and oligosaccharides) and complex carbohydrates (dextrins, starch, and glycogen), often considered as metabolisable carbohydrates or carbohydrates that do not include dietary fibre.

The macronutrients have different energy and nutritional contents. Thus, they are processed by the body and influence appetite in different ways [3]. There are recommendations as to the proportions of energy intake which should come from each macronutrient source, and these are described on the Dietary Adequacy and Nutritional Requirements.

Micronutrients

Micronutrients include vitamins and minerals that are needed for the production of enzymes, hormones and other substances that are essential for growth and development. Deficiencies in vitamins and minerals, such as iron, iodine, vitamin A, folate and zinc result in severe illnesses [4, 5]. Recommendations for vitamins and minerals vary by age. There are recommendations for micronutrient intake, and these are described on the dietary adequacy and nutritional requirements page.

Food groups

Diet has traditionally been analysed chemically into nutrients, but we consume our diet as foods. Thus, this classification is more intuitive to the general population. Public health dietary recommendations are increasingly made in terms of foods and food groups, such as the Public Health England's 5-A-Day message and the Eatwell Guide shown in Figure D.1.1.

Figure D.1.1 The UK Eatwell Guide.
Source: [6]. Download the Eatwell Guide as a PDF (2.41 Mb).

Food components

All foods can be classified broken down into their nutrient components; however, there may be considerable variation in nutrient content between different varieties of the same food [7]. In addition, it is important to remember that foods are not just packages of nutrients. Typical components of everyday foods include:

1. Nutrients, such as proteins, fats, carbohydrates, vitamins, and minerals
2. Naturally-occurring non-nutrient compounds, such as flavonoids in berry fruits or phytosterols in vegetable oils
3. Naturally-occurring toxins, such as hydrogen cyanide in almonds or aflatoxins in peanut butter
4. Intentionally-added compounds or food additives, such as preservatives, colourants or flavourings - all commonly found in processed foods
5. Industrial pollutants, such as heavy metals or polychlorinated biphenyls
6. Microbiological factors, such as foodborne pathogens or commensal bacteria
7. Non-nutritive compounds generated in a cooking process, such as advanced glycated end-products, acrylamide, and specific trans unsaturated fatty acids.
8. Other chemical residues, such as antibiotics, insecticides, fungicides or herbicides

The characterisation of dietary patterns has emerged largely as a reaction to concerns about the over-simplification of diet-disease relationships by concentrating on single nutrients or foods. People, after all, do not consume nutrients or even foods in isolation; in reality, they consume many combinations of foods in a variety of quantities and proportions. Dietary patterns represent overall patterns of food consumption, which may result in synergistic or antagonistic effects on health above and beyond effects of single nutrients or foods. Examples of well-studied dietary patterns are Dietary Approaches to Stop Hypertension (DASH) and the Mediterranean diet, both of which have been associated with lower risk of cardiovascular diseases [8, 9].

Beyond the analysis of total diet, researchers are interested in understanding how meals are consumed and how this may influence diet and health. This involves analysis of diet data by time of consumption, for example. There have been changes in meal patterns over recent decades [10] and their impact on health outcomes remains to be elucidated. As an example, while breakfast has been often promoted as the most important meal of the day in the media or perceived as a healthy behaviour, available evidence from randomised controlled trials has not supported its benefit at least for weight loss [11] or clinical events. Other examples of dietary behaviours include snacking in front of the TV, having meals with family members, eating outside, the speed of eating, drinking at a pub, binge drinking, and cooking practice.

A pattern of dietary behaviours is sometimes called a dietary pattern. The use of these terms has not been standardised in the field of nutrition.

1. Scientific Advisory Committee on Nutrition. Dietary Reference Values for Energy London: The Stationery Office; 2011 [27 October 2021]. Available from: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/339317/SACN_Dietary_Reference_Values_for_Energy.pdf.
2. Widdowson, E. M., McCance, R. A. (2014). McCance and Widdowson's the Composition of Foods. United Kingdom: Royal Society of Chemistry.
3. Carreiro AL, Dhillon J, Gordon S, Higgins KA, Jacobs AG, McArthur BM, Redan BW, Rivera RL, Schmidt LR, Mattes RD, et al. The Macronutrients, Appetite, and Energy Intake. Ann Rev Nutr. 2016;36:73-103
4. Sight and Life. Micronutrients, Macro Impact: The Story of Vitamins and a Hungry World Basel: Sight and Life Press; 2012. Available from: http://www.sightandlife.org/fileadmin/data/Books/Micronutrients_Macro_Impact.pdf.
5. Investing in the Future: A United Call to Action on Vitamin and Mineral Deficiencies Ottawa: Micronutrient Initiative; 2009 [6 January 2017]. Available from: http://www.unitedcalltoaction.org/documents/Investing_in_the_future_Summary.pdf.
6. Public Health England. The Eatwell Guide London: UK Government; 2016 [6 February 2017]. Available from: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/551502/Eatwell_Guide_booklet.pdf.
7. Ruth Charrondière U, Stadlmayr B, Rittenschober D, Mouille B, Nilsson E, Medhammar E, Olango T, Eisenwagen S, Persijn D, Ebanks K, et al. FAO/INFOODS food composition database for biodiversity. Food Chem. 2011;140:408-12
8. Mozaffarian D, Appel LJ, Van Horn L, Components of a cardioprotective diet: new insights. Circulation. 2011;123:2870-91
9. Tong TY, Wareham NJ, Khaw KT, Imamura F, Forouhi NG, Prospective association of the Mediterranean diet with cardiovascular disease incidence and mortality and its population impact in a non-Mediterranean population: the EPIC-Norfolk study. BMC Med. 2016;14:135
10. Huseinovic E, Hörnell A, Johansson I, Esberg A, Lindahl B, Winkvist A, Changes in food intake patterns during 2000-2007 and 2008-2016 in the population-based Northern Sweden Diet Database. Nutr J. 2019;18:36
11. Sievert K, Hussain SM, Page MJ, Wang Y, Hughes HJ, Malek M, Cicuttini FM, Effect of breakfast on weight and energy intake: systematic review and meta-analysis of randomised controlled trials. BMJ. 2019;364:l42