There are three principles that capture the scientific rationale when determining healthy and sustainable diet recommendations, they are dietary: variety; balance; and moderation. The principles need to be considered as a coherent whole with all three being equally important and interlinked. A description of each principle and how they relate to a healthy and sustainable diet follows.

Principle 1: Dietary variety – to help achieve a nutritionally adequate diet and help protect the biodiversity of the food supply

Dietary variety refers to eating a variety of nutritious foods every day. Apart from breast milk in the first six months of life, no single food can supply an appropriate balance of all the nutrients necessary for health. Foods with similar characteristic nutrients and nutrient profiles can be grouped into distinct food groups based on their type and nutrient composition [5]. Many countries with food-based dietary guidelines identify 4–6 ‘core’ food groups: starchy staples, vegetables, fruits, dairy foods, other ‘protein foods’ and fats and oils [5]. A nutritionally adequate diet can best be achieved by selecting foods from both across and within the core food groups while prioritising minimally processed foods. These foods should be consumed in amounts recommended in dietary guidelines. A number of countries have undertaken dietary modelling activities to estimate a minimum number and reference size of food serves to be eaten daily from each food group to enable nutrient intake recommendations to be met [20,21,22].

From a health perspective, foods can differ substantially in their composition of nutrients and more than an estimated 26,000 other bioactive compounds [23]. Dietary variety increases the likelihood of consuming an adequate amount, type and combination of nutrients and other bioactive compounds for promoting nutritional health and preventing nutritional deficiency diseases. Synergistic effects among nutrients within and between foods are also instrumental in varied diets positively impacting on health [24]. Conversely, risk of consuming excessive amounts of certain nutrients and other bioactive compounds that may be present in high concentrations in some foods may be reduced through dietary variety.

From a sustainability perspective, consuming a variety of foods helps to protect the biodiversity of food systems by promoting the production of a wide range of genetically diverse food crops and species. Food crop and species biodiversity helps increase the resilience of food production to threats from pests and diseases in agroecosystems [25]. This biodiversity may also help contribute to higher and more stable yields as well as lower land clearing and use of harmful agrochemicals [26].

Principle 2: Dietary balance – to help reduce risk of diet-related non-communicable diseases and excessive use of finite environmental resources and production of greenhouse gas emissions

Dietary balance refers to the relative dietary proportions of the different food groups from which foods are selected. Dietary imbalances arise when the total amount of foods from one or more food groups is consumed in excessive or inadequate amounts relative to the total amount of foods consumed from other food groups. Two dietary imbalances are receiving particular attention. First, in many high-income countries, it is reported there is too low an intake of nutritious plant-source foods relative to animal-source foods [27, 28]. Second, in an increasing number of countries around the world, it is reported there is too low an intake of minimally processed nutritious foods relative to UPFs [10, 27, 29,30,31].

From a health perspective, dietary imbalances are associated with an increased risk of diet-related NCDs such as obesity, cardiovascular disease, certain cancers and diabetes [32, 33].

Concerning the imbalance of nutritious plant-source foods relative to animal-source foods, the World Health Organization (WHO) has advised that a healthy diet includes [34]:

Fruit, vegetables, legumes (e.g. lentils and beans), nuts and whole grains (e.g. unprocessed maize, millet, oats, wheat and brown rice).

At least 400 g (i.e. five portions) of fruit and vegetables per day [33], excluding potatoes, sweet potatoes, cassava and other starchy roots.

Currently, the WHO (and other UN agencies) does not provide explicit advice on quantities of animal-source foods to include in a healthy and sustainable diet.

Concerning the imbalance of minimally processed nutritious foods relative to UPFs; UPFs are not an essential component of a healthy and sustainable diet. These foods often contribute superfluous energy, ‘risk’ nutrients (added sugar, salt and industrial trans fatty acids) and industrial ingredients, while displacing minimally processed nutritious foods, from dietary patterns [18]. The WHO has advised that a healthy diet includes [34]:

Less than 10% of total energy intake from free sugars.

Less than 30% of total energy intake from fats. Unsaturated fats are preferable to saturated fats and trans-fats of all kinds.

Less than 5 g of salt per day. Salt should be iodized.

Reducing excessive dietary intake of UPFs will help correct dietary imbalances not only directly by reducing dietary intake of risk nutrients and industrial ingredients, but also indirectly by increasing dietary intake of minimally processed nutritious foods (assuming the maintenance of a relatively constant dietary energy intake). The amount of UPF that might be accommodated within a healthy diet will vary with a person’s nutrient and energy requirements.

From a sustainability perspective, consuming an excessive amount of animal-source foods relative to nutritious plant-source foods can have significant adverse impacts in terms of greenhouse gas emissions, land use and degradation, water use and pollution linked to nutrient-rich fertilisers, and other environmental indicators [35, 36]. Consuming an excessive amount of UPFs relative to minimally processed nutritious foods is associated with biodiversity loss and soil degradation, excessive use of finite environmental resources such as water and food packaging waste, especially plastics [37, 38]. These multiple adverse sustainability impacts resulting from excessive consumption of UPFs are even more concerning given these foods are surplus to nutritional requirements.

Principle 3: Dietary moderation – to help achieve a healthy body weight and avoid wasting finite environmental resources used in providing food surplus to nutritional requirements

Dietary moderation refers to consuming enough food to provide for but not exceed the body’s energy needs. It is essential for optimal growth and development (until physical maturity is reached), to maintain a healthy body weight and composition (post-maturity), and to allow for a level of physical activity consistent with long-term good health (all ages) [32].

From a health perspective, people who consume enough food to provide for but not exceed their body’s energy needs generally have a healthy body weight and composition, and a lower risk of experiencing many diet-related NCDs, such as cardiovascular disease and type 2 diabetes, relative to people whose energy needs are exceeded. People who consume an excessive number and/or size of food serves from each food group beyond that necessary to meet their body’s energy needs may become overweight or obese. Living with overweight or obesity is associated with metabolic problems and an increased risk of many diet-related NCDs [39]. Conversely, children unable to consume enough food to meet their energy needs may have their growth and development stunted. Stunting is also associated with poor physical, mental and functional health problems [40]. Adults unable to consume enough food to meet their energy needs over an extended period will suffer from undernutrition and will be vulnerable to adverse health outcomes [41]. Both stunting and underweight are a consequence of challenges with food accessibility, affordability and availability [42].

From a sustainability perspective, consuming excessive amounts of food not only contributes to overweight and obesity, but also because it is surplus to meeting the nutritional needs of the consumer it is a waste of the finite environmental resources used in producing, processing, distributing, storing and preparing that food [43]. Also, excessive food consumption usually involves extra food being purchased and subsequently leading to extra amounts of food packaging requiring disposal.