Measures and metrics of sustainable diets

With a focus on dairy products

Designing food patterns that are more sustainable and healthy is more challenging than it first appears. Especially when aspects such as availability, affordability and food culture are also taken into account. How do you determine a more sustainable and healthy diet? Drewnowski (2018) is using linear programming models in his research.

Summary | Meeting the nutritional needs of a growing population and ensuring that future generations have access to healthy diets is driving the focus towards a more sustainable global food system. A sustainable diet requires foods and dietary patterns to be nutrient-rich, affordable, culturally acceptable, and sparing of natural resources and the environment. Drewnowski (2018) measures these aspects in his recent publication. One of his conclusions: the environmental impact of dairy farming should be weighed against the high nutrient density of milk, yogurt and cheese.

A broader definition of sustainable diets was developed by the Food and Agriculture Organization (2010) based on 4 principal domains. Foods and food patterns need to be:

  1. Nutritionally adequate
  2. Affordable
  3. Culturally acceptable
  4. Sparing of natural resources and the environment

Each domain has its own measures and metrics.

Nutrient density metrics

Measures and metrics of sustainable diets 2To assess the nutrient density of foods, a system for ranking foods by their nutritional value has been developed called nutrient profiling (NP). NP models are used to separate foods that are energy-dense from those that are nutrient-rich and this concept of nutrient density can be applied to individual foods, composite meals, and the total diet.

Figure 1 illustrates the concept of nutrient density with a simple crude NP model that is based only on the content of 2 index nutrients, protein (g/100 kcal) and calcium (mg/100 kcal). From this over-simplified model it becomes clear that few foods naturally provide high amounts of protein and calcium at relatively low energy cost (100 kcal), except for milk and dairy products that could therefore be classified as naturally nutrient-rich.

Figure 1 Nutrient density model: relation protein and calcium

Adapted from Drewnowski (2018).

Published NP models are generally quite complex and use at least 5 and up to 40 index nutrients, for example the Nutrient-Rich Foods (NRF) family of scores which includes a varying number of qualifying nutrients and 3 disqualifying nutrients. The qualifying nutrients typically include protein, fibre, and selected vitamins and minerals and the disqualifying nutrients, known as nutrients to limit, are saturated fat, added sugars, and sodium. One of the best described  NRF models is the NRF9.3 score. This index is based on 9 qualifying nutrients (protein, fibre, vitamin A, vitamin C, vitamin E, calcium, iron, potassium, and magnesium) and 3 disqualifying nutrients (saturated fat, added sugar and sodium). The NRF score has been validated against the Healthy Eating Index which is an independent measure of a healthy diet.

Food affordability metrics

The concept of nutrient density allows for further analysis of the financial cost of calories and nutrients known as the food affordability metrics. Food affordability has been determined for different food groups and is a measure of both calories and nutrients per penny. When used in conjunction with the NRF9.3 score, the lowest-cost foods with the highest nutrient density can be identified. The data for 2342 foods from the US Food and Nutrient Database for Dietary Studies (FNDDS 2009 – 2010) was used to calculate  the median cost per 100 kcal. The 2342 foods were aggregated into 9 major food groups, namely milk and dairy; meat, poultry and fish; eggs; dry beans and legumes; grains; fruits; vegetables; fats and oils, and sweets, including sugar-sweetened beverages. The analysis found that vegetables, fruit, meat, poultry and fish cost more per 100 kcal, whereas sweets, grains, and fats cost less. Fats and sweets had the lowest NRF scores and vegetables and fruit had the highest NRF scores, followed by beans. Within the milk and dairy group, low-fat milk, and low-fat yogurt had the highest NRF scores whilst cheeses generally had lower scores due to their sodium and saturated fat content. The per-calorie cost of the milk and sweets groups were similar however the milk group had the higher overall nutritional value.

Figure 2 Food affordability

Adapted from Drewnowski (2018).

Food acceptance measures

Identifying nutrient rich foods at an affordable price is only a part of the challenge. Sustainable food patterns also need to be socially and culturally acceptable. Tradition, society, religion and culture can influence food choices, in particular protein sources. In some countries meat consumption has increased whereas others continue to have traditional plant-based diets within which milk and dairy products may become the naturally preferred source of animal protein. However in the search for alternative proteins there remains some trade-offs with nutrient density, cost, environmental impact, and cultural acceptability. For example, proteins from pulses and soy are more widely accepted than proteins from insects. Whilst the production of meat and dairy has a higher environmental cost, the amount and quality of the protein they naturally provide is higher than what can be obtained from plant foods.

Environmental impact metrics

Global food production, transportation and storage contributes to greenhouse gas emissions (GHGEs) also known as the carbon footprint or carbon cost. The carbon cost of diets is typically expressed per calorie and can also be expressed per nutrient. In analysis of Drewnowski, foods with a higher nutrient density, were associated with a higher carbon footprint and an increase in diet quality increases the carbon cost of diets. Meat, milk, and dairy products have higher GHGE values per 100 g but much lower values per 100 kcal. Grains and sweets had the lowest GHGEs using both methods but were high in energy and low in nutrients. It is therefore important to consider the basis of these calculations. Expressing carbon cost per 100 g or 100 kcal  makes a significant difference. Vegetables, for example, may have a low carbon footprint per unit weight, but many vegetables are 90% water and provide almost no calories and nutrients. Therefore greenhouse gas emissions associated with the production of vegetables may be low when expressed per 100 g but become much higher when expressed per 100 kcal. Given the differences in energy density across food groups (see Figure 2), continuing to express carbon cost per 100 g makes little sense. Therefore the carbon footprint of diets is normally expressed per calorie.

A recent article entitled “Decreasing the Environmental Footprint of our Diet” published in Nutrition Magazine (the Netherlands) described the use of the Optimeal® model to calculate the environmental impact of various diets. The authors concluded that the menu based on the common Dutch diet, including meat, dairy (350 grams), fruit and vegetables from Dutch farms scored the lowest environmental impact and is sufficient to achieving recommended nutrient intakes. Drewnowski (2018) proposed that “Determining the point at which the higher carbon footprint of some nutrient dense foods is offset by their higher nutritional value is a priority area for additional research”.

Milk and dairy foods in sustainable diets

Milk is a natural source of high quality protein and micronutrients, including calcium, phosphorus, potassium, iodine and vitamins B2 and B12. Drewnowski concludes that based on the sustainability measures and metrics, milk can be described as nutrient-rich, affordable, acceptable and appealing. Modern farming practices have also lowered the environmental impact of dairy production.

Few food groups satisfy all 4 domains of sustainability. Drewnowski (2018) concluded that in order to meet the nutrient needs of the world’s population, the environmental impact of dairy farming should be weighed against the high nutrient density of milk, yogurt and cheese.

Reference

  1. Drewnowski A. (2018). Measures and metrics of sustainable diets with a focus on milk, yogurt, and dairy products. Nutr Rev. 2018;76(1):21-8.
  2. Peters et al (2017). Hoe verkleinen we de ecologische voetafdruk van ons bord? 9 eetregels om duurzamer te eten. Voeding Magazine 2017.