Beyond ‘healthy’ – what are sustainable diets and how can we measure them?
It is commonly agreed that a ‘healthy’ diet is one that contains the right amount of food for how active you are (enough calories but not too many) and ensures sufficient amounts of all the necessary nutrients through consumption of a diverse diet. In practice this means: eat a lot of varied fruit, vegetables and wholegrain cereals; and go easy on fat, meat, dairy products and processed foods.
Healthy diet guidelines exist in many countries, an example of which is the Eatwell Plate that was developed in the UK, which provides a visual illustration of the amount of foods from different food groups one should consume every day. Measures have been developed to capture how healthy a diet is. These range from indices that measure adherence to national dietary guidelines, to measures of diet quality such as the dietary diversity score (DDS).
But is ‘healthy’ good enough?
Our food production systems affect and are affected by the natural environment. Feeding a growing, urbanising global population creates pressure on natural resources through land clearance, use of fresh water and greenhouse gas (GHG) emissions. Meanwhile, biodiversity loss, falling water tables and changing climates all threaten to undermine the ability to produce sufficient, healthy food.
Although a healthy diet is an essential requirement to tackling nutritional challenges, it is becoming increasingly evident that a more holistic approach is required; one that also encompasses the environmental effects of what the growing global population eats.
Enter sustainable diets
The concept of ‘sustainability’ refers to meeting our needs without compromising the future resources of the planet. This implies that a sustainable diet is also a healthy diet, but in addition it considers impacts on the environment. ‘Healthy’ is an essential but not a sufficient condition for a diet to be ‘sustainable’.
But how can we measure the sustainability of diets?
How to measure impact of diets on the environment
Food production affects the environment through various pathways, including greenhouse gas (GHG) emissions, use of land for agriculture (which can result in loss of biodiversity), water use, and application of fertilisers and pesticides, among others.
Many of these pathways have been used to measure the sustainability of diets, though GHG emissions have received by far the most attention in the past. The environmental impacts are commonly assessed through a method called life cycle analysis (LCA), which examines the effects of food production on the environment over various stages, such as on-farm production, processing, transport, and up to retail and consumption at home. The environmental effects are usually measured as the amount of environmental impact (e.g. amount of land used, amount of GHGs emitted) per gram of food that ends up on our plate.
These measures have shown that some foods tend to have a markedly higher impact on the environment than others. For example, animal products, in particular from ruminants (such as cows and sheep), have high environmental impacts compared to plant-based foods because of the amount of methane produced by the animals and the amount of plant-based food it takes to feed a farm animal.
Some foods may also have relatively different impacts across the different environmental indicators, for example having low GHG emissions but high water use, or vice versa. Combining the various indicators into an aggregated score or index would allow for a more comprehensive measure of environmental impacts, but unfortunately we are so far lacking such measures. Other problems with assessing the sustainability of diets include a lack of good environmental data for many countries and regions, and the difficulty of comparing impacts when different measures have been used.
Marrying the ‘healthy’ with the ‘sustainable’
How do both goals of health and sustainability fit within the context of what we eat, and is there a trade-off between the two?
We recently explored this in a dietary optimisation model (Green 2015) for application in the UK. Our model found that if we were to modify the average UK diet so that it met healthy guidelines (set out by the World Health Organisation), then average consumption of meat, dairy and sugar would need to decline while intake of fruit, vegetables and cereals would increase. Looking at the environmental impact, we saw a 17% reduction in GHG emissions compared to the current average diet. We also found that by further reducing consumption of high-GHG foods, the UK could achieve a 40% reduction in GHG emissions from diets without radically changing what we are eating (i.e. without needing to convert completely to vegetarianism or veganism).
However, health and sustainability do not always go hand-in-hand. Many unhealthy foods may have low GHG emissions, such as sugar and some oils, and increasing consumption of fish, which is recommended in health guidelines, can stress some fragile fisheries. Our study also found that reducing GHG emissions by more than 40% in the UK would lead our diets to become very restricted and lacking in some fruits and vegetables that we currently import. This shows that the relationship between ‘healthy’ and ‘sustainable’ can be a complex one.
Once better measures of sustainability are developed, dietary guidelines need to follow suit and include these environmental priorities in food recommendations. Such guidelines already exist on a limited scale, such as the Livewell Plate, a visual representation of foods that are both healthy and sustainable.
More effort is needed though, both on the research and policy front, to understand how we can feed the growing population of the globe in a way that is healthy AND sustainable.
Green R, Milner J, Dangour AD, et al. The potential to reduce greenhouse gas emissions in the UK through healthy and realistic dietary change. Climatic Change 2015; 129: 253-65.
By Dr. Rosemary Green, Dr. Edward Joy and Mr. Lukasz Aleksandrowicz, London School of Hygiene & Tropical Medicine/ LCIRAH
Photo: Wikimedia Commons