Yogurt and sustainable diet

Choose your low-emission diet with care

YINI_Choose your low-emission diet with care

Diets that have the least impact on the environment also tend to be those that are the healthiest, this latest study from the USA shows. But as you take the leap to switch to a diet with the smallest possible carbon footprint, pause for a moment to make sure you’re not missing out on some vital nutrients. You may decide to add a few extra items, such as dairy products, to your shopping basket.

The authors of this study examined the food choices made by thousands of Americans and found that those diets with the lowest carbon footprint generally have the best diet quality. In many ways such diets are more nutritious than those that place a heavier burden on the environment. But some notable nutrients were in short supply and these will need to be taken into account in future guidance on how we can eat to boost both our own health and that of our planet, say the authors.

Our food contributes to climate change

Our food production is one of the largest contributors to climate change, say the authors. But previous research has estimated that if we all change our diets we could cut greenhouse gas emissions from food by as much as 50%.

Expert organisations advise that we shift towards more plant-based diets as these are good both for our health and for the environmental sustainability of our food provision. However, say the authors, most studies have been carried out on certain types of diets such as vegetarianism or on sustainable diets devised by researchers. We know little about how individual dietary choices are linked to environmental impact and nutrition quality.

Comparing greenhouse gas emissions with diet quality

The authors therefore looked at the link between greenhouse gas emissions and diet quality using data from 16,800 adults taking part in the National Health and Nutrition Examination Survey (NHANES). The participants were asked to recall everything they’d eaten and drunk over a 24-hour period.

The authors ranked the diets into five groups according to their greenhouse gas emissions. The diet groups with the highest and lowest emission were then compared for diet quality using the US Healthy Eating Index (HEI) and for the amounts of specific nutrients that are known to be eaten too much or too little in the USA and so are of public health concern.

Diets vary widely in their carbon footprint

Their results revealed a huge variation in the size of carbon footprint in these one-day diet samples. The group with the lowest greenhouse gas emissions was responsible for only 8% of the total emissions from diet – five times less than the 41% contributed by the highest emission group.

Women tend to have a lower dietary carbon footprint than men

As women generally consume less energy than men, they can be expected to have a lower carbon footprint, and indeed the study found that the low-emission diets were more likely to be eaten by women than men. Low-emission diets were also more popular among young people aged under 30 years, and African-Americans.

Vegetarians had a smaller carbon footprint from their diet than did non-vegetarians, on average – as did people who read food labels or had tried dietary guidance.

Low-emission diets have pros and cons

When the researchers looked at the nutrient content of the diet groups, they found the low-emission diets contained more fibre and vitamin E and less sodium and saturated fats than the high-emission group.

But the low-emission diets were far from perfect: not only were they higher in sugars than the high-emission diets, but they were also lower in several nutrients of public health concern, such as iron and calcium. They also contained less vitamin A and D, and less potassium than the high emission group. These nutrients are found in animal-source foods – meat and dairy products – which were eaten much less in the low emission diets than the high emission group.

Low emission diets are healthier overall – but lack some key components

Overall, the diets with the lowest greenhouse gas emissions had a better diet quality as shown by their average total HEI scores. But when the researchers assessed the separate components of the HEI scores, they found some important drawbacks.

Low-emission diets scored significantly higher on the whole fruit component of the HEI, as well as the whole grains, seafood and plant proteins, fatty acids, and sodium components. However, high-emission diets scored higher on protein foods, dairy, vegetables, and refined grains.

 ‘Low-GHGE [greenhouse gas emissions] diets in our sample were not only higher in sugars, but also lower in several nutrients of public health concern, such as iron and calcium.’ – Rose D et al, 2019.

Tailor diets to balance nutritional quality with greenhouse gas emissions

Our diets are a cocktail of many different ingredients, each of which can influence the nutritional quality and environmental impact. This implies that we can tailor what we eat to improve the diet quality and shrink our environmental footprint, say the authors.

Further studies should include the impact of diet on other environmental factors, such as water and land use, the authors suggest. In the meantime, they conclude, this research adds to a growing body of evidence on which to base new dietary guidelines that incorporates sustainability as a key factor in choosing what we eat.

‘It is clear from our work that acceptable diets can be crafted that both reduce GHGE [greenhouse gas emissions] and improve overall nutritional quality.’ – – Rose D et al, 2019.

Find out more: read the original paper
Source: Rose D, Heller MC, Willits-Smith AM, Meyer RJ. Carbon footprint of self-selected US diets: nutritional, demographic, and behavioral correlates. Am J Clin Nutr. 2019 Mar 1;109(3):526-534.

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