Consumers are increasingly aware of the environmental impacts of their diets and lifestyles, with everything from transport to dinner coming under scrutiny for its footprint. Pets are also coming under the microscope, principally for the role of their diets in the wider food system.
Quantifying the environmental impact of cat and dog food is challenging since the majority of ingredients are by-products of human food production, e.g., bone meal or grain leftovers. In the DELTA Model®, some of these ingredients are classified under “Other uses”, while some fit in the “Inedible portion” class, showing some of the challenges around assessing these commodities. Some studies allocate all impacts of production to the primary product, making the by-product footprint-free, while others allocate impact based on the mass or economic value of the ingredients.
A recent study used the economic approach to calculate that 1-3% of global agricultural emissions are on account of pet food production, with lower percentages for land and water use. Another calculated the impact of the US pet population’s diet as around 25-30% of the human population’s, including land, water, and fossil fuel use.
One estimate stated that around 140 million people could be nourished using the energy currently entering the US pet food system. However, this was purely an energy calculation, and did not include full human nutritional requirements. Moreover, it does not account for the fact that the food sources demanded by people do not match the lower quality ingredients used in pet food. However, there are increasing purchasing trends towards premium products that do include substantial proportions of human edible food.
As the impact of pet food is not negligible, there have been calls to reduce this pawprint. This impact is affected by many of the same issues as the impact of the human diet: food waste, overconsumption (and consequent non-communicable disease), and the differing impacts of different food sources. Thus, similar solutions can be tried, such as minimising waste, correct portion sizing, and inclusion of environmental impact alongside nutrition in ingredient selection.
Pet ownership is on the rise globally. While the benefits of pets are clear to any pet owner, and have measurable benefits for human wellbeing, they cannot be left out of any holistic approach to measuring or reducing environmental impact.
Helping consumers to make healthier, more sustainable food choices is a target for many retailers. A recent scientific articlehas investigated the efficacy of various “nudging” techniques in cafeteria settings.
Nudging involves deliberately changing how choices are presented to consumers to influence their decision. In this study, the authors changed how cafeteria meal choices were presented to consumers in eight cafeterias in schools, universities, businesses, and hospitals in Germany, on two occasions three years apart.
The authors used an existing sustainability calculator that includes both environmental and health data to determine the most sustainable choice on a menu.
The nudges trialled were placing the most sustainable dish in the best position on the service counter; placing it at the top of the menu; giving it a more descriptive name; and adding sustainability labels with or without full explanation of the sustainability of the dish.
Simply placing an item in the best counter position led to a 22.5% increase in selection above baseline in the first study, but this dropped to 11.6% in a second study three years later (2019/2020). Extensive sustainability labelling and explanations had no effect on sales in the first study but resulted in a 15.9% increase in sales in the second study. The remaining nudges either had no significant impact, or actually led to sales decreases.
These results can be useful to those in the food service industry with an interest in increasing sustainable choices. The authors cite increasing sustainability awareness and consumer interest as a possible reason for the success of the extensive sustainability labels in the second study only. Counter position was by far the most successful approach, and also the easiest for the cafeterias to implement.
This Thought for Food is part of a series on key nutrients identified as global challenges by the SNi DELTA Model®.
Calcium is an essential dietary nutrient: a trace element that is vital for healthy bodily function. However, a recent reportcomposed by the Calcium Task Force (created by New York Academy of Sciences in partnership with the Children’s Investment Fund Foundation) has identified widespread global calcium deficiency. This matches results of the DELTA Model®, which show that global calcium availability from food is only around two-thirds of the global population’s requirement. Here we provide a closer look at the role of dietary calcium, levels of deficiency, and ways forward to improve intakes.
The Task Force was composed of scientists with expertise in nutrition, particularly around micronutrients and child health. Their report aimed to gather the existing data on calcium deficiency and associated health outcomes to assist policy and dietary interventions to reduce the prevalence of deficiency.
About 99% of calcium in the body is in our bones. The rest plays a host of roles in our muscular, hormone and nervous system function, as well as in normal cell function. Calcium has a key skeletal role; therefore, calcium intake is particularly important in childhood and adolescence during bone growth. For all of these functions, calcium becomes important again in later life, such as during pregnancy and for bone retention in older adults.
Dietary calcium requirements are set largely based on bone health outcomes; that is, the intakes of people who have healthy bones are deemed sufficient intakes. This is due to the difficulty of understanding exactly how much calcium a person needs each day. When calcium intake is insufficient to meet bodily requirements, calcium stored in bone is used to maintain the other bodily functions requiring calcium, so no immediate effects are obvious. However, a sustained deficient calcium intake over a long period will to lead to depletion of bone calcium stores.
Another challenge in setting recommended intakes is the role of diet. Food sources of calcium, as well as what else is being eaten at the time of calcium ingestion, impacts calcium bioavailability. Plant antinutrients, such as phytic and oxalic acid, can bind to calcium and make it less easily absorbed by the gut. Vitamin D (another dietary micronutrient) is essential for active calcium absorption, and low vitamin D can increase bone calcium turnover and calcium requirement.
As a result of these challenges, there is variation in recommended intakes depending on which authority you ask: for example, the recommended intake range for infants in the Unites States and Canada is around double that recommended in the United Kingdom. Recommended intakes change as you age, and there is less variation in recommendations for adults – around 800 mg per day for most authorities.
Using the DELTA Model® Nutrient Supply and Density tools, the richest sources of calcium per unit mass are various oilcrops, spices and aquatic plants, all of which make minor contributions to global dietary calcium (<25 mg per person per day). The largest contributor to global calcium availability is dairy, accounting for 49% of global food calcium, or around 300 mg per capita per day. Next are vegetables, at around 100 mg per capita per day, cereals at around 50, and meat at around 25.
The Task Force report stated that around 3.5 billion people worldwide are at risk of inadequate calcium intake, with the vast majority of these individuals located in Africa and Asia. Calcium deficiency is more widespread in low- and middle-income countries than in high-income nations. For those countries where data is available, it is children, adolescents, and individuals over 50 who are most commonly deficient, with women more likely to be affected than men.
We used the DELTA Model® approach to compare 2018 national nutrient availability data for demographically weighted national calcium requirements. We found that only 33 out of 170 countries had sufficient calcium availability to meet the needs of their populations. This sufficiency ranged from just enough to a 90% calcium surplus in Finland. Most of these countries were in Europe, with a few from Western Asia, as well as the US and Australia.
We found the lowest calcium availability compared to requirement in lower income countries in Africa, the Caribbean and Southeast Asia. In some cases, as little as 16% of population calcium requirement was nationally available (Timor-Leste). While data quality is often poorer for these regions compared to Europe and North America, 77 out of 170 countries had less than half of their calcium requirements available, thus these calculations echo the results found by the Task Force.
The greatest focus of studies on the health outcomes of calcium deficiency relate to bone health.
Osteoporosis (brittle bones) is linked to extended low calcium intake and increases the risk that bones will break under stress. Osteoporosis disproportionately affects women, particularly over the age of 50. Calcium and vitamin D supplementation have been shown to reduce fracture rates in older people in some clinical trials, but this affect is not always observed. Rickets is a similar condition mostly observed in children and linked to low calcium and/or vitamin D intakes.
Interestingly, despite the fact that lower income countries tend to have lower calcium intakes, they also have lower rates of bone fracture than high intake countries. However, the report authors caution that these lower income countries have poorer quality health data and smaller elderly populations, among other confounding factors. Studies on bone mineral density and fracture risk often show contrasting results between and even within regions. Furthermore, many regions lack data on bone health, making causative associations difficult.
In lower income parts of the world, the impacts of low calcium intake are often overshadowed by shorter-term outcomes of malnutrition, such as anaemia, wasting and stunting, as well as infectious diseases. Of current interest is the potential role of calcium intake in pregnancy disorders of hypertension and related mortalities.
Other research has investigated relationships between calcium intake or supplementation and a broader variety of health outcomes, such as cardiovascular disease, colorectal cancer, and kidney stones, with varied and sometimes contradictory outcomes. Dosage and target population appear to play a role, but there remains much to be understood about the role of calcium in health.
Increasing calcium availability and consumption
While there is still a lack of consensus on the exact calcium intakes necessary for consumption, the number of individuals globally not meeting even the most conservative recommendations merits intervention.
From a food perspective, those countries with the lowest calcium intakes are most commonly those with low dairy intakes. High European dairy consumption matches our results that these countries had the highest 2018 calcium availability. Increasing the consumption of calcium dense foods, such as dairy, nuts, seeds, and whole fish will not only improve calcium intakes, but also the intakes of other essential nutrients, due to the nutrient dense nature of these foods.
The final recommendations of the Task Force report for future work were: to progress towards consensus on the true global prevalence of calcium deficiency; to obtain more data on global calcium intakes, particularly in lower-income, higher-risk countries; and, to clarify the role of calcium in fracture risk and bone mineral density. The authors also noted the challenging absence of some method to determine the calcium status of an individual, given there is currently no single biomarker or health outcome that does so.
What can be said incontrovertibly is that deficient calcium intakes are common worldwide. While the full extent and impact of this deficiency is uncertain, that should not prevent action to improve intakes toward recommended levels.
The SNi team thank Professor Connie Weaver, expert in mineral bioavailability and the role of calcium in human nutrition, for her contributions to this Thought for Food. Prof Weaver was an author on the Task Force report.
The European Regional Office of the World Health Organisation has released a fact sheet reviewing the evidence for the impact of plant-based diets on health and sustainability. The document was released in response to increasing discussion of the concept of “plant based” foods and diets in the European region. They define such diets as “emphasizing foods derived from plant sources coupled with lower consumption or exclusion of animal products”.
In Europe, non-communicable diseases (NCDs) are the major health concern, with cardiovascular disease alone accounting for more than half of all European deaths. Many NCDs can be linked to diet, and specifically to low consumption of vegetables and fruit. Indeed, in more than half of the European countries, daily vegetable and fruit consumption recommendations are not met.
Several studies have identified the reduced burden of many NCDs, especially diabetes, in the vegetarian and vegan population. They note the generally lower BMI and all-round healthier lifestyle of individuals who choose these diets as a contributing factor, as well as the diets themselves.
Reductions in NCDs from more balanced diets would be expected to have benefits for health, and thus reduce health-care expenditure. Environmentally, there may be benefits to reducing the impacts associated with high consumption of animal-sourced foods, such as greenhouse gas emissions and biodiversity loss.
The greatest risks associated with increasingly plant-based diets are for nutrient intakes. The fact sheet encourages proper planning of these diets to account for the reduced supply and bioavailability of nutrients such as iron, vitamin A, B12 and D, and zinc, as well as choosing foods fortified or supplemented with these nutrients.
The authors also warn against blanket associations between plant-based and healthy. Many highly or ultra-processed foods are plant-based, but not all such foods can be described as healthy. Examples such as imitation meats and milks are given, as highly processed plant-based foods containing added sugars, flavours, colours, emulsifiers, and salt. Little is currently known about the nutritional or health impacts of such foods if forming a major part of the diet, as this is still an emerging product group.
The report concludes that the adoption of plant-based diets can be beneficial, and that even incremental changes towards such diets may have benefits. They recommend that foods in such a diet be chosen that are minimally processed and ensure adequate nutrient intakes. Increasing vegetable and fruit consumption, particularly for those not meeting recommendations, should certainly be a target for European authorities.
Two recent correspondence articles published in The Lancet have highlighted some of the challenges in global dietary intake and risk estimation studies. These studies are held in high regard and widely used, so their quality is of great importance.
The first article highlights the differing estimates of two global dietary intake studies: the Global Dietary Database and the Global Burden of Disease (GBD) study 2017. Both report country-level intakes of food groups among adult populations, but are calculated quite differently: the former largely from aggregated dietary survey data, and the latter largely from national level food availability data.
The outcomes of the two approaches showed wide variation in many countries: more than 10-fold differences in several countries across multiple food groups. Among these were consumption of sugar-sweetened beverages, an important target for reduction. Inaccurate estimation of consumption can lead to inaccurate or disproportionate estimates on public health.
The second article also considers the GBD study, but compares the dietary disease burden estimated in the 2017 study with the recently published 2019 GBD study. The burden on human health of diets low in fruit, nuts and seeds, vegetables, and specific fatty acids had more than halved. Meanwhile, diets high in red meat moved from the 15th largest contributor to diet related disease burden, to 5th, with 36 times higher deaths attributed to these diets than were found in the 2017 study.
The cause of these changes was altered methods in calculating disease burden. The latest study assumed relationships between high red meat diets and heart disease, breast cancer, stroke, diabetes, and colon cancer, whereas the earlier study only included the last two conditions. It also set zero intake as the optimum for minimising health risk, while the previous study used a moderate intake value. These health associations are contended in the scientific literature, as described fully by the authors . A response has since been published, explaining that the upcoming 2020 study will wind back its estimates on meat following further changes to methodology.
The importance of these global studies of dietary intake and associated health outcomes cannot be questioned: there is an urgent need for this data to inform public health decision making. However, these studies are often among the most widely cited and reused in public health policy, thus their conclusions have far-reaching consequences. The authors of these articles emphasise the importance of having multiple models to identify areas of uncertainty. The results also highlight the weaknesses of relying solely on food availability data, rather than consumption. The articles feature calls for stronger dietary data in low- and middle-income countries; standardisation of food group classifications and disease burden estimation; and caution when interpreting the reliability of such estimates.
Many researchers have proposed scores and methods for reducing the nutritional value of foods down to a single number – as covered in a recent Thought for Food. A new method takes an approach rooted in population dietary intakes.
There are many challenges to this: which nutrients to include? How to weight components without introducing bias? A recent paper has avoided these issues by including all nutrients in the Australia and New Zealand Nutrient Reference Values, weighted by the degree to which the Australian population under- or overconsumes them.
The NRF-ai metric (Nutrient-Rich Food Index – adequate intake) means that foods containing under consumed nutrients like calcium, magnesium, vitamin B6 and zinc will receive higher scores than those containing the same amount of vitamin C or phosphorus, which are consumed at adequate levels by most of the population. Conversely, foods containing free sugars will be penalised, as intake of these is above recommendations in most populations. The score can be made specific to age and gender groups, as the prevalence of deficiency for each nutrient varies between these groups. Ultimately, this leads to a metric that ranks a food item on its ability to address the nutritional needs of the population.
There are many applications for this metric for comparing foods. In the paper, the author considers the score per $ retail price, to understand the cost-effectiveness of a food for meeting nutritional needs. Per environmental impact examples are also given.
NRF-ai represents an unbiased approach to reducing the nutritional value of a food down to a single number. While this approach still loses information compared to the full nutritional composition, it is still valuable for comparisons between like products. A similar approach is applied by the DELTA Model® for the nutritional value of food items for meeting global nutrient requirements.
The environmental impact of a food, be that carbon footprint, water use, land use or some other factor, can be estimated by life cycle analysis (LCA). With the environmental impact of food an increasingly important consideration for many consumers, industry and policymakers, the FAO have recently published a report on the challenges and opportunities of nutritional LCAs – those that attempt to capture the nutritional value of food alongside its environmental impact.
LCAs are strongest when used to identify hotspots or areas for improvement within the supply chain for a single item. They can be used to answer industry questions like: where should we act first to lower the footprint of our product? They can also be used in comparisons between two otherwise identical products for consumers: which one should I buy? However, challenges arise when LCAs are used to compare the impacts of very different products.
Take Energy Rating labels on electrical appliances as an example. Analogous to LCAs, these are an indication of the relative energy usage of a particular model compared with other appliances of the same type. These are useful for comparing two refrigerators, but do not really help when comparing refrigerators with freezers. They are even less useful when comparing a refrigerator with a washing machine: the appliances have completely different functions, and a purchaser would be unlikely to use them to choose which of the two to take home.
Even within the category of refrigerators, ratings become less relevant when comparing different size models, as they provide a different level of service. Without considering the service or benefit provided by the product we do not have a fair basis on which to compare the footprint or cost of providing that service.
The same problem exists when comparing foods. When we look at the footprint of food products and start making comparisons, we need to be clear on the service or benefit being provided by the products to ensure we are making a valid comparison. However, the service provided by a food item depends on the purpose for which it is consumed.
Food is consumed for a variety of reasons: as a source of nutrition, for sensory experience or pleasure, or for social and cultural purposes. Accounting for these different purposes is not straightforward. For example, from a nutritional perspective, alcoholic beverages provide very little benefit, but many consumers may still place high value on their sensory or social purposes.
The FAO report focuses on nutrition, rather than the other services provided by food, and looks at how nutritional information can be combined with environmental impact data.
One approach is to try and bring together the “benefit” and “cost” into a single analysis: the development of a nutritional LCA (nLCA), a life cycle analysis that includes nutrition.
There are two different methods by which this can be done:
As part of the definition of the functional unit (e.g., land use per 100 kcal)
As part of the human impact assessment, what is often thought of as the cost side of the analysis (e.g., likely impact on human health)
Neither of these approaches is easy.
Shifting functional units
Often, an LCA uses mass as the functional unit. For example, if considering the water use needed to grow rice, an LCA might report results as “litres of water used per kg of rice”. In this case, the functional unit is “1 kg of rice”.
Putting nutrition into the functional unit moves away from just using mass. In the simplest form, this may be evaluating a set of foods based on the amount of a particular nutrient they contain. Protein is often used for this purpose. Our rice example would then change to “litres of water used per kg protein in rice”.
However, protein is not a single nutrient needed by the body, but rather a collection of amino acids, which are the essential nutrients. Not all proteins are created equal, having both different concentrations of these amino acids and varying in their digestibility. Rice protein is therefore different to soy protein, for example. Thus, comparing water use per kg protein does not capture this information. Sophisticated methods that include protein quality exist, but are challenging and rarely used.
Most food items provide more than one nutrient, and we need a broad range of nutrients to remain healthy. The DELTA Model® estimates the ability of the global food system to supply a basket of 29 nutrients, and would include more given suitable data. Evaluating a food item based on only one target nutrient misses this complexity.
An alternative to selection of a single nutrient as the functional unit is to use a basket of nutrients to create some form of nutrient reference score. The intention of this score would be to provide a more “balanced” view of the nutrition provided by foods. However, what nutrients should make up this score? Do they all have equal weighting? Or are some more important than others? And how does this relate to the needs of an individual? The scientific literature contains many different suggestions, each with their strengths and weaknesses. Each is at risk of introducing some form of bias into the assessment.
Another important consideration is portion size. Once we move away from a functional unit based on mass, we lose some of the context around the amount of food that needs to be consumed to deliver a particular nutrient or group of nutrients, and how that relates to the size of a normal serving. Functional units “per serving” have also been explored, but face the same problems as mass based units.
Bringing human health into the assessment
The alternative approach is to leave the functional unit as the mass of the food item and build the nutritional assessment into the impact side of the LCA. This requires having data on the expected impact of consuming a food for human nutrition or health. The main approach that has been considered to date uses epidemiological data on diets, health, and mortality. This is usually of the kind captured in the Global Burden of Disease (GBD) study, which calculates statistical links between consumption of food groups and expected lifespan or quality of life.
Unfortunately, this data is limited to comparatively coarse effects. The GBD study reports statistical measures for 15 health aspects related to diet and 3 related to nutrient deficiency. The statistical associations are the result of a complex analysis that attempts to isolate the impact of individual food factors on overall outcomes. Changes in assumptions used in the analysis between the 2017 and 2019 data sets resulted in significant changes in the apparent impact of several food groups. These have been highlighted in a recent letter to The Lancet, and would have a major effect on any nLCA employing this data.
In general, the benefits of consumption of food or nutrients follow a curve. Initially there is a positive impact on health, with increasing consumption providing nutrients essential to bodily functions and growth. This benefit is reduced once daily requirements are met, and, if consumption continues to increase, may eventually have negative health outcomes.
This is illustrated with the energy content of diets: eating insufficient calories leads to wasting, but eating too many leads to obesity and a range of related health conditions, and just how much is too few or too many depends upon the need of the individual. Sodium is another example: a diet deficient in sodium can have serious health consequences. However, many diets contain a considerable excess of sodium, carrying health risks for many individuals.
Putting food and nutrients in context
Food items are consumed as part of meals and diets, and it is at this level that we need to apply considerations of nutritional sufficiency. The relative nutritional benefit of consuming a food item varies based on the dietary context of the individual. For example, the protein or amino acid content of a food item may be of limited value in a diet that is otherwise oversupplied with this nutrient, but of immense value in a diet that is deficient.
Within the DELTA Model we have implemented a simple nutrient contribution measure for food items. This is based on the sum of the relative contribution the food item makes to each of the nutrients captured in the model. As such, it gives a higher weighting to nutrients that have low global availability and a lower weighting to nutrients that are abundant.
For example, the default 2018 DELTA Model scenario has a 34% deficiency for calcium against global requirements (achieving 66% of target), whereas phosphorous has a 150% excess (250% of target). Thus, a food that provides 33% of the daily target for calcium gets a score of 0.5, whereas 33% of the daily target of potassium scores only 0.13 – approximately ¼ the importance. A similar approach has recently been published for the individual dietary context.
The right use of nLCA
The challenges described above stem from trying to compare refrigerators with washing machines, and lead us to the fact that nutrition does not easily collapse into a single score.
The scope of comparisons, or the grouping of foods into groups becomes important. If food items are grouped with others that provide or purport to provide similar nutritional benefits, we can make more realistic comparisons that better reflect the real choices facing us.
As an example, we might compare the nutritional LCA of milk with that of a plant beverage and use a nutritional functional unit that reflects the role of these items within the overall diet. Milk products make a significant contribution to the global supply of calcium, phosphorous, and potassium, six indispensable amino acids, dietary fat, overall protein, and vitamins A, B2, B5, and B12. A nutritional functional unit could be designed that reflects this nutritional value to enable us to compare milks and milk-alternatives when consumed as a source of nutrients. However, this same approach would not necessarily be appropriate if the purpose of the product was simply to whiten a cup of coffee. The intended service or benefit of foods must be understood when deciding how to compare costs.
Whilst the concept of a universal nutritional LCA that provides all the information necessary to support a wide range of decisions is attractive in its apparent simplicity, the reality is that nutrition and environmental impacts are too complex, and too important, to be reduced to a single number.
Sustainability considerations are increasingly featuring in dietary recommendations from official sources. The detail of these recommendations varies, but a recent review argues the need to take a holistic view of sustainability.
The paper highlights the complementary and synergistic benefits of consuming both plant-sourced foods (PSF) and animal-sourced foods (ASF) as part of a sustainable diet, and the false dichotomy that PSF are good and ASF are bad – sometimes propagated in dietary recommendations. A holistic approach is recommended when broadening guidelines, such that multiple dimensions of environmental and socio-economic factors are considered.
The authors provide comprehensive tables on the macronutrient, micronutrient and bioactive components found in PSF and ASF. They also make the obvious, but often neglected point, that not all PSF and ASF have the same nutritional content, environmental or sociology-economic impacts and that variety and redundancy (overlapping nutrient profiles of different foods) will be required for nutritional adequacy, food security and sustainability.
Also covered is the impact of food composition and structure – the food matrix – on nutrient bioavailability. Broader factors such as food processing and meal preparation, waste, impact of and on the microbiome and food synergy: the improvements in nutrient bioavailability as a result of nutrient interactions, are also included in the analysis.
Although fruit, vegetables and dairy food groups are most prevalent in dietary recommendations, they are under-consumed throughout the world. At a global level the most commonly under-consumed nutrients are calcium, iron, zinc and vitamin A. From the DELTA Model® we know that not only is calcium under-consumed but it is also under-produced by the global food system.
When guiding consumers, clear and simple messaging is often retained. However, sustainable nutrition is far from simple, so we must ensure clarity of guidelines if we are to help individuals make sustainable, healthy choices.
A recent study and accompanying editorial have compared dietary nutrient density and diet-related greenhouse gas emissions to understand whether individuals consuming more nutritious diets with lower carbon footprints have a longer lifespan than others.
The short editorial highlights some of the issues in interpreting data for the relationships between mortality and food. Nutrient density, as measured here, can be assessed through multiple metrics. Similarly, climate impact can be measured in many ways, here via greenhouse gas emissions in CO2-equivalents.
Although the impact of nutrition on death rate is an important consideration in assessing the impact of diets on health, it is limited and does not account for broader lifestyle impacts on mortality.
Similarly, although climate impact is an important consideration in assessing the impact of diet on the environment, land, water, and fertiliser use (among other factors) must also be considered in assessments of the environmental efficiency of diets.
Within these limitations the study found several interesting associations, with notable differences between women and men.
In the female cohort, mortality was 13% lower for those consuming diets with a high nutrient density and a low climate impact compared to those on low nutrient density and high climate impact diets. However, the same reduction in mortality was found for those consuming diets with a high nutrient density and high climate impact, indicating that nutrient density was the most important factor in the reduced mortality rate.
For men, mortality was 11% higher for diets having a low nutrient density and low climate impact compared to diets having a low nutrient density and high climate impact. It was suggested that this was due to sugar content: sugar has a small climate impact, but a clear negative impact on health when overconsumed.
The editorial makes some useful suggestions for how improvements can be made to such studies, such as considering food intake rather than reducing the assessment to individual nutrients. These results emphasise that climate-friendly diets are not always healthy, and vice versa, but that healthy, climate-positive diets can be achieved.
Linear programming was combined with supermarket food price data, food composition data and adult nutrient requirement data to find daily diets that met all nutrient requirements, optimised for cost.
Similarly to the original USA study, the least cost diet contained both plant- and animal-sourced foods. At NZ$3.23, the price difference to the plant-only diet (NZ$4.34) was reduced compared to the USA results.
In both the NZ and USA diets, milk was a large contributor to nutrient targets in the least cost diet. A soy beverage was the largest contributor to both mass and cost in the plant-only diets. Eggs, legumes, and cabbage were also important in both country settings. While fish was present in the USA least cost diet, this was replaced with green mussels in NZ.
The relative changes to the price of animal-sourced foods before they became priced out of the least cost diet differed between the two settings. Milk was removed when its price was 2.2x current NZ retail price (compared to 8x in the USA study); eggs under price increases of 1.8 times (11.5x in the USA study); and meat items under price increases of 1-2x (3-5.5x in the USA study). In contrast, the mussels remained in the least cost diet even if ten times more expensive than current prices.
This work reinforces the conclusions of the original paper: plant- and animal-sourced foods contribute to affordable, nutritious diets, and it can be more expensive to achieve adequate nutrient intakes if restricted to plant foods alone. However, the reduced resilience of most of the animal-sourced foods to price increases in the NZ setting is reflective of the differing relative retail prices of food in the two countries, which the authors state may be reflective of the influence of government subsidies in the USA.