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20 Aug 2024 — Researchers project that plant-based diets in the US and New Zealand that meet healthy adults’ energy and nutrient requirements are 34–45% more expensive than diets including animal-based foods. This is due to the need to include fortified plant-based foods for essential nutrients, mainly pantothenic acid, zinc and vitamin B12.
Through linear programming, the Massey University, New Zealand research team determined the role of animal- and plant-sourced foods in modeling a “least-cost” or most affordable diet for adults. The study considers the nutritional quality of foods, food prices and diet affordability.
“Sustainable healthy diets should be considered around four interrelated dimensions: environment, society and culture, nutrition and economy,” Dr. Sylvia Chungchunlam, the study’s lead author, tells Nutrition Insight.
“Recently, the perceived negative environmental impact of animal food production and consumption has prompted a shift in opinion toward developing a global sustainable and healthy diet based predominantly on plant-based foods.”
“This raises the question of whether there is an optimal balance between animal- and plant-sourced foods in the complicated connection between nutrition and cost. The impact of our recent studies is to provide objective evidence of the respective roles of animal- and plant-based foods to answer that imperative question.”
The authors underscore that nutritional adequacy and economic implications of plant-sourced sustainable healthy dietary patterns must be considered in this shift to plant-based diets. However, diet affordability has not been sufficiently researched.
The cost of plants
The research, published in Frontiers in Nutrition, compares modeled least-cost diets with and without plant-based products in the US and New Zealand. The linear programming model minimized the cost of the optimal dietary solution by changing the quantities and corresponding costs of selected foods while controlling for daily estimated energy requirement, minimum and upper intake limits of nutrients and maximum limits on daily food serving sizes.
“Our research provides the evidential basis that shifting from diets that include animal-based foods to solely plant-based diets comes at a higher dietary cost,” says Chungchunlam.
Animal-sourced foods are generally the lowest-cost sources of protein, calcium, iron and vitamin B12.In the US, a modeled nutrient-adequate least-cost diet with animal-based foods was projected at US$1.98 daily. The main contributors to total diet cost were milk (26%), fortified breakfast cereals (14.2%), potatoes (12.6%) and legumes (12.4%).
Meanwhile, a plant-only least-cost diet cost US$3.61 daily, with fortified soymilk (37%), legumes (13.3%), fortified breakfast cereals (12.7%) and cabbage (9.7%) being the main contributors to that cost.
The researchers highlight that the US government provides economic subsidies to the animal-sourced food sector, which may distort the market and affect the prices of animal-based foods compared to plant-based commodities. Therefore, they included another modeling study on New Zealand “where food subsidies imposed on animal-sourced foods are not found.”
Here, the most affordable plant-only diets were also more expensive than those including animal-sourced foods, albeit at a smaller difference, US$2.87 instead of US$2.14 daily. Primary foods in the modeled plant-based diet included fortified soymilk (47%), seeds (12.6%), pasta (10.5%) and legumes (7.3%).
Animal-sourced foods
Chungchunlam explains that animal-sourced foods are generally the lowest-cost sources of essential nutrients like protein, calcium, iron and vitamin B12. Although plant-based foods provide some vital components at a lower cost, such as dietary fiber and vitamin C, they tend to be nutrient-poor.
“Factors like bioavailability — the proportion of an ingested nutrient that is absorbed and available for utilization in metabolic functions — may also play a role. When setting recommendations for nutrient intake levels for a population, bioavailability is considered, and the recommended intakes are generally given in bioavailable amounts.”
She highlights that nutrient bioavailability differs widely between foods sourced from animals and plants, with animal-based foods generally providing more bioavailable nutrients than their plant-based counterparts.
For example, the researchers note that protein quality is an essential factor for assessing the inclusion of animal or plant sources in least-cost diets. Generally, diets with animal proteins had a higher Digestible Indispensable Amino Acid Score — a method assessing protein quality — than diets with only plant-based protein. According to the authors, “animal proteins play a critical role in ensuring sufficient provision of utilizable protein and indispensable amino acids.”
“Thus, it is challenging to adequately meet the recommended requirements for protein, essential fatty acids, fiber, vitamins and minerals, at a similar or lower cost, through consumption of plant-based foods only,” Chungchunlam continues. “In this instance, plant-based diets have a higher cost per unit of nutrient than mixed diets that include plant- and animal-based foods.”
Chungchunlam says that the monetary cost of foods is a critical determinant of food choice, diet quality and nutrition security.Researching economic impact
As diet sustainability focuses on health and environmental sustainability impacts, Chungchunlam stresses that the “economic dimension of sustainable diets has often been overlooked.”
“It is crucial to include the economic perspective of sustainable diets as the monetary cost of foods is a critical determinant of food choice, diet quality and food and nutrient security.”
“However, economic indicators are less established in the literature than environmental or health indicators. To characterize the economic cost of foods comes with its challenges, such as geography, regional or seasonal differences at a country national level and time,” she explains.
“Food price markets may also be influenced by subsidies or benefits, which will affect the outcomes of research studies on the economic aspect of sustainability.”
Chungchunlam adds that researchers also need to assemble social indicators to evaluate diet affordability, such as household income levels or distribution of income toward spending for foods at a population or national level.
Essential nutrient intake
The researchers did not set out to create realistic diets for consumption. Instead, they selected foods in the computer model to fulfill energy and nutrient recommendations requirements for an average adult population aged 19–50. Further modeling research should consider people most susceptible to inadequate nutrient intakes and food price increases.
The study finds several vitamins and minerals as “first-limiting nutrients” — calcium, potassium, selenium and vitamins A, C, D and E. Additional first-limiting nutrients in the plant-only diets were zinc, pantothenic acid and vitamin B12. Most of the plant-sourced foods included in the model were enriched with such nutrients, such as soy milk fortified with vitamin B12.
To lower daily costs of healthy diets, the researchers suggest combining plant-based with animal-sourced foods.Chungchunlam explains that the research also examined the use of supplements to account for these missing nutrients by including scenarios where these products supplied particular vitamins and minerals. “These least-cost nutritious dietary patterns were, as expected, less expensive.”
“However, the cost of the dietary supplements must be considered, which will impact the total cost of meeting nutrient needs. A pertinent question may arise that the combined cost of dietary supplements and least-cost nutritious diet may be more expensive than the cost of the least-cost nutrient adequate dietary pattern alone,” she underscores.
“The outcomes that animal-based foods need to be present in least-cost nutrient-adequate dietary patterns were not altered. This emphasizes the high value of animal-based food products to provide sufficient essential nutrients, even with dietary supplements.”
Combining animal-sourced foods and plants
Chungchunlam considers several developments to help reduce the cost of nutritious plant-based diets, such as “any processing method to increase the bioavailability of essential nutrients in plant-based foods.”
She also points to the “fortification of plant-based foods to enrich key nutrients that are frequently lacking in the diet.”
In addition, she underscores the need to examine the impact of combining plant- and animal-based foods in dietary patterns.
“While animal-derived foods contain most essential nutrients in higher concentrations and greater bioavailability, plant-based foods contain essential compounds, such as vitamin C and fiber, that are generally not present naturally in animal-based foods. Likely, adding even a relatively small amount of nutrient-rich animal-based foods to nutritious plant-based diets can result in a big difference in terms of lowering cost.”
“An opportunity exists to bring together the best attributes of both plant- and animal-based foods, to complement each other and interact synergistically, to produce a new range of food products,” Chungchunlam concludes.
By Jolanda van Hal