The Role of Human Diet in Phosphorus Demand23 January 2013
GLOBAL - Because meat production accounts for more than 70 per cent of the global phosphorus footprint on average, decreasing meat consumption in already high phosphorus-footprint countries could play an important role in sustainable phosphorus management strategies, according to a new study from Canada and the US.
Over the past 50 years, there have been major changes in human diets, including a global average increase in meat consumption and total calorie intake, according to Geneviève Metson of McGill University in Canada and co-authors there and at Arizona State University in the US.
In their paper in Environmental Research Letters, they report their study in which they quantified how changes in annual per capita national average diets affected requirements for mined phosphorus between 1961 and 2007, starting with the per-capita availability of a food crop or animal product and then determining the phosphorus needed to grow the product.
The global per-capita phosphorus footprint increased 38 per cent over the 46-year time period, but there was considerable variability among countries. Phosphorus footprints varied between 0.35kg phosphorus per person and year (DPR Congo, 2007) and 7.64kg phosphorusper person and year.
"Meat consumption was the most important factor affecting phosphorus footprints; it accounted for 72 per cent of the global average phosphorus footprint."
Temporal trends also differed among countries; for example, while China's phosphorus footprint increased almost 400 per cent between 1961 and 2007, the footprints of other countries, such as Canada, decreased.
Meat consumption was the most important factor affecting phosphorus footprints; it accounted for 72 per cent of the global average phosphorus footprint.
The results show that dietary shifts are an important component of the human amplification of the global phosphorus cycle. These dietary trends present an important challenge for sustainable phosphorus management.
Metson and co-authors report their results indicate that dietary choices, especially those related to meat consumption, have a large impact on the demand for phosphorus in food production. Approximately 28 per cent of the total increase in phosphorus demand between 1961 and 2007 was due changes in the global average diet, including increasing meat consumption.
As diets vary around the world, so do phosphorus footprints.
Their analyses show that each country's phosphorus footprint changed uniquely though time, and that these changes were significantly correlated with wealth and development status. Citizens in poorer nations eat fewer calories and less meat and thus require less phosphorus to produce their diets. For example, China's around 400 per cent increase in phosphorus footprint during the study period follows the country's rapid increase in wealth and changing lifestyle. Nevertheless, China's per-capita phosphorus requirements remain much lower than those for most of North America and Europe.
The potential for dietary modifications to enhance global phosphorus sustainability will differ considerably from country to country and region to region, depending on the role of diet in phosphorus demand.
Dietary changes have considerable potential to change the demand for mined phosphorus. In particular, reduced consumption of meat, and especially beef, could result in dramatic declines in phosphorus demand.
Based on predictions of future diet and population, phosphorus requirements to feed humanity are expected to increase between 68 and 141 per cent between 2007 and 2050. However, changes in diet and population will vary widely across the planet. Many developing countries will increase their nutritional and caloric intake, and thus their phosphorus footprint, in an effort to improve food and nutritional security.
At the same time, data from these researchers indicate that developed countries have considerable scope to reduce their intake of phosphorus-intensive food groups, particularly meat, a shift that would have both environmental and human health benefits. Measures for creating a more sustainable social and environment food system will require different strategies in different places, and context should also be considered in evaluating diet and changes in diet.
Diet mitigation is only one strategy in a suite of options available to better manage the relationship between food systems and phosphorus cycling.
For example, increasing Phosphorus Use Efficiency (PUE) can reduce the amount of phosphorus required to produce each crop, and increasing recycling can reduce demand for newly mined phosphorus.
In particular, increased recovery of phosphorus in manure and less phosphorus-intensive methods of animal husbandry could enhance the phosphorus sustainability of the developing world's dietary transition towards a more meat and calorie intensive state.
Another study estimates that if residual soil phosphorus was taken into account (which has accumulated between 1965 and 2007), the use of manure and mineral phosphorus fertiliser to meet crop requirements is between 1.68 and 2.08×107 tonnes of phosphorus, while estimates that do not include residual soil phosphorus are 50 per cent higher. This is more than the amount of phosphorus required for the researchers' "vegetarian and low population growth" scenario without a change in PUE, indicating that both farm phosphorus management and diet mitigation will be important strategies in the future.
However, PUE alone will not ensure long-term sustainability for phosphorus management. Diet modulation can reduce phosphorus demand throughout the food production and processing chain, and thus can be an important addition to phosphorus management shifts at the farm level.
Synergies between phosphorus sustainability and other sustainability priorities may be key to implementing change. Throughout the food and ecological footprint literature, increases in meat and processed food production are associated with increased environmental impairment. In particular, meat production produces more greenhouse gases (GHG) and requires considerably more fossil fuels, water, land and nitrogen than similar levels of plant-based caloric and protein production. A less phosphorus-intensive diet could thus also help address other sustainability challenges, and conversely, motivations to change diets for a variety of site-specific environmental reasons may help address phosphorus sustainability goals.
Metson and co-authors' calculated demand for phosphorus was similar to the amount of mined phosphorus used as fertiliser.
It is important to note, however, that crop and animal feed requirements are also met by manure application or use of weathered or accumulated phosphorus from the soil. If, for example, phosphorus inputs of pasture and grass feed to animal product phosphorus conversion efficiencies are included, the average global phosphorus footprint is approximately 47 per cent higher than when calculated including only mined phosphorus sources.
Manure application and residual phosphorus, when considered as sources of phosphorus for crops, are higher than the phosphorus requirements Metson and co-authors calculated to meet human diet choices globally. Their estimates of per-capita phosphorus footprints are approximately 50 per cent to 170 per cent higher than in the existing literature.
Most of the other studies used human phosphorus excretion data as a base for consumption while Metson and co-authors worked from food availability data, which permitted more detailed temporal and spatial comparisons and the inclusion of the phosphorus required for all crops produced for food, as opposed to relying only on estimated waste through the food system as in previous studies.
This comprehensive approach also permitted a systematic comparison of national food phosphorus requirements as well as analysis of the impact of specific food groups composing the dietary phosphorus footprint, they added.
"Decreasing meat consumption in already high phosphorus-footprint countries could play an important role in sustainable phosphorus management strategies."
Dietary choices have played an important role in the increased demand for mineral phosphorus fertiliser over the past 50 years, concluded Metson and co-authors.
The global phosphorus footprint increased between 1961 and 2007 but the magnitude and direction of these changes varied among countries.
Furthermore, there is a positive correlation between Human Development Index (HDI) and national per-capita food phosphorus footprints, likely because increases in HDI are associated with a more meat-intensive diet.
Because meat consumption is the biggest diet contributor to phosphorus footprints, future meat consumption may play an important role in the demand for phosphorus resources. Decreasing meat consumption in already high phosphorus-footprint countries could play an important role in sustainable phosphorus management strategies, and in synergies with other health and environmental sustainability priorities.
Metson G.S., E.M. Bennett and J.J. Elser. 2012. The role of diet in phosphorus demand. Environmental Research Letters. 7(4) 044043. doi:10.1088/1748-9326/7/4/044043
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