Food and Water
Meat consumption, as a share of total diet, has been increasing worldwide and varies considerably by country and region.
Historically, increased wealth has been associated with a "nutrition transition", or a move to diets heavier in meat and dairy . The evidence is that humans have an innate preference for energy-dense foods, particularly those heavy in fats and sugar .
Animal products, especially beef, tend to have much greater land use, greenhouse gas, and water impacts than plant-based products.
The feed conversion efficiency of an animal (or substitute) is the ratio of input feed to edible output product, measured by mass. Feed conversion efficiency is highly correlated with lifecycle land use and greenhouse gases .
Following is an estimate of the number of animals that are slaughtered, or that are required to produce over their lifetimes, to provide 5% of a human's lifetime nutritional needs.
In the United States, the trend has been toward more efficient beef production, though not at a rate that is sufficient to fundamentally change beef's high impacts relative to other foods.
On metrics such as yield per animal , meat production tends to perform better in wealthier countries than in poorer countries.
Following is a comparison of estimated impacts of grass-fed beef, relative to feedlot beef.
Grass-fed cattle may play a role in sequestering carbon in the soil, though the magnitude of this effect is of considerable controversy. Even with the sequestration effect, the lifecycle emissions of grass-fed beef is likely to be only marginally lower than those of feedlot beef .
The world is trending toward aquaculture production of seafood, though primarily as an augmentation, rather than a replacement, to wild catch.
Farmed fish require protein, which in turn comes in part from wild fish as follows.
Following are estimates of greenhouse gas emissions from four methods of seafood production.
The high impact of recirculating aquaculture systems (RAS) is driven primarily by electricity consumption. On other metrics, RAS tends to perform better than other forms of aquaculture due to recycling of nutrients. If electricity is produced from a low-carbon source, RAS may also have lower greenhouse gas emissions.
Integrated multitrophic aquaculture--or the mixing of species at different levels in the food chain --is of interest as a means of conserving feed  and reducing overall environmental impacts .
Price is not necessarily a barrier in shifting to established meat alternatives.
It is unclear if emerging meat alternatives, such as cultured meat, will be better able to gain consumer acceptance than established alternatives. Such acceptance depends on politics and culture, in addition to cost, taste, and nutrition . Furthermore, in wealthier countries, protein and fat intake already exceed healthy levels .
National Geographic goes into greater detail on the outlook for aquaculture.
Modern Farmer explains how animal products generally have greater environmental impacts than plant products and circumstances under which the reverse can be true.
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