Comment: In their efforts to protect animal welfare do vegans run the risk of damaging the planet? Dr Jacqueline Rowarth and Canterbury-based nutrition scientist Dr Graeme Coles investigate.
Vegans face a moral dilemma - animal welfare or climate change. Dr Graeme Coles, a Canterbury-based nutrition scientist, has done the calculations.
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He estimates that someone getting their minimum essential amino acid requirements from plant-based foods excrete more climate-damaging material than the same person getting them from animal products.
"Avoiding animal protein in the diet means excreting 32g/day more nitrous oxides than people on omnivorous diets" he says.
"The extra nitrous oxides from a vegan's diet are equivalent to the emissions associated with two return journeys annually from New Zealand to London".
The problem for vegans is that plant-derived protein is of markedly lower nutritional value than animal-derived protein.
This is because of its low content of essential amino acids. A combination of amino acids is needed in order to maintain muscle and the immune system; if there is a limiting amino acid then other amino acids (and the nitrogen they contain) will be wasted.
Further problems are associated with the measures that plants have evolved to prevent predation. These are termed "antinutritional factors" – the nutrients appear to be present but can't be accessed by the digestive system.
In order to overcome this, humans apply treatments such as soaking, heating, acidification, or pulverisation. Treatment takes time and energy, which increases the greenhouse gases associated with the food.
Soybeans are the only staple crop also considered a protein crop. Soy ranks sixth for provision of calories globally, and soybeans are approximately 36 per cent protein when dry. After soaking overnight and cooking for a couple of hours, the protein is 17 per cent.
But soy's high concentrations of antinutritional components mean that only a portion of the soybean protein is digested; the estimate is approximately 73 per cent, in comparison with 80-100 per cent from animal proteins.
Furthermore, the antinutritional factors delay the digestion of soy and other proteins in the diet. Other legumes have considerably lower protein quality and are generally less than 60 per cent digestible.
In contrast, most of the protein in meat, egg and dairy is digested in the first part of the small intestine, which enhances rate of absorption. Processing animal products is easier too: beef is 26-34 per cent protein, and cooking takes five minutes (depending on the taste of the consumer).
There is another problem with plants, even before food reaches the human mouth: wastage. Food that is suitable for humans to eat is also suitable for other consumers – termed pests.
Whether the pests have legs (mammals, reptiles, birds, insects), no legs (nematodes, slugs snails), or are fungi, bacteria or viruses, they destroy plants and stored plant products.
Glasshouses or vertical farms mitigate against predation but are not practicable for staple crops such as maize, rice and wheat (the top three global staple crops). Nor are they suitable for potatoes, cassava or soybeans (the next three).
In fact, glasshouses and vertical farming are suitable only for relatively high-value crops, which provide vitamins and variety but not carbohydrates or proteins.
Technological developments might change this in the future, as might the price that people are prepared to pay, but in the meantime the bulk of crops are grown outdoors and protected by the farmer with nets and sprays.
This protection is only partially successful.
Listen: Dr Graeme Coles talks to The Country's Jamie Mackay about the vegan diet - and discloses what he thinks is the most efficient form of food:
The Institution of Mechanical Engineers (IMechE) reported in 2013 that as the development level of a country increases, food losses generally move further up the supply chain.
In South-East Asian countries for example, rice losses range from 37 per cent to 80 per cent of total production, depending on development stage (China loses approximately 45 per cent of the crop and Vietnam loses over 70 per cent).
In the developed world wastage occurs at and after harvesting. In the UK, up to 30 per cent of vegetable crops aren't harvested at all because they do not meet exacting market standards for appearance.
Harvesting itself leaves much material in the field – over 80 per cent of broccoli, 6 per cent of wheat, 50 per cent of pineapple. More is lost in storage.
Of the produce that does reach the supermarket, promotions encourage customers to purchase excess, which, in the case of perishable foodstuffs, inevitably generates wastage in the home. ImechE estimates that between 30 per cent and 50 per cent of what has been bought in developed countries is thrown away by the purchaser.
Therefore, an unintended consequence of moving to a plant-based diet is increased waste including all the resources that went into the wasted food.
This includes the nutrients, water and fossil fuels, and the land area that has produced waste, not food.
Dr Coles notes that food waste and the components of the plant that we can't digest can be converted into useful foods with the help of animals, particularly ruminants.
Furthermore, ruminants can make good use of arable land when food crops can't be grown such as during the winter, or during fallow periods aimed at restoring soil fertility and structure.
"This integrated approach can easily halve food wastage and intensify food production in a sustainable way," says Dr Coles.
"Using animals to improve the efficiency of a plant-led food economy by improving nutritional effectiveness and reducing waste is a sustainable ethical compromise".
- Dr Jacqueline Rowarth has been vegetarian for over 40 years. Dr Graeme Coles enjoys all forms of food in moderation.