In the Anthropocene, food systems are the main reason for changes in the Earth's ecosystems, including climatic changes. They involve a major loss of biodiversity, damage to entire ecosystems, and alteration of the global carbon, phosphorus and nitrogen cycles. At the same time that food wastage is soaring, 820 million people are dying of hunger (United Nations 2021), and 2 billion suffer from diseases related to per diem expenses (WHO 2021; FAO 2019). Current food systems do not appear to be healthy or sustainable.

Livestock production is a major cause of climate change, soil loss, water and nutrient pollution, and the decline of wild predators and herbivores. But plant-only food production is not without its associated environmental impacts: they appear to be more environmentally sustainable - they use fewer resources - and have a smaller ecological footprint, but require large areas, treatments and share the need for long-distance transport.

Impacts of agriculture and livestock
Agriculture and livestock use 30 and 40 % of the land area and are responsible for 30 % of greenhouse gas emissions, and 70 % of freshwater use. Fisheries have fully exploited 60 % of the wild population and overfished another 30 %(FAO 2018).

The impact of ruminant farming is greater than that of other animals: higher methane emissions and higher feed production for them. According to some estimates(Treu et al. 2017) greenhouse gas (GHG) emissions for organic chicken and pig production may be higher than conventional ones (slower growth and lower feed efficiency). Organic systems require more land than conventional ones, albeit with - and due to - lower total nitrogen inputs per hectare(Our World in Data 2017).

The main crop manager of forest loss in the Amazon is soybeans(Mongabay 2017), used to produce animal feed and as source vegetable protein in processed vegetarian diets. In Brazil, soybean production today is four times higher than it was two decades ago. Oil palm crops are causing entire ecosystems to disappear(Marinova and Raphaeli 2018).

Vegetarian-vegan diets generate lower environmental impacts (GHG emissions and land occupation), although they also produce direct impacts on wildlife(Fischer and Lamey 2018). Organic systems provide environmental services, do not use pesticides, increase the resilience of agriculture, and can mitigate future effects of Climate Change on yields. Environmental impacts of diets should not only be assessed in terms of dietary patterns, they should integrate production systems looking towards conservation(Chai et al. 2019).

Ethical motivations
The field of conservation is based on ethics, including an appreciation of nature, an understanding of the need to protect it, and a belief that the Earth should be shared between humans and nature(Leopold 1949).

Utilitarians propose that it is wrong to consume products from factory farms because pain and suffering are caused, appealing that animals have the right to a respectful attention , not to be considered a resource. But it is not true that wild animals live in natural paradise and that only human action causes them suffering (Rousseaunian view) (Fischer 2019; Animal Ethics 2019). Food and water scarcity, predation, disease, and intraspecific aggression are factors in the natural environment that cause suffering to wild animals on a regular basis(Ben Ami 2017).

Many plants (roots, stem, foliage and green fruits) are protected from being eaten prematurely (they are bitter, acidic, even poisonous). In such cases only ripe fruits should be eaten, and that is why they are sweet or have umami taste, flavors that humans desire at birth. Sometimes there are psychological, physiological, social and cultural barriers to eating vegetables and fruits in quantity(Wrangham 2009).

Health effects
The human species has evolved as an omnivore, as evidenced by its morphological and physiological characteristics, including its teeth. Vegetarian and vegan diets are low in n-3 PUFA, protein, calcium, zinc, iron, iodine, selenium, vitamins B12 and D, taurine or creatine (essential nutrients), and can be dangerous for children, adolescents, the elderly and sick people if not supplemented(Garcia-Maldonado et al. 2019; Petti et al. 2017).

The report EAT-Lancet proposes a sustainable, nutritious and healthy per diem expenses composed of vegetables, fruits, whole grains, legumes, nuts and unsaturated fats, a small amount of fish and poultry, and little or no red meat, processed meat, added sugars, refined grains and starchy vegetables. With it should be possible to meet the United Nations Sustainable development Goals.

Keys to sustainable food
Sustainable per diem expenses is composed of "food brought to market with production processes that have little environmental impact (preferably local), is respectful of biodiversity and ecosystems, and is nutritionally adequate, safe, healthy, culturally acceptable and economically affordable"(FAO 2010).

The omnivorous per diem expenses has worse carbon, water and ecological footprints, but some vegetarians and vegans may even generate a higher environmental impact(Rosi et al. 2017).

A shift towards the adoption of "more" vegetarian diets would improve both human well-being and the health of the planet. It would result in lower GHG emissions, less demand for freshwater and land, and less biodiversity loss. Acceptable land use outcomes would be achieved when the per diem expenses includes 35% less meat (24% decrease in land use)(Lacour et al 2018).

We must consider each case, without proposing a radical ban on the consumption of animal products. Let us reflect: can we produce meat without suffering? do plant-based diets cause harm? can a per diem expenses that includes meat reduce more deaths (of wild animals) than a per diem expenses without it?

Impacts can be reduced if the following conditions are met:

The demand for animal products is reduced and the proportion of plant-based foods in diets increases (ideally to a global average of 90% of food consumed).

2. Ecologically inefficient ruminants (cattle, goats and sheep) and bushmeat are partially replaced by monogastrics (poultry and pigs), integrated aquaculture and other more efficient protein sources.

3. Livestock production is directed away from fossil fuel-based systems and towards systems linked to the structure and functions of ecosystems that conserve energy and nutrients.