Diet and greenhouse gas emissions
Food production accounts for 20–30% of the UK’s greenhouse gas emissions (GHGE) and therefore plays an important role in climate change1. Whereas agriculture contributes directly to GHGE, particularly methane and nitrous oxide production, indirect contributors include transportation, energy supply, and waste management2. Greenhouse gases are emitted at every stage of the food chain, from agricultural production (farming methods and land use) through to processing and manufacturing (packaging and transportation), consumer activities (storage and cooking), and food waste disposal. Meat and, to a lesser extent, milk and dairy are the largest contributors to diet-related GHGE3: ruminants belch methane (more than 80x as powerful a greenhouse gas as is carbon dioxide in its first 20 years; and pasteurisation of milk, and refrigeration of dairy products and meat, are especially energy intensive). Further, rainforest destruction for growing animal feed, or for grazing, removes a major carbon sink. Accordingly, most of the focus on diet and the climate change effort has centred around reducing dietary intake of meat (primarily red meat and processed meat) and dairy. Such a change in dietary pattern will have a significant nutritional impact on patterns of dietary protein intake given that more than 60% of protein consumed in the UK is derived from animal proteins, of which meat and dairy make significant contributions4.
Protein and ageing muscles
The biological ageing process is associated with a gradual loss of skeletal muscle mass and function, collectively termed sarcopenia. Muscle loss typically begins during the 4th/5th decade of life at a rate of 1% per year until 70 years, increasing to 1.5% per year from 80 years onwards. Once a critical level of muscle loss is reached, detrimental health consequences include reduced mobility, a loss of independence, and increased risk of falls.
Skeletal muscle is comprised of muscle proteins that are continuously remodelled. A key metabolic process of muscle remodelling is muscle protein synthesis (MPS) whereby amino acids are incorporated into new functional muscle proteins. The MPS response to food and/or exercise is impaired in older adults; a concept coined “anabolic resistance”. To overcome anabolic resistance, dietary choices should consider the most anabolic protein sources. A combination of factors determines the anabolic potential of a protein source. First, the digestibility of the protein source determines the bioavailability of amino acids as substrate for the synthesis of new muscle protein. Second, the protein source must constitute a complete profile of all nine essential amino acids (EAA), of which a high leucine content is necessary for maximal stimulation of MPS.
Characteristic differences in EAA profile and leucine content exist between plant- and animal-based protein sources5. The total EAA content of animal proteins typically exceeds plant proteins. Plus, animal-based protein sources typically boast a complete EAA profile, whereas most plant proteins are deficient in one or more EAA, usually lysine ormethionine. The leucine content of animal proteins is typically greater than that of plant proteins.
Several studies have compared the capacity for plant and animal proteins to stimulate MPS, A seminal study by Wilkinson and colleagues recruited trained young men who consumed either fluid skimmed milk or a soy protein beverage immediately following exercise6. Both drinks contained 18 grams of protein. Milk protein was shown to stimulate a 34% greater MPS response than soy protein. This outcome was attributed to the greater digestibility, superior EAA profile and higher leucine content of milk.
Two other studies in older adults demonstrate that ingesting a 4 oz (113 g) steak stimulated a greater MPS response compared with a soy-based beef replacement, whereas casein protein (the main milk protein) stimulated a greater MPS response than wheat protein7. Based on these scientific studies, a general consensus has been reached that animal proteins, in particular dairy proteins, are more potentin stimulating MPS compared with plant proteins. However, this statement is limited to the comparison of dairy proteins with soy and wheat only.
Why study alternative plant-based protein sources?
Plant-based protein-rich foods may be considered more sustainable for the environment than animal based protein-rich foods. Based on estimates of GHGE associated with the UK’s supply and production of different food groups, plant-based food sources fall into low and moderate carbon footprint categories, whereas animal proteins fall into the high category2.
So, where next for future study into the anabolic properties of plant-based proteins?Combinations of potato and rice protein, corn and pea protein, and soy and hemp protein appear to be complementary in providing a complete EAA profile for supporting MPS. In practical terms, a bean and quinoa bowl provides an example meal to “cover” amino acid requirements from a single vegetarian dish. At the risk of opening up a can of worms, a similar gap in knowledge exists for insect-based sources of protein. Based on amino acid profile, beetles, mealworms, ants and termites are promising sources.
How do we tailor protein recommendations for muscle health and the environment?
Moving forward, a multidisciplinary approach is required where we combine our knowledge of nutrition and physiology, with a mathematical modelling approach. A study by Macdiarmid and colleagues modelled the dietary changes required to achieve nutritional requirements for health while meeting target reductions in GHGE derived from dietary means8. Findings revealed that a balanced, rather than extreme approach, is possible, i.e. a sustainable and healthy diet can be achieved without completely eliminating meat and dairy from the UK diet.
Diet clearly makes significant contributions to global warming and climate change. Regarding protein nutrition, the anabolic potential of a protein source is dictated by three key factors, namely the digestibility of the protein, the EAA profile and the leucine content. On the basis of current scientific knowledge, animal-based protein sources are more anabolic than plant based protein sources, at least on a dose-matched basis. Future protein recommendations should take a holistic approach by considering muscle health, appetite and other nutrients of concern. But climate change threatens our very survival and that of our ecosystems and consideration of this fact must drive food policy above all else. These recommendations will likely include a marked increase in plant-based foods, but without elimination of meat or dairy.
This piece was originally published in The Physiology Society's Physiology and Climate Change booklet.
- Macdiarmid J. 2013. Is a healthy diet an environmentally sustainable diet?Proceedings of the Nutrition Society. Feb.72 (1):13-20. doi: 10.1017/S00296651120028933.
- Lonnie M., et al. 2018. Protein for life: Review of optimal protein intake, sustainable dietary sources and the effect on appetite in ageing adults. Nutrients. Mar. 10(3): doi: 10.3390/nu100303605.
- van Vliet S., et al. 2015. The skeletal muscle anabolic response to plant- versus animal-based protein consumption. J Nutr. Sept. 145(9):1981-91. doi: 10.3945/jn.114.204305.6.
- Wilkinson S., et al. 2007. Consumption of fluid skim milk promotes greater muscle protein accretion after resistance exercise than does consumption of an isonitrogenous and isoenergetic soy-protein beverage. Am J Clin Nutr. Apr.85(4):1031-40. doi: 10.1093/ajcn/85.4.10317.
- Gorissen S., et al. 2016. Ingestion of Wheat Protein Increases In Vivo MuscleProtein Synthesis Rates in Healthy Older Men in a Randomized Trial. J Nutr. Sept.146(9):1651-9. http://doi.org/10.3945/jn.116.2313408.
- Macdiarmid J., et al. 2012. Sustainable diets for the future: can we contribute to reducing greenhouse gas emissions by eating a healthy diet? The American Journal of Clinical Nutrition. Sept, 2012. (96)3. 632–639. https://doi.org/10.3945/ajcn.112.038729