In this article I’m going to address whether plant protein is just as good or better for your health than animal protein and share with you the evidence from scientific studies. Find out about potential problems with plant proteins. I’ve not considered which protein source is better for the environment, just the implications for human health.
Protein – animal or plant, does it matter which?
Are all protein sources created equally, or is there a difference? While there are many environmental and ethical advantages to a vegan diet, this article is just about how protein sources impact muscle mass. Proteins are all made from little building blocks called amino acids. If all proteins are made from these amino acids, how do they have a different effect on our bodies? Read on to find out …
Recent evidence has suggested that plant-based proteins in soy and wheat lead to a lower muscle protein synthetic response than animal based proteins1.
Skeletal Muscle Mass
Skeletal muscle is regulated by both the production and break down of muscle. Muscle mass is either increased, maintained or decreased depending on the balance between protein breakdown and production. The balance of muscle mass is thought to be influenced most by muscle protein synthesis (MPS) i.e. the production of new muscle than degradation of existing muscle mass1. MPS is increased by food intake especially dietary protein, and physical activity. While most forms of exercise lead to increased MPS, resistance training has the greatest impact. However, very heavy training may have the reverse effect, by increasing protein breakdown, necessitating a compensatory further increase in protein intake to maintain muscle mass. Loss of muscle mass may occur from a lack of protein because of protein breakdown occurring at a faster rate than muscle protein synthesis.
Proteins and Muscle Synthesis
When you eat proteins, they are digested and broken down into amino acid building blocks. These amino acids can either be used to synthesize new proteins, but may also act as signalling molecules, inducing MPS. This response only appears to last for a few hours after eating, so is one of the reasons why it is important to regularly eat protein both after exercise and also while resting1. Eating protein after exercise has been found to increase the synthetic properties, leading to greater muscle mass.
There is growing consumer interest in plant based food sources, partly because they are considered healthier, but also better for the planet, with lower requirements for water, land and energy than production of animal based foods.
However, there is evidence that plant-based protein sources may exhibit lower digestibility than animal-based proteins1,2. Studies have primarily used milk, egg, beef to assess the MPS, but more recently the effects of soy have been studied. Using matched quantities of protein, soy did not increase the MPS as much as animal sources1,2.
Why Are Animal Proteins Different?
There are a number of differences between plant and animal based proteins, that account for their differences in MPS.
The digestibility of proteins can be determined by calculating the proportion of amino acids that are effectively digested and absorbed, becoming available in a form suitable for body protein synthesis.
Animal-based protein sources, including dairy, eggs, and meat, are highly digestible (>90)1. Plant based protein sources have a much lower level of digestibility ranging from 45-80%1. This is partly because of the processing methods and anti-nutritional factors, which are compounds that interfere with digestion and absorption of the protein. However, purified plant proteins where the antinutrients are removed, such as soy protein isolate, and pea protein concentrate, have a digestibility similar to animal based protein sources (>90%).
Once digested, plant derived amino acids are also more easily converted to urea, which is then excreted in urine, in contrast to milk derived amino acids, that are more available for MPS. It’s not fully understood why this happens, but may be because plant based proteins contain less essential amino acids, that can’t be synthesised in the body and must be eaten.
Essential Amino Acids
All proteins are made up of smaller building blocks called amino acids. There are 22 different amino acids. Of these 13 can be synthesised within our bodies, while 9 have to be eaten. These are called essential amino acids, as we need them to survive. The non-essential amino acids can be synthesised by the body in the liver from nitrogen, fats and carbohydrates.
Meat and fish contain all the essential amino acids and are therefore called ‘complete’ protein sources. Most vegetables are not complete protein sources, except for chia seeds, buckwheat, soya, hemp and quinoa. This means that vegans must combine protein sources in order to get adequate amounts of all the essential amino acids during each day.
Lysine and methionine are two essential amino acids that are found in lower concentrations in plant proteins than animal alternatives. The World Health Organisation, and Food and Agriculture Organisation of the United Nations3 recommend eating 30mg of lysine for every kilogram of body weight per day, and 10mg per kilogram of methionine.
For lysine this equates to approximately 4.5% of total protein intake per day, and methionine is 1.6%. These percentage requirements are based on a recommended adult protein intake of 0.66g per kilogram of body weight.
Across a range of plant proteins essential amino acids comprised between 30-41% of the total protein, with lysine between 2.8-7.3%, and methionine 0.9%-2.6%. Whereas animal protein essential amino acid content comprised 40-52%, with lysine between 7.1-10.6% and methionine 2.2-3.0%1. Most plant-based proteins that have sufficient levels of lysine, have particularly low methionine levels. Conversely, most of those plant-based proteins that have sufficient levels of methionine, have low levels of lysine.
Quinoa is a high quality protein source with relatively high levels of lysine (6.5%) and methionine (2.6%), but it has yet to be investigated whether this would be associated with a higher amino acid availability, or MPS.
Leucine, another essential amino acid, has also been found to be an important predicator of the capacity to stimulate MPS1. Whey protein has one of the highest concentrations of leucine, and combined with it’s speed of digestion, accounts for why it commonly used post exercise to stimulate MPS. Animal based protein sources generally contain more leucine (8.5-9% and >10% for dairy proteins), than plant-based proteins (6-8%).
Long-term Muscle Mass
A snap-shot of MPS only reflects skeletal muscle reconditioning – i.e. muscle repair and remodelling, it does not estimate longer-term maintenance and growth (hypertrophy). So while in the short term, plant based proteins appear to have lower muscle synthetic properties, is this also seen longer-term? Are people who follow a vegan diet less muscly than those who eat meat?
Looking at studies over a 12 week period, drinking milk, was found to lead to greater gains in muscle than a comparable (isonitrogenous) amount of soy protein1,2. Eating a compensatory higher amount of plant-based protein in some cases seem to improve MPS, but in some groups this may be challenging. Sarcopenia, is an age related decline in muscle mass and skeletal function4. Elderly people have lower MPS5 and therefore need higher protein requirements in order to preserve muscle mass4. Males are more likely to have a higher baseline body mass, and since protein needs are calculated on the basis of body mass, elderly males would therefore need the highest quantities of protein in their diet. Therefore, increasing this further may not be a feasible solution. In addition, some studies have found that increasing the protein led to a greater percentage being transformed to urea, and excreted in the urine, instead of greater muscle synthesis1. Also kidney function naturally declines with age, and a high protein diet is contraindicated in those people with impaired renal function6.
This led to the hypothesis that instead of just increasing protein in general, would supplementary leucine, lysine and methionine increase muscle synthesis instead? Increasing ingestion of leucine was found to be of synthetic benefit in the short-term, but studies haven’t found that this was replicated over 3-6 months in elderly men1. However, fortification of wheat with free lysine was associated with greater gains in height and weight1.
Selective Plant Breeding
If essential amino acid is so important, is it possible to selectively breed plants to increase their digestibility and also essential amino acid composition, thereby increasing their muscle synthesis properties? Maize has been selectively bred using a strain with a single gene mutation that results in higher levels of lysine (nearly double those seen in generic maize)1. The new maize is called quality maize protein (QMP), and has been shown to improve growth in children where maize comprises a large majority of daily protein intake1.
Protein Combination in Vegan Diets
Combining plant proteins that are lower in lysine yet higher in methionine (e.g., wheat, rice, hemp, and maize) with plant proteins that are higher in lysine yet lower in methionine (including black bean, oat, soy, lentil, potato, and pea) may help the muscle synthesis of plant-based protein intake1.
Although the optimal dietary amino acid composition to support muscle health in cancer is yet to be established, a diet where animal-based proteins form the majority (>65%) of proteins appears to be beneficial during active cancer treatment7. During active cancer treatment, it is a time of increased energy needs, with a pro-inflammatory state and catabolic state (where molecules are broken down)7. While a diet rich in plant-based sources of food is associated with cancer prevention, it may be inadequate to support these increased needs. Treatments for cancer are also frequently associated with vomiting and reduced appetite, so nutrient dense foods are preferred. Cancer can also be associated with cachexia, where general wasting of muscle and fat is seen, usually more frequent in advanced cancer. Therefore, a diet that promotes muscle synthesis (anabolism) is best obtained by a combination of both protein from animal and plant sources7. Since a vegan diet would require considerable planning to ensure that protein needs were met7.
This article has highlighted the potential problem with plant based proteins, that a plant based diet isn’t the best for muscle mass, but by increasing protein consumption, especially those with higher levels of leucine, methionine and lysine, will help increase MPS. While there are benefits of a plants with regards reduced risk of cancer, after a cancer diagnosis, plant proteins alone are unlikely without considerable planning to meet the metabolic needs during treatment, and so a combination of animal and plant based proteins are recommended7.
2. Wilkinson, S. B. et al. 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 Nutrition 85, 1031–1040 (2007).