Protein and vegetarian diets

Kate A Marsh, Elizabeth A Munn and Surinder K Baines
Med J Aust 2013; 199 (4): S7-S10. || doi: 10.5694/mja11.11492
Published online: 29 October 2013

This is a republished version of an article previously published in MJA Open

Amino acids — the building blocks of protein

Amino acids are classified as being either essential (or indispensable), meaning the body cannot adequately synthesise them and must obtain them from the diet, or non-essential (or dispensable), indicating that the body can make them (Box 1). However, a number of the latter are conditionally indispensable under certain physiological conditions and in certain disease states. In particular, biosynthesis and/or intake of amino acids such as glutamine, arginine and cysteine may be inadequate in meeting physiological requirements during times of stress such as illness, surgery or injury.2

Infants and growing children have relatively higher requirements compared to adults for indispensable amino acids and some conditionally indispensable amino acids.1,3

Recommended dietary intakes

Nutrient reference values (NRVs) for Australia and New Zealand include a recommendation for an acceptable macronutrient distribution range (AMDR) for protein of 15%–25% of energy intake.10 The AMDR is an estimate of the range of intake for each macronutrient for individuals (expressed as a percentage of total energy intake) that would allow for an adequate intake of all the other nutrients. The NRV document notes that while, on average, only 10% of energy need be consumed as protein to meet the physiological need for protein, this level is insufficient to allow for estimated average requirements (EARs) for micronutrients when consuming foods commonly eaten in Australia and New Zealand.10 In other words, while consuming lower amounts of protein-rich foods could meet the body’s protein needs, it would not provide sufficient amounts of other nutrients found in these foods including iron, zinc, calcium and vitamin B12. Recommended dietary intakes (RDIs) for protein for different sex and age groups are shown in Box 2.

The 1995 National Nutrition Survey (NNS) for Australians found the mean daily protein intake for those aged 19 years and over was 91 g or 17% of energy.11 Mean intakes for those aged 19 years and over were 109 g for men and 74 g for women — amounts well above the RDI. Intakes were at least 60% greater than the RDI for most groups, except those aged 65 years and over, whose mean intakes were 84 g for men and 64 g for women; although relatively lower, these amounts were still adequate in terms of RDI. Children and adolescents were eating close to or more than double their RDIs and, while pregnant women were not surveyed separately, the average intake for women would be adequate to meet the RDI during pregnancy or lactation.

In general, studies of Australian vegetarians have found that their protein intakes are significantly lower than those of omnivores. A study of Australian men aged 20–50 years found that those on a lacto-ovo-vegetarian (LOV) diet consumed 80 g of protein per day (16% of energy) and vegans consumed 81 g of protein per day (12% of energy) compared with 108 g (17% of energy) for omnivores.12 Among women aged 18–45 years, those following a vegetarian diet (LOV and vegan) had a mean protein intake of 54 g per day (14% of energy) compared with 67 g per day (18% of energy) for omnivores.13 While the reported protein intakes of vegetarians are significantly lower, it is clear from these studies that most vegetarians and vegans still meet the RDI for protein, and intakes are within the AMDR.

Do protein requirements differ for vegetarians?

Protein requirements for healthy adults have not been found to differ according to whether dietary protein is predominantly from animal, vegetable or mixed protein sources provided soy protein or a variety of other vegetable proteins is consumed.14 However, studies comparing single sources of protein have found significant differences between plant and animal sources, particularly with cereal proteins such as wheat and rice,4,15-17 as their low lysine content may be a limiting factor. Consequently, if protein intake was to be restricted to a single plant source, such wheat, rice or legumes (other than soy), then the amount of protein required to meet essential amino acid needs may be increased.7

The benefits of plant protein

While the lower protein intake and quality of protein in a vegetarian diet is often believed to be a concern, there is increasing evidence that consuming protein from plant rather than animal sources may, in fact, be one of the reasons why vegetarians generally have a lower risk of overweight, obesity and chronic disease. In comparison to protein foods of animal origin, most plant protein sources are lower in saturated fat, free of cholesterol and haem iron, higher in fibre, and are good sources of antioxidants and phytochemicals, all of which may contribute to a reduced disease risk.

A number of studies have shown that a higher intake of protein, particularly animal protein, in infants and early childhood may increase the risk of overweight and obesity in later life.20-22 In adults, the European Prospective Investigation in Cancer and Nutrition (EPIC)-Oxford study compared weight gain over 5 years among almost 22 000 meat-eating, fish-eating, vegetarian, and vegan men and women; it found that weight gain was lowest in the vegan group and in those who, during follow-up, had changed to a diet containing fewer animal foods.23 The study also found that meat-eaters had the highest body mass index (BMI) and vegans the lowest BMI, while fish-eaters and vegetarians had similar, intermediate mean BMIs.24 Differences in macronutrient intakes accounted for about half the difference in mean BMI between vegans and meat-eaters, with high protein and low fibre intakes most strongly associated with increasing BMI.24 More recently, a study of several cohorts from the EPIC study participating in the diet, genes and obesity (Diogenes) project reported that in contrast to plant protein intake, total protein and protein from animal sources was positively associated with subsequent weight gain although there was no overall association between dietary protein and change in waist circumference.25

While the safety of high-protein, low-carbohydrate diets is debated, the type of protein in such diets may be important. A report of two cohort studies found that a low-carbohydrate diet based on animal sources was associated with higher all-cause mortality, while a vegetable-based low-carbohydrate diet was associated with lower all-cause and cardiovascular disease mortality rates.26 Other studies have shown benefits of plant protein compared with animal protein for lowering blood pressure27,28 and the risk of type 2 diabetes29,30 and of ischaemic heart disease in healthy men.31 Furthermore, consumption of soy protein may slow the progression of kidney disease compared with consumption of animal protein, particularly red meat.32


Vegetarians who eat a range of plant foods can easily meet their protein requirements, even though the protein content of vegetarian diets is usually lower than that of omnivorous diets. Most Australians eat significantly more protein than is required. The consumption of plant protein rather than animal protein may play a role in weight management and reducing chronic disease risk.

Provenance: Commissioned by supplement editors; externally peer reviewed.

  • Kate A Marsh1
  • Elizabeth A Munn2
  • Surinder K Baines3

  • 1 Northside Nutrition and Dietetics, Sydney, NSW.
  • 2 Sanitarium Health and Wellbeing, Berkeley Vale, NSW.
  • 3 School of Health Sciences, University of Newcastle, Newcastle, NSW.


Competing interests:

Kate Marsh previously consulted for Nuts for Life (Horticulture Australia). Elizabeth Munn consults for Nuts for Life and Sanitarium Health and Wellbeing, and has previously consulted for Go Grains Health and Nutrition, Avocados Australia, Healthy Food Guide magazine, and the Almond Board of Australia. Go Grains Health and Nutrition and Nuts for Life are providing a contribution towards the cost of publishing this supplement.

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