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Sustainable healthy diets are dietary patterns that promote all dimensions of individuals’ health and wellbeing; have low environmental pressure and impact; are accessible, affordable, safe and equitable; and are culturally acceptable.1
A key part of a more sustainable diet is to consume more plant sources of protein in place of animal proteins. This will help reduce both the environmental and some of the health burdens associated with animal protein sources.
Principles of sustainable eating can also be applied in the context of clinical nutrition. In the clinical management of malnutrition, the food first principles focus on maximising energy and protein intake. High energy and protein containing foods primarily include dairy foods. However, due to changes in personal preferences for vegan or plant-based diets along with environmental concerns. There is a greater need for dietitians to be able to support patients with making sustainable dietary changes, including those patients who are malnourished.
This article discusses the basis of a plant-based diet, the different plant proteins and how plant-based diets can be applied to clinical nutrition.
The British Dietetic Association defines a plant-based diet as “a diet which is based on foods derived from plants, including vegetables, wholegrains, legumes, nuts, seeds and fruits, with few or no animal products.2
UK consumption of plant-based drinks such as those based on soya, oat, almond and rice is increasing.3 Changes in consumption trends to plant-based drinks have been attributed to lactose intolerance, cow’s milk protein allergy, taste preference and wishing to follow a sustainable diet.4,5
Increasing the proportion of proteins consumed from plant-sources is an important means of improving sustainability. Many dietary recommendations focus on this for both health and environmental reasons, including EAT-Lancet, EWG, World Cancer Research Fund and Heart UK.6
Plant-based milk substitutes, or plant extracts, are water-soluble extracts of legumes, oilseeds, cereals or pseudocereals that resemble cow’s milk in appearance.7 However, these substitutes exhibit different sensory characteristics, stability and nutritional composition from cow's milk. Plant protein sources include cereals (wheat, rice, millet, maize, barley, and sorghum), legumes (pea, soybean, bean, faba bean, lupin, chickpea, and cowpea), pseudocereals (buckwheat, quinoa, and amaranth), nuts, almonds, and seeds (flaxseed, chia, pumpkin, sesame, and sunflower).8
The inclusion of plant proteins from whole sources such as legumes, nuts, pulses and grains has been shown to result in an improved fat profile, lower energy density and significantly increased fibre content.9 Processed plant-based protein sources are growing in popularity but can be high in fat and salt, so consumers should consider this when adding them to the diet, as an environmentally sustainable diet is not necessarily a healthy one.10
Protein quality and quantity is not compromised when switching to more plant sources. Plants contain all essential amino acids and diets entirely based on a variety of plant proteins which meet daily energy requirements can also meet all essential amino acid requirements.11,12 However, there are also other factors that affect the nutritional quality of crops, including soil condition, crop maturity, post-harvest handling, storage, use of fertilisers and pesticides, crop variety, and climatic condition.8
Two requirements for a protein to be considered high quality, or complete, for humans are having adequate levels of indispensable amino acids to support human growth and development and being readily digested and absorbed.13
Given that many whole food sources of plant-protein are less calorie-dense than animal sources of protein, greater overall food intake is needed to meet energy requirements which, in turn, helps meet indispensable amino acid requirements. In addition, it has now become much easier for consumers to boost intake of plant proteins via the availability of multiple plant-based protein isolates and concentrates (soy, pea, canola, potato, fava, etc.) in the food industry.12
Dietary protein intake stimulates muscle protein synthesis and the response to protein intake can vary substantially between different dietary protein types or sources. The muscle protein synthetic responses to the ingestion of plant-based proteins such as soy, compared to animal proteins, has been previously shown to be lower. However, a balanced combination of different plant-based proteins may provide a higher quality protein blend, thus improving muscle protein synthesis.14
As plant-based diets are becoming increasingly common, it is likely that the number of patients under the care of a dietitian that may require guidance on how to make dietary change whilst considering the need for plant-based food choices will increase. One key area in dietetic practice where adaptations to advice is required for those following a plant-based diet is nutrition support.
In order to facilitate nutrition support in those individuals who wish to follow a plant-based diet, there are a number of considerations:
There are several plant-based milk alternatives available and there are some nutritional differences. Table 1 outlines the different plant-based milk alternatives compared to cow’s milk.15
| Milk | Energy content (average kcal/100ml) | Protein content (average g/100ml) |
|---|---|---|
| Oat milk | 47 | 0.8 |
| Soya milk | 42 | 3.2 |
| Almond milk | 37 | 0.6 |
| Coconut milk | 25 | 0.1 |
| Rice milk | 52 | 0.2 |
| Cow’s milk (whole) | 63 | 3.4 |
Dairy is a key provider of protein, vitamins B2 and B12, iodine and calcium.6 Therefore, non-dairy milk alternatives are commonly fortified with additional vitamins and minerals such as calcium, vitamins A, D, B-2, and B-12.16
There are a limited number of plant-based oral nutritional supplements. However, the plant-protein source used is soya protein isolate.
The nutritional value and abundance of branched chain amino acids (BCAAs) in dairy proteins means they still have a considerable role to play in the development of nutritional products and foods, especially in foods for special medical purposes (FSMP).17 The Protein Digestibility Corrected Amino Acid Score (PDCAAS) is the standard measure of protein quality.18 Milk proteins have a PDCAAS score of 1.0. With the exception of soy proteins (PDCAAS 0.91–0.95), individual plant proteins have lower PDCAAS scores (<0.9).10 More recently the Digestible Indispensable Amino Acid Score (DIAAS) has been adopted to quantify dietary protein quality. However, there have been limitations of DIASS in relation to plant based proteins.19
Plant proteins are often described as incomplete, due to the insufficient amounts of all nine essential amino acids. Although protein content and amino acid composition vary between plant species, in general, protein found in legumes are limited in methionine and cysteine; cereals (lysine, tryptophan); vegetables, nuts and seeds (methionine, cysteine, lysine, threonine); seaweed (histidine, lysine).20
Claims that foods are a “source” or a “high source” of protein are permitted if protein contributes 12% and 20% of energy from protein, respectively.21
There are a number of plant-based protein sources that can be used in food fortification. These foods include nut butters, soya products (e.g. milk, yoghurts, cheese), vegan cheese, coconut yoghurt, plant-based and sustainable oils and hummus. In addition, plant-based protein powders such as soy or pea protein can be used to prepare nourishing drinks.
As dietary patterns change for both healthy and vulnerable people, it is important that dietitians and industry recognise the need for tailoring advice and products to meet the requirements of a plant-based diet. A key consideration in plant-based diets is ensuring an adequate nutritional intake and ensuring nutrient fortified foods are available. In the management of malnutrition through nutrition support, where protein requirements can often be raised, it is important to be aware of the different plant-based protein sources and the protein quality of these alternative foods.
