If you have read one or many nutrition articles of recent years you would be forgiven for thinking that potatoes are nothing but carbohydrates with no other purpose as a food than converting to sugar, spiking blood glucose and creating havoc to our metabolic systems. Once a proud staple food, it has been attacked by dieters and professors alike, even relegated to the top tier (use sparingly) in an alternative Healthy Eating Pyramid.
You would also be forgiven for thinking of other foods, usually animal foods, as protein.
In food guides, foods are placed into various groups of similar nutrient value and these are usually fruit, vegetables, cereals, dairy and then a fifth group often named ‘protein‘ foods. Here ‘protein’ foods refer to meat, eggs, fish, poultry, legumes and nuts. The UK Eatwell Guide (1), although listing examples, refers to this group as ‘proteins’; the US Dietary Guidelines (2) refers to this group as ‘protein’ and ‘protein foods’; and the US Choose MyPlate as ‘protein’ (3). I note the Australian Dietary Guidelines (4) do not refer to this food group as ‘protein’ but rather lists the foods within the group ie: meat, poultry, fish, egg, tofu, nuts, seeds, legumes/beans. The Australian example aside, such messages of ‘protein’ as a food group from nutrition authorities in the UK and US, has a flow-down effect to health professionals and the public and this theme of ‘protein’ as a food group is common in the lay-press. One could mistakenly assume that those foods (meat, eggs, fish, poultry, legumes, nuts) are solely or predominantly protein; and that cereals, grains, tubers, potatoes, vegetables and dairy foods (which appear in different food groups) are thus low, inferior or poor quality sources of protein. Neither assumption is correct. .
Foods are often thought of as ‘carbohydrates’ or ‘proteins’ or ‘fats’. However, with some rare exceptions (5), foods are not solely one macro-nutrient but rather mixtures of protein, carbohydrate, fat and water. In point of fact, some foods often thought of as ‘protein’ (eggs, cheese, fatty fish, nuts, some meats) have a higher percentage of their energy value derived from fat. On the flip side, foods often regarded as ‘carbohydrate’ (potatoes and cereals) have moderate levels of protein. As they are generally eaten in large quantities, they can substantially contribute to protein intake. In some traditional cultures these are regarded as staple foods providing a high percentage of overall protein intake.
Returning to potatoes and the false assumption that they provide only carbohydrates … potatoes provide high quality protein in sufficient proportion that if eaten in sufficient quantities to fulfill energy needs, potatoes alone would provide enough protein.
Here are some key facts about protein and potatoes …
(references are given in detailed section below).
- Food proteins are long complex polymers comprised of amino acids. There are twenty amino acids we require, nine of which are essential and must be obtained from the food we eat.
- Recommended protein intakes range from 33 to 81 g per day, dependent on age, gender and activity level. Current Australian adult daily intakes are 77.9 g females and 104.6 g males, well above recommended minimum requirements.
- The World Health Organisation (WHO) guideline on safe protein-energy ratios (minimum) from diets for various ages and activities varies from 8.1-12.8%. Adult Australian levels of intake in the 2011 dietary survey were about 18%.
- Protein-energy ratio in selected protein-containing foods range from 7.6% (rice) to 75% (white fish). Potatoes derive 12-16% of their energy value from protein. This is above WHO guidelines for a safe protein-energy ratio.
- Potatoes provide all essential amino acids above requirement patterns. That is, potatoes provide high quality ‘complete’ protein.
- Potatoes consumed whole, undiluted with fat, provide adequate protein for adults and growing children, if consumed up to energy (calorie/kilojoule) requirements.
For those wanting more in-depth information or references on the above – keep reading …
Proteins and amino acids in foods
Proteins are complex structures comprised of long chains of amino acids. The body breaks down food protein into amino acids during digestion. After absorption, the amino acids are ‘reassembled’ into the desired body proteins for a variety of functions such as muscle mass and other body tissues, enzymes and hormones. There are twenty amino acids required as the building blocks of body proteins and eight of these (nine in infants) are considered essential and must be obtained from the diet. Protein quality and availability in food is dependent on many factors including protein digestibility, the presence or absence of certain factors that may interfere with absorption, and the proportion of essential amino acids in the food proteins. Requirement depends on age, activity level and general health. (6)
The World Health Organisation (WHO) provides guidelines on a considered safe level of protein intake (7) – that is; daily amounts required to provide maintenance of body tissue in adults and growth in children. The safe level of intake for adults over 18 years is 0.83g per kilogram a day which translates to ~33 to 66 g per day, depending on weight.
Figure 1: World Health Organisation. Safe level of protein intake in adults.
The Australian National Health and Medical Research Council (NH&MRC) Nutrient Reference Values provide a guide for an Estimated Average Requirement (EAR) and a Recommended Daily Intake (RDI) relevant to the Australian context. (8) The EAR is the estimated daily intake in healthy individuals to meet requirements of half the people in a particular life stage and gender group; whereas the RDI would meet the needs of 97-98% of each group. EARs range from 37 – 46 g/d (women), 52-65 g/d (men) and RDI range from 46-57 g/day women, 64-81/day men as given in Figure 2. Recommendations for infants, children and pregnant women and lactating women can be found at this link. (8)
Figure 2: Australian N&MRC Protein Intake Recommendations for Adults.
In the Australian Health Survey 2011-2012, conducted by the Australian Bureau of Statistics, intakes were reported to be 104.6 in males aged 19-64 and in females intakes were 77.9 (9). This is well above considered minimum requirements and is above intakes reported in the previous 1995 survey (intake was males 101.1 and in females 69.5) (10).
The WHO have provided guidelines on the protein-energy ratio in diets in order to provide a safe level of intake of protein. Protein-energy ratio means the ratio of energy derived from protein as a proportion of total energy (or calorie / kilojoule) intake. At a safe level of protein-energy ratio it is considered that as long as sufficient energy is consumed, the protein content of the diet will be adequate. This estimated safe ratio varies for different ages, gender and level of activity. (7)
Figure 3: WHO Mean and reference protein-energy ratios.
Figure 3 from WHO sources (7), provides mean and reference protein:energy ratios. The latter is considered a safe level of intake. I have highlighted the reference for those with moderate activity. From the data in the table, males aged 18 to over 60 years, a safe protein-energy ratio varies from 0.072 to 0.104 (7.2-10.4% of total energy derived from protein). For females it is 8.3-11.3%. A slightly higher percentage is required for sedentary individuals (8.1 – 12.8%). This is because sedentary people require relatively the same protein but less overall energy. Thus the protein-ratio needs to be higher.
Protein-Energy ratios in traditional global diets
Global traditional diets range above those requirements with an Indian village diet providing 11.1% energy from protein, a UK vegetarian 12.7% and UK omnivore diet 14.2%. (7) This is shown in figure 4. In the Australian Health Survey in 2011, the average protein-ratio of the adult diet averaged 18.3% for adult men and 18.5% women (11), an increase from 17.0% men and 17.2% female in the 1995 survey (12).
Figure 4: Available protein from global diets in terms of protein-energy ratios.
In the 1930s-70s, due to reported cases of malnourished infants in Africa, the Food and Agricultural Organisation described a global deficiency of protein and set recommended levels of protein intake at high levels. The assumptions this was based on were later found to be incorrect. Even though protein deficiency in the presence of adequate calories can occur, this is much rarer than the more common cases of malnutrition due to deficiency of calories ie: lack of food. It was discovered provision of additional calories to malnourished children, corrected the malnutrition. The incorrect assumptions spanning four decades is known as ‘The Great Protein Fiasco’ (13). This led in the 1970s to a massive recalculation of protein requirements and a drop in recommended protein-energy ratios. Nevertheless, the assumption we require high levels of protein persists.
Protein-energy ratio in selected foods
In terms of foods, animal food protein sources such as egg, meat, fish and poultry; as well as soy and legumes are considered ‘protein foods’. However, cereals, and starchy vegetables provide protein above WHO reference values for a safe protein intake. I have calculated average protein, energy and protein-energy ratios of selected foods and tabled them in ascending order of protein-density, including five potato varieties. This is summarised in Figure 5. The detailed list with calculations are provided as Ref (14).
Figure 5: Protein-Energy ratios of selected foods.
Protein-energy ratio in selected foods range from 7.6% (rice) to 75% (white fish). Cereals, potatoes and nuts provide protein-energy ratios of ~ 7-20%; dairy foods ~20-27% (except skim 42%); legumes ~ 35-38%; eggs, fish, poultry and red meat ~26-42%; lean beef ~ 60-70% and white fish 75%. Note that lean meats and low-fat dairy have higher protein ratios than fatty varieties as the fat content dilutes down the protein density (See Ref 14).
The protein-energy ratio in the five potato varieties varies from 7.1 – 15.9% and averages 13.5% (Figure 5). This is above the UK vegetarian intake (12.7%) and a fraction below UK omnivore (14.2%) intake. It is above what is considered a safe level of intake (7.2 – 12.8%) in WHO guidelines, even for sedentary individuals. In other words, if you ate only potatoes (and I am not suggesting that you should), as long as you ate enough potatoes to supply energy needs, potatoes alone would provide enough protein. For example a daily intake of 8000 kilojoules in potatoes (about 3100 g or 21 potatoes) would provide approximately 65g of protein which is above daily EAR for Australian adults and above RDI for adults aged 19-70. (Elderly adults are recommended slightly higher levels). Such a high level of intake of potatoes is possibly unrealistic, although historically potatoes provided a higher proportion of daily food intake than today, for example in Ireland. In some traditional diets potatoes provide a substantial proportion of protein and energy (15). In many countries cereals provide a high proportion of protein in the diet. In the PURE study on 18 countries; cereals dominated as main protein source in lower income countries; and middle-income nations had a mixture of cereals and animal-based foods as major protein sources. It was only in the high income nations that animal foods became the top five sources of protein. (16)
Protein quality of a food is based on protein content, protein-energy ratio (as described above), and essential amino acid content of the protein. In order to synthesise body protein, we require the essential amino acids to be delivered in a certain pattern. If a food delivers essential amino acids at or above that pattern, it is considered to be a complete protein. Figure 6 shows the distribution pattern of essential amino acids from selected foods (7).
Figure 6: Distribution of Amino Acids in Food Proteins
Animal protein foods are complete proteins as they provide all essential amino acids above requirement patterns. Soy beans and other legumes are considered the best protein foods for plant-based diets as they are high in protein and provide all essential amino acids above requirement patterns. Although lower in overall protein content, potato protein is high quality providing each of the essential amino acids above 100% of requirement patterns. This is in contrast to cereals, cassava and yam which are limiting in one or two essential amino acids. Historically it was considered plants low in one essential amino acid had to be combined with other plant foods to complete the protein (eg eating rice and beans). This concept is now considered out-dated. It is not considered necessary to eat foods with complementary amino acids at the same meal as long as adequate protein and calories/ kilojoules are eaten over the course of a day. (17)
Summary: Protein in Potatoes
The protein-energy ratio in potatoes varies from 7.1 – 15.9% and averages 13.5%. This is above the WHO reference value of a safe level of intake (7.2 – 12.8%) even for sedentary individuals. It is above the UK vegetarian intake (12.7%) and just a fraction below UK omnivore level of 14.2% (7). Potatoes provide high quality protein. In other words, as long as you ate enough to supply energy needs, potatoes alone would provide enough protein.
And while you are getting enough protein … you would also be getting substantial amounts of vitamins and minerals.
To be continued
Disclaimer: Nothing in this article or on this website should be taken as medical or dietary advice. Anyone reading any information provided within should seek advice from their own medical practitioner for any issue, disease, illness or health-related problem they may have. Always seek your own advice from a medical practitioner or dietitian before changing your own diet.
This is a series of posts on staple foods:
- Good foods, bad foods
- Potatoes protein power
- Super-spuds super-foods
References and Notes –
1. UK NHS Eat Well Guide. Retrieved 07 December 2018. Public Health England contains public sector information licensed under the Open Government Licence v3.0
2. US Dietary Guidelines 2015. Retrieved 07 December 2018. US Department of Health and Human Services and U.S. Department of Agriculture. 2015-2020 Dietary Guidelines for Americans. 8th Edition. December 2015.
3. US Plate. Reference as per Reference # 2.
4. National Health and Medical Research Council (2013) Australian Dietary Guidelines Summary. Canberra: National Health and Medical Research Council.N55a. Licensed under Creative Commons 4.0.
5. Most foods are a combination of macronutrients. Exceptions include vegetable oils which are solely fat, butter which is 99.475% fat, sugars which are solely carbohydrate, egg white which is 96.667% protein; and gelatine which is 98.952 % protein.
6. Understanding Nutrition. Australia and New Zealand 2nd Edition. 2014. Eleanor Whitney, Sharon Rolfes, Tim Crowe, David Cameron-Smith, Adam Walsh.
7. WHO Technical Report Series 035: Protein and amino Acid Requirements in Human Nutrition. report of a joint WHO / FAO / UNU Expert Consultation. 2007.
8. Australian Government. National Health and Medical Research Council. Nutrient Reference Values for Australia and New Zealand.Nutrients: Protein.
9. Australian Bureau of Statistics. 43640DO001_20112012. Australian Health Survey: Nutrition First Results – Food and Nutrients. 2011-12. Table 1:1 Mean daily energy and nutrient intake. Protein
10. Australian Bureau of Statistics. 4805.0. Nutrient Intakes and Physical Measurements. 1995. Table 2. Median Daily Energy, Moisture and Macronutrient Intake. Page 21.
11. Australian Bureau of Statistics. 43640DO002_20112012. Australian Health Survey: Nutrition First Results – Food and Nutrients. 2011-12. Table 2:1 Mean contribution to energy intake. Protein (%)
12. Australian Bureau of Statistics. 4805.0. Nutrient Intakes and Physical Measurements. 1995. Table 3. Mean contribution of Macronutrients to Energy Intake (Percent). Page 22.
13. The Great Protein Fiasco. Donald McLaren. The Lancet. Volume 304, Issue 7872, P93-96, July 13, 1974
14. Protein-Energy Ratio in selected foods. Leonie Elizabeth. July 2018.
Figure 7: Protein-Energy ratio in selected foods per 100g
15.Potatoes and Human Health, Part III. Potato-eating Cultures. Whole Health Source. Nutrition and Health Science. Stephan Guyenet. 02 October, 2010.
16. Associations of fats and carbohydrates intake with cardiovascular disease and mortality in 18 countries fro five continents (PURE): a prospective cohort study. M Deghan, A Mente et al. The Lancet. Online 29 August 2017. Supplementary appendix.
17. Understanding Nutrition. Australia and New Zealand 2nd Edition. 2014. Eleanor Whitney, Sharon Rolfes, Tim Crowe, David Cameron-Smith, Adam Walsh. Page 176.