These findings are largely correct. We do not necessarily need newer findings to confirm that. If you look back a few years and consider one of largest experiments in this regard it becomes clear that this is not ideal but possible. The Irish have survived as long as they had enough potatoes available.
[…] [the Irish Potato Famine, because about two-fifths of the population was solely reliant on this cheap crop for a number of historical reasons.]1
Potatoes alone have no fat soluble Vitamins A, D, and no B12. While D might not be a problem in central latitudes because humans may synthesise adequate amounts of this vitamin through sun exposure alone, vitamin A and B12 must be acquired from other sources. The amounts of biotin and of calcium in potatoes is equally quite low. Potatoes are also quite low in essential fatty acids and overall fat content. The protein content is of relatively good quality but with comparably low digestibility and mass.
These deficiencies of potatoes are very well balanced when the diet also contains cow's milk, since it provides all of these "missing ingredients" that are generally recognised as essential.
Otherwise the potato is quite unusual in its measured content of nutrients:
Carbohydrates:
Potato carbohydrates may be classified as starch, non-starch polysaccharides, and sugars.
What potatoes are best known for: source of energy in the form of starch.
Lipids:
The lipid content of potato is low. Galliard (1973) found 0.08 to 0.13% (FWB) in 23 varieties. This range is too low to have any nutritional significance but contributes towards potato palatability (Kiryukhin & Gurov, 1980), enhances tuber cellular integrity and resistance to bruising and plays a part in reducing enzymic darkening in tuber flesh (Mondy & Mueller, 1977).
Organic acids:
The major organic acids identified in the potato are citric and malic acids (Jadhav & Andrew, 1977; Bushway et ai, 1984). Others present are oxalic and fumaric (Bushway et al., 1984), chlorogenic and phosphoric (Schwartz et aL, 1962), as well as ascorbic, nicotinic and phytic acids, amino acids and fatty acids. All these contribute to flavour and help to buffer the potato sap (the pH of the tuber is 5.6 to 6.2); the level of some, especially that of malic acid, can be used to indicate tuber maturity. Ascorbic and nicotinic acids influence directly, and phytic acid indirectly, tuber nutritional value (see pp. 45 and 49).
Phytic acid is an anti-nutrient that diminishes the value of the potato and everything eaten with it. Other problematic substances found in potatoes are lectins, glycoalkaloids and proteinase inhibitors.
Pigments: Anthocyanin pigments in the periderm and peripheral cortex produce totally or partly pigmented skins in potatoes. In some South American varieties, the pigment is so dark that tubers may appear black and others dark purple.
These secondary nutrients are often touted as very healthy constituents in potatoes. Whether they are or not, in commercially grown potatoes they are almost absent.
Potato flesh may be white or various shades of yellow, depending upon the variety. Yellow coloration is generally due to presence of carotenoid pigments. The major carotenoid identified in 13 German varieties was violaxanthin, followed by lutein and lutein-5,6-epoxide and, in lower concentrations, by neoxanthin A and neoxanthin (Iwanzik et al., 1983); beta-carotene was detected in only trace amounts or was totally absent. One cultivar had an intense yellow flesh colour, but a relatively low level of carotenoids. It is therefore possible that, in some varieties, the yellow colour is due to other, unidentified, pigments as well as to carotenoids. In some places (e.g. Peru) yellow-fleshed varieties are highly prized and command higher prices than those with white flesh.
Varieties differ substantially in their contents. Growing and harvesting conditions influence their value, as do factors like freshness, storage conditions, and how they are prepared and eaten. Most of the most important nutrients are present in potatoes. There may very well be other nutrients missing for something approaching anything like an 'optimal diet', but potatoes already offer a large chunk of what a human typically needs.
Vitamins:
Potatoes are substantial sources of several vitamins: ascorbic acid (vitamin C) and the B vitamins thiamin (B1), pyridoxine (B6) and niacin. Riboflavin (B2), folic acid and pantothenic acid are also present. Small amounts of vitamin E have been reported (Paul & Southgate, 1978). Biotin is present in traces. The vitamin A precursor beta-carotene is absent or present only in trace amounts.
Factors affecting contents
Values can vary considerably, as the ranges, determined by different authors, shown in Table 2.10 demonstrate, but relatively little work has been carried out to determine the sources of variation. Different methods of analysis can lead to varying results: Finglas & Faulks (1984, 1985) attributed differences in their determined values for thiamin, niacin, riboflavin and total folate from those previously reported for the potato in food composition tables to analytical methods that were more reproducible than those used earlier.
The thiamin content of potatoes depends upon variety (Swaminathan & Pushkarnath, 1962; Leichsenring et al., 1951) and location of growth (Leichsenring et al., 1951). Tubers from loamy soil contained more thiamin than tubers from sandy soil, and thiamin content is greatly increased by nitrogen fertilization (Augustin, 1975).
However, although the potato has been shown to be a source of good-quality protein, to have a favourable ratio of protein calories to total calories and to be an important source of vitamins and minerals, its overall value in the diet nowadays is generally greatly underestimated.
Energy value:
The potato has a lower average carbohydrate content than do other roots and tubers, and also a comparable fat content (Table 2.2). Raw potato has a somewhat lower average energy content than other raw roots and tubers with 335 kJ (80 kcal) per 100 g. However, the large variation in tuber DM content produces a range of energy contents also, e.g. 264 to 444 kJ (63 to 106 kcal) per 100 g was found for the energy values of North American commercial varieties (Toma et al., 1978fl). The energy content of raw potato is considerably less than that of raw cereals and legumes; however, when cooked, the latter staples absorb large quantities of water, which changes their composition significantly. The potato, when boiled in its skin, retains its energy value almost unaltered. A fairer comparison of the potato and the cereals or legumes, therefore, is either on a dry, raw basis or on a cooked, 'as eaten', basis. […] Bread and tortillas, however, provide substantially more energy than cooked potatoes. […] The potato's low energy density (energy content per gram of food) is advantageous when potatoes are included (without added fat or energy- rich sauces) in diets of the developed world, where obesity, as a state of malnutrition, is found increasingly. In parts of the developing world where diets are energy deficient, this attribute may be a disadvantage, particularly in the diet of infants and small children, whose digestive systems cannot cope with large intakes. Too much potato would be needed to supply all the energy requirements of small children, so they need an energy-rich supplement. […] Although adults would also have to consume large quantities to meet all their daily energy needs, their digestive systems have a greater capacity. Up to 4.5 kg per capita were consumed daily in Ireland in the seventeenth to nineteenth century (Pimental et al., 1975). This would have provided approximately 15.06 MJ (3600 kcal) and 94 g of total protein.
Dietary fibre:
Boiled potato flesh has a dietary fibre content similar to that of cooked white rice and a much lower content than that of boiled green plantains or of boiled Phaseolus beans. Potatoes cooked as french fries or chips are a more concentrated source of fibre (Table 2.8). It can be calculated that 100 g of boiled potato supply 1.0, 0.7, and 0.5 times the fibre that can be found in a 35 g 'medium' slice of white, brown or wholemeal bread, respectively; a 25 g packet of chips supplies 1.9,1.4 and 1.0 times the respective bread fibre contents.
There is no recommended daily allowance (RDA) for dietary fibre at present. It has been suggested (Brodribb, 1983) that about 40 g/day should be consumed to maintain correct colonic function. Recently an ad hoc working party of the NACNE (National Advisory Committee on Nutrition Education, 1983) recommended an increase in British dietary fibre intake to 30 g per person per day. When potatoes are consumed in quantity on a regular basis, they make a significant contribution to dietary fibre intake. At present, for example, fresh potatoes contribute 15% of the dietaryfibreintake in British households and rank as a primary source (Finglas & Faulks, 1985).
At present, potato may be seen from the tables to compare favourably on a raw basis with all the listed staples and vegetables in terms of thiamin, riboflavin and niacin, and with most of the vegetables in pyridoxine and pantothenic acid contents. It has a much lower biotin content than the other vegetables, but it may be a comparatively richer source of folic acid than was previously thought. Fresh potatoes may contain 30 mg or more of ascorbic acid per 100 g when newly harvested, with an average value of 20 mg/100 g, although values decline when potatoes are stored, cooked or processed.
Comments on protein contribution from potatoes:
Potato protein is of sufficiently high quality for maintenance purposes in adult man and for growth of infants and children. The relatively low digestibility of potato protein is a disadvantage when potatoes are used for feeding to children; potatoes have to be consumed in large quantities to satisfy both protein and energy requirements, a characteristic they share with other root and tuber staples. Potatoes are rarely consumed as the sole source of N in the diets of either adults or children, but it is clear that they can make a valuable contribution to the protein content and quality of a mixed diet, provided present levels of protein in potato are maintained.
Source: Jennifer A. Woolfe,: "Potato in the human diet", Cambridge University Press: Cambridge, New York, 1987.
Returning to the Irish experiment with John Reader: "Potato. A History of the Propitious Esculent", Yale University Press: New Haven, London, 2008:
"I have heard [the potato] stigmatized as being unhealthy, and not sufficiently nourishing for the support of hard labour; but this opinion is very amazing in a country, many of whose poor people are as athletic in their form, as robust, and as capable of enduring labour as any upon earth. When I see the people of a country, in spite of political oppression, with well-formed vigorous bodies, and their cottages swarming with children; when I see their men athletic and their women beautiful, I know not how to believe them subsisting on an unwholesome food."
This quotes an English survey of the time. While these observations seem to be the result of relatively subjective impressions, there are quite some numbers to back that up:
The population of Ireland more than doubled between 1687 and 1791, rising from 2.16 million to 4.75 million. During the fifty years from 1791 to 1841 another 3.4 million were added, taking the population to 8.15 million. Another 1.75 million emigrated to North America, Scotland, England and even Australia during the same period, which brings the total born in Ireland to nearly 10 million — almost a five-fold increase in 154 years (1687 to 1841). ‘Probably in no other western country has so rapid a rate of natural increase been so long sustained,’ writes the historian Kenneth H. Connell.
