What would happen if you only ate potatoes?

How a diet of mash, chips and crisps would affect you in the long run

Jennie Jackson
Thursday 18 February 2016 09:49 GMT
Chips are being consumed more than ever - but not in the once-popular combination of fish and chips
Chips are being consumed more than ever - but not in the once-popular combination of fish and chips (Adam Berry/Getty Images)

In the film The Martian, Matt Damon’s character, Mark Watney, is stranded on the red planet with nothing to eat but spuds. Now, a 36-year-old Australian is following the same diet, voluntarily.

In an attempt to lose weight and improve his relationship with food, Andrew Taylor has decided to eat nothing but potatoes for a year. But is this approach likely to work, or will he run out of nutrients? And could he have chosen a better single food to live on?

Just over a month into the diet, Taylor posted a photo on Facebook of some unfinished mash potato on his plate. This is a nice illustration of “sensory-specific satiety” – the theory that the pleasure we take in consuming a single food goes down as we eat more of it, so we stop eating so much.

He is probably feeling the benefits of his weight loss (10kg in the first month), with increased energy levels, and already will have reduced his chances of developing diabetes and other chronic conditions. But what will happen in the long term?

Enough protein and fat?

Eating around 3kg of potatoes a day will provide just over 2000kcal, a reasonable amount for a man of his size aiming to lose weight. But while potatoes are an excellent source of carbohydrates and fibre, he may struggle to get enough protein. A 120kg man may need up to 90g of protein, but this diet will provide only 60g.

Proteins are made up of a range of amino acids, including some that must be supplied in our diet, and potatoes contain a surprisingly good balance of these. But, despite spuds providing a good balance of amino-acids, there simply won’t be enough of them in Taylor’s diet.

Potatoes are also low in fat (only 9g per 3kg), so Taylor’s diet doesn’t supply enough of the two essential fatty acids (linolenic and linoleic acid), nor does it provide enough fat to aid the absorption of fat soluble vitamins: A, D, E and K.

We should also eat foods containing ready-made long-chain omega-3 polyunsaturated fatty acids, of which the only reasonable dietary source is oily fish. These fats have specific structural and functional roles in cell membranes, can act as hormones, and help to control blood pressure.

Charging a phone with a potato?

Vitamins and minerals

Taylor will get enough thiamine, niacin, vitamin B6, folate, vitamin C, potassium, magnesium, phosphorus and copper, even allowing for losses of these nutrients during cooking. Also, he has boosted his chance of getting enough vitamins A and E, iron and calcium by agreeing to include sweet potatoes in his diet. But his diet lacks vitamins D and B12.

Exposure to sunshine in Australia means that he should be making enough vitamin D, but unless he takes a supplement his stores of vitamin B12 may well run out before the end of the year. Prolonged deficiency will result in pernicious anaemia and possibly even irreversible nerve damage. Since he is allowing himself some seasonings, yeast extract would be a good choice, to top up levels of some of the other B vitamins, including biotin and riboflavin (vitamin B2). A lack of these will affect the way that he can use the energy from his food.

Also, his diet will provide only around 6mg of zinc and he will need up to 9.5mg a day. Zinc deficiency would become obvious in tissues with rapid turnover, such as the linings of our mouth, intestine and skin, resulting in reduced immunity and wound repair, and perhaps a loss of taste buds.

Other minerals that might be lacking in this diet include chloride, selenium and iodine, since levels of these depend on the soil in which the potatoes were grown. Using an iodine-enriched salt would be helpful in terms of chloride and iodine, but his diet would probably supply only 30μg of selenium, low enough to cause deficiency in most people. This could reduce immunity and lower reproductive capacity, as well as affecting thyroid function and antioxidant status.

Something better than potatoes?

Eating only one food probably won’t do any harm in the short term. However, there is no known food that supplies all the needs of human adults on a long-term basis. Since Taylor is determined to follow a one-food diet, then potatoes are probably as good as anything, as they contain a wider range of amino acids, vitamins and minerals than other starchy foods, such as pasta or rice. If he had chosen a single animal-derived food, he would have had no fibre in his diet and a poor intake of various vitamins, minerals and antioxidants. Fruits and non-starchy vegetables are very low in protein and fat, meaning he would have to plough through a huge amount to get enough sustenance.

Green is not always healthy

As a final caution, potatoes produce solanine, a glycoalkaloid poison. The amount in tubers of commercial varieties is generally low, but potato tubers that have been damaged in some way, or stored in the light, become green and produce more solanine. Eating even small quantities of green potatoes can cause nausea and vomiting, cramps, fever, dizziness, headaches, convulsions.

The toxic dose doesn’t seem to have been definitively determined, and it’s not clear how well solanine is absorbed and metabolised, nor whether it builds up when eaten in small amounts over a long time. It’s clearly safe to eat “normal” quantities of potatoes (up to around 300g) on a daily basis, but the safety of eating ten times this, for a whole year, has not been established. But Taylor should be comforted by the fact that potatoes are a staple food throughout the world, albeit part of a slightly more varied diet.

Eat to live, or live to eat?

One food is not enough, but we don’t need an enormous range of foods. My great great grandfather, living in rural Aberdeenshire, had, like most of his contemporaries, a very limited diet of mainly potatoes, with oatmeal, kale and small amounts of fish and boiled beef. He lived into his nineties and claimed never to be bored with his diet, saying: “It’s just maet” (where the Scots word “maet” refers to food in general, not just meat). Food was fuel, rather than a form of comfort and entertainment. Perhaps that is the sort of relationship with food that Andrew Taylor is trying to achieve.

Jennie Jackson, Lecturer in Human Nutrition and Dietetics, Glasgow Caledonian University

This article was originally published on The Conversation. Read the original article.

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