PERSPECTIVES WITH DR ALAN BARCLAY

HOT POTATOES 
While they are a pariah in many parts of the developed world, due to the current popularity of low carbohydrate diets, potatoes have been eaten and enjoyed all over the world for centuries. Even today, roots and tubers are the third largest carbohydrate-containing food source in the world, with potatoes representing nearly half of those consumed.

Potatoes are classified as a starchy vegetable, due to the fact that they are predominantly composed of carbohydrate (86% of energy), and most of that carbohydrate is starch. Whatever the variety of potato, potato starch consists of amylopectin and amylose in a fairly constant ratio of 3:1, which is one of the reasons why the average potato has a high GI.

The average GI of potatoes is 77, with a range between 54 to 101, depending on the variety and how they are cooked and eaten. A medium-sized (around 150g) boiled potato provides approximately 23 g of available carbohydrate, so the average GL is 15, which is moderate.

Despite their generally high GI, research from single meal studies suggests that boiled potatoes are more filling (satiating) than equal kilojoule (calorie) portions of other common carbohydrate-rich foods like bread, rice and pasta.

A small proportion of the starch found in potatoes is resistant to enzymatic degradation in the small intestine and therefore reaches the large intestine essentially intact, where it becomes fuel for the microbiome. The amount of resistant starch in a potato depends on how it is cooked and eaten: 

• boiled potatoes – 2.4g per 100g 
• cooled-and-reheated potatoes – 3.5g per 100g 
• baked potatoes – 3.6g per 100g 
• cold potatoes (whether originally baked or boiled) – 4.3g per 100g. 

The protein content of potatoes is comparable to that of most other root vegetables and tubers with approximately 2–4 g per serve (medium-sized potato). On a dry-weight basis, this is comparable to that of cereals and, with the exception of beans, exceeds that of other commonly consumed vegetables. Potatoes have a relatively high biological value (BV) of 90 compared with other key plant sources of protein (e.g., soybean with a BV of 84 and beans with a BV of 73). Unlike many vegetables, potatoes contain all nine essential amino acids and are therefore a complete protein.

Potatoes contain a variety of essential vitamins and minerals most notably vitamins B (B6, riboflavin, thiamin and folate), C and the minerals potassium, magnesium, and iron. A medium-sized boiled potato provides 17 mg of vitamin C – more than half of the estimated average requirement for adults. Potatoes provide one of the most concentrated sources of potassium with a medium-sized boiled potato providing 647 mg, or nearly 20% of the Adequate Intake for adults – significantly more than those foods commonly known as high in potassium, like bananas, oranges, and broccoli.

While they are not particularly high in iron compared to meats, poultry and seafood, potatoes are a reasonable source of non-heme (i.e., plant sourced) iron, and importantly, the bioavailability of iron in potatoes exceeds that of many other iron-rich vegetables owing to extremely low or non-existent levels of antinutrients, chelators and ligands that inhibit iron absorption in the gut (e.g., tannins, oxalates, phytates) and high levels of vitamin C, which enhances iron absorption from the gut.

Frying potatoes (French fries and potato crisps/chips) increases their energy (kilojoules or calories) density, and acrylamide levels, and may also make them high in saturated and trans fats (depending on the fats/oils used) and sodium (from added salt). Also, the kind of potato used to make hot chips commercially has a very high GI, so the average GI of French fries is 75.

So, the answer to “should I be eating potatoes?” is yes. Boiled, baked or cold potatoes (potato salad) are a delicious, nutritious and very affordable staple that can be included in moderation in a healthy balanced diet. However, save fried potatoes for special occasions because they are a treat not daily fare – and be fussy about the fats/oils they are fried in.

Read more:

• Potatoes, Nutrition and Health 
• A satiety index of common foods 
• Nutrient Reference Values 
• Biological value 
• Acrylamide Questions and Answers

   
Alan Barclay, PhD is a consultant dietitian and chef (Cert III). He worked for Diabetes Australia (NSW) from 1998–2014 . He is author/co-author of more than 30 scientific publications, and author/co-author of  The good Carbs Cookbook (Murdoch Books), Reversing Diabetes (Murdoch Books), The Low GI Diet: Managing Type 2 Diabetes (Hachette Australia) and The Ultimate Guide to Sugars and Sweeteners (The Experiment, New York).

Contact: You can follow him on Twitter or check out his website.