Starch: The final frontier
Dr Alan Barclay is an Accredited Practising Dietitian and Nutritionist with over 20 years of experience in clinical dietetics, public health and academia. The amount and type of carbohydrate in food is his particular area of expertise, having completed a PhD in this area in the mid 00s. He is author of several books: Reversing Diabetes, Low GI Diet Diabetes Handbook, Low GI Diet Managing Type 2 Diabetes and The Ultimate Guide to Sugars and Sweeteners; and he is regular blogger for GI News. We asked Dr Barclay about the lesser understood topic of starches in the modern food supply.
There have been several shifts in the way we think about carbohydrates. The first was the idea of classifying sugars as simple carbohydrates and starch as complex carbohydrates to describe their physical structure but perhaps the real revolution came with the discovery of the glycemic index (GI). The GI went a step better to describe the effects of carbohydrate-rich foods on the body, and specifically the impact on blood glucose levels. The GI overturned many misunderstandings about differences between sugars and starches, including the erroneous idea that starches had a lesser impact on blood glucose levels than sugars. We know now that while glucose has the highest glycemic impact of all carbohydrates with a GI of 100, fructose and lactose are low GI and sucrose (table sugar) is medium. What is less known is the glycemic impact of starches and maltodextrins and this is important considering the increasing use of starches and maltodextrins as food additives, particularly in this age of demand for sugar-free and no-added-sugar foods.
What is the difference between starches, maltodextrins and sugars?
Chemically, they are much more similar than most people realise. They are composed of single sugar (monosaccharide) molecules, but where they differ is in the number of molecules joined together.
Sugars are mono and di-saccharides, or 1 or 2 sugars joined together, and are sweet. The most common monosaccharides are fructose, galactose and glucose, and when combined together they form the disaccharides lactose (glucose + galactose), maltose (glucose + glucose) and sucrose (glucose + fructose). All occur naturally in foods like fruits, vegetables, milk and yoghurt and they are all added to foods as ingredients although in Australia sucrose (table sugar) is most frequently used.
Maltodextrins are chains of 3 - 10 units of the sugar glucose and have varying degrees of sweetness. They are found naturally in alternative sweeteners such as rice malt syrup, and are commonly added to foods as thickeners.
Starches are simply chains of 10 or more units of the sugar glucose and are not sweet. There are 2 main forms – amylose and amylopectin. Amylose is composed of simple linear chains whereas amylopectin chains are branched – much like a tree. Both kinds of starch are found naturally in a wide range of foods including breads, cereals, rice, pasta, potato, sweetcorn, yams, taro, beans, lentils, chickpeas, nuts and seeds. They are also commonly added to foods as thickeners – gravies, for example, are thickened with starch typically from wheat or rice.
Are starches healthier than sugars?
Sugars, maltodextrins and starches provide essentially the same amount of kilojoules per gram and can all contribute to dental caries (tooth decay). When found naturally in whole foods like fruits, vegetables, legumes, wholegrains, milk and dairy all can be considered a part of a healthy diet.
Refined sugars, maltodextrins and starches can contribute to health problems because the refining process removes dietary fibre, vitamins and minerals, which are all important for good health. We eat refined sugars, maltodextrins or starches as ingredients or food additives. Foods high in refined sugars include sugar-sweetened beverages and confectionery, while maltodextrins and refined starches are present in high amounts in potato/rice/corn based extruded snacks.
How do starches vary in their glycemic impact?
We don’t eat pure starch and it’s not really possible to measure its GI on its own. However, starchy foods cover the full spectrum from low (≤ 55) to high GI (>70), depending on the source and method of processing.
GI of a selection of starchy foods
|White, glutinous rice, cooked in rice cooker||98 (high)|
|Tapioca, boiled||93 (high)|
|Brown rice pasta, boiled||92 (high)|
|Potato, Pontiac, boiled, mashed||91 (high)|
|Burger rings (extruded savoury packet snack)||90 (high)|
|Rice bubbles||87 (high)|
|Sweet potato, purple skin, white flesh||75 (high)|
|Sliced white bread||71 (high)|
|Basmati rice, white, boiled||58 (medium)|
|Potato, Nicola, boiled||58 (medium)|
|Taro, yam||54 (low)|
|Lentils, canned||42 (low)|
|Chickpeas, canned||40 (low)|
Why do some starchy foods such as rice and potato have a high GI?
This is the subject of ongoing scientific research. We know that amylose takes longer to digest than amylopectin, and the ratio of these different kinds of starch varies between different foods. Most varieties of rice are high in amylose and some think this is why they typically have a high GI, for example. The kind of dietary fibre found naturally in starchy foods also has an effect; soluble fibres found in legumes, for example, has a more powerful effect than insoluble fibres found in wheat. The amount of processing also makes a difference – kibbled wheat has a lower GI than highly refined wholemeal wheat flour for example.
Has there been an increase in the use of starches and maltodextrins as food additives, and if so, why?
A recent study from the University of Wollongong found that maltodextrins are now the number 1 sweetener added to foods in Australia (Probst et al. DAA, 2015). It’s likely that they have been used to replace added sugars.
Does using maltodextrins or starches to replace sugars in foods make them healthier?
Not necessarily. They provide the same amount of kilojoules and can have a high GI depending on the source. They are also cariogenic as salivary amylase can break them down to glucose in our mouths which cariogenic bacteria can use for fuel.
What starches should people look out for on food labels?
Industrial starches are often partly modified, which affects the available carbohydrate, digestibility and therefore GI, so these additives are likely to vary in their GI although until they are tested we can't know for certain. I would assume many would be high
|(1400) Dextrin roasted starch|
|(1401) Acid-treated starch|
|(1402) Alkaline-treated starch|
|(1403) Bleached starch|
|(1404) Oxidized starch|
|(1405) Starches, enzyme-treated|
|(1410) Monostarch phosphate|
|(1412) Distarch phosphate|
|(1413) Phosphated distarch phosphate|
|(1414) Acetylated distarch phosphate|
|(1420) Starch acetate|
|(1422) Acetylated distarch adipate|
|(1440) Hydroxypropyl starch|
|(1442) Hydroxypropyl distarch phosphate|
|(1443) Hydroxypropyl distarch glycerol|
|(1450) Starch sodium octenyl succinate|
|(1451) Acetylated oxidized starch|
|Rice malt syrup|