Going with the WHOLE grain
Grains are the seeds of cereal plants and include amaranth, barley, buckwheat, bulgur, maize (corn), millet, oats, quinoa, rice, rye, spelt and wheat. These staple foods are the most concentrated source of carbs in our diet, provide us with protein, are low in fat, packed with essential vitamins and minerals and rich in fibre when you eat the wholegrain varieties. Studies around the world show that eating plenty of wholegrain cereals reduces the risk of certain types of cancer, heart disease and type 2 diabetes. In addition, a higher fibre intake, especially from whole cereal grains, is linked to a lower risk of cancer of the large bowel, breast, stomach and mouth. Although all wholegrains are healthy, nutritious foods, it’s only the low GI ones that reduce your blood glucose and insulin levels throughout the day and increase your sense of feeling full and satisfied because they are the ones that slowly trickle glucose into your bloodstream. We like to say that your body is doing the processing, not the manufacturer.
By the way, GI Group, I've got a question for you. How would you explain this ‘contradiction’? Both wholemeal bread and sweet corn contain lots of insoluble fibre but the GI of wholemeal bread tends to be high while sweet corn's GI is relatively low. Why do you think this happens?
Why do some ‘wholegrains’ have a low GI and not others? Prof Jennie Brand-Miller explains
Some people use ‘wholegrain’ and ‘low GI’ as though they are interchangeable terms. That’s not the case at all. Wholegrain foods can be high or low GI, and it’s essential to understand this and know what’s low and what’s not to manage your blood glucose levels. In fact many processed wholegrain foods such as wholemeal bread have a high GI. Why? It all comes down to the physical state of the fibre and the starch in the food. When wheat fibre has been finely divided as it is in wholemeal bread, it does little for either constipation or blood glucose levels. That’s why we say to choose your carbs carefully, and if your favourite wholegrain food has a high or moderate GI, combine it with a low GI food to reduce the glycemic load of the meal or snack. Just for the record, here are five factors that can slow digestion of those nutritious wholegrain carbs. (For tips on reducing the GI of your diet see November GI News.)
- Starch gelatinisation
The starch in raw food is stored in hard, compact granules that make it difficult to digest. During cooking, water and heat expand these starch granules to different degrees – some actually burst freeing the individual starch molecules inside. If most of the starch granules have swollen and burst during cooking, we say that the starch is fully gelatinised. These swollen granules and free starch molecules are very easy to digest. The less gelatinised or swollen the starch is after cooking, the slower the rate of digestion.
- Physical entrapment
The fibrous coat around foods like beans, chickpeas, lentils, barley and seeds acts as a physical barrier. It slows down access of the digestive enzymes to the starch inside and thus slowing digestion. That’s why we say to look for lots of grainy bits when buying bread.
- How much amylose starch there is in the food
Amylose and amylopectin are two types of starch found in foods, but the ratio of one to the other varies considerably. Here’s how it works. Amylose is a straight chain molecule, like a string of beads. These tend to line up in rows and form compact clumps. The more amylose a food contains, the less easily the starch is gelatinised and the slower its rate of digestion. Legumes have lots of amylose as does basmati rice. Amylopectin on the other hand is a string of glucose molecules with lots of branching points, such as you see in some sorts of seaweed. Amylopectin molecules are larger and more open and the starch is easier to gelatinise and digest. So if a food has more amylopectin than amylose, it’s going to be moderate or high GI.
- Particle size
The larger the particle size, the lower the GI. When you eat a starchy food in ‘nature’s packaging’ – whole intact grains like barley that have been softened by soaking and cooking – the food will have a lower GI value. It’s the grinding or milling of cereals that reduces the particle size that makes it easier for water to be absorbed and enzymes to attack during digestion. That is why cereal foods made from fine flours like many breakfast cereals tend to have a high GI value.
- The type of fibre
The effect of fibre on the GI value of a food depends on the type of fibre (soluble or insoluble). Soluble fibres are the gel, gum and often jelly-like components of foods like oats, legumes and apples. By slowing down the time it takes for food to pass through the stomach and small intestine, soluble fibre can lower your body’s glycemic response to a food.
Insoluble fibres are dry and bran-like and often referred to as roughage. All cereal grains and products made from them that retain the outer coat of the grain are sources of insoluble fibre. But not all foods containing insoluble fibre are low GI. Why? Insoluble fibres will only lower the GI of a food when they exist in their intact, original form, for example in whole grains of wheat. Here they act as a physical barrier, delaying access of digestive enzymes and water to the starch within the cereal grain.