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Carbs – good or bad?

It’s 20 years since I last wrote an article about carbohydrates for the KV Voice. At that time, I referred to the popular sport of ‘carb bashing’. Sadly, nothing much has changed in that respect. However, I live in hope that more people are prepared to think about foods rather than concentrating on a particular nutrient. Context matters and that’s particularly relevant to foods. Let’s take a closer look.

Carbohydrates exist in two forms – sugars and starches. Sugars are mostly categorised as ‘simple’, with the common sugars found in foods consisting of one or two components. Starches are referred to as ‘complex’, with long chains of sugars arranged in different ways.

Sugars

The common sugars found in foods are grouped as monosaccharides and disaccharides:

Monosaccharides include:

  • Glucose – also referred to as dextrose
  • Fructose- sometimes called fruit sugar
  • Galactose – formed in the intestine when we digest the sugar in milk.

Disaccharides include:

  • Sucrose – made up of one molecule of glucose and one of fructose
  • Maltose – comprising two molecules of glucose 
  • Lactose – one molecule of glucose and one of galactose.

Starches

Starches are made up of long chains of simple sugars. These may be linked in different ways and such linkages influence how rapidly the starches are broken down by the digestive enzymes in the small intestine.

Processing can change how rapidly starches are broken down to sugars. For example, if bread is made the traditional way with the dough being allowed to rise slowly, the enzymes in the small intestine take longer to break the starch down to its component sugars than occurs with the rapid dough method that is used in many packaged breads. The more rapid breakdown of starch in bread flour can cause problems for people with diabetes. A high content of dietary fibre found in grainy breads will also slow down this stage of the digestive process and may help reduce likely problems. The fact that the dietary fibre will then pass to the large intestine where it will help ‘good’ bacteria to multiply adds even more reason to choose wholegrain breads.

Starches can also change with cooking and cooling. For example, when potatoes are cooked and served hot, their starch is digested fairly quickly. If you leave the potato to cool down, the starch changes and when consumed it will take much longer to be broken down to glucose. The digestion of rice also changes if the rice is cooked by the absorption method rather than being boiled in lots of water and then drained. If rice is cooked, then cooled and used for fried rice, its carbohydrates are also broken down more slowly. Pasta cooked to the al dente stage is also digested more slowly than if it’s overcooked.

The starch that is broken down more slowly is called ‘resistant starch’. As well as leading to a slightly slower release of glucose in the small intestine, it increases healthy bacteria in the large intestine. CSIRO has developed a variety of barley with an increased content of this type of starch.

Energy

Under normal circumstances, the energy used for metabolism and for most types of physical activity comes from a mixture of glucose and fatty acids (from fat). Glucose is absorbed into the blood and delivered to cells throughout the body. A small amount is stored in the liver in the form of glycogen and can be rapidly released if the level of blood glucose falls. You may become aware of this process hours after the last meal when you feel the sensation of a hunger pang. (Note this is not the kind of ‘hunger’ you may experience when you smell hot chips or spot a tempting pastry.) After a few minutes, a genuine hunger-rumbling pang stops. This is because the body has released liver glycogen to restore the blood glucose level. That doesn’t last long and, if you still haven’t eaten, you may get a second pang. Once the small store of liver glycogen is used up, if you don’t eat any food containing carbohydrate, the body can restore normal blood glucose by breaking down some protein, either from food or from lean muscle tissue.

Larger quantities of glycogen are also stored in muscles and are available only for the muscle to use for physical activity. The more physical activity you do, the greater the store of glycogen in muscles.

Insulin, a hormone produced by the pancreas, is needed when the body uses blood glucose for energy. Some ‘low carb’ enthusiasts regard insulin as something undesirable. In fact, it is entirely normal – and essential – within limits.

If you’re not about to engage in physical activity, a sudden influx of a large quantity of glucose from rapidly digested starches or sugars will result in the release of a high quantity of insulin. This then decreases the amount of fat being used in the body’s fuel mix.

This is unlikely to be a problem if you are engaging in plenty of physical activity, as the insulin will help the body use glucose for the energy needed. But if you are largely sedentary, and don’t need that sudden influx of energy, a high insulin level will mean you burn less energy from the fat you are eating, or from your body’s fat stores.

Good or bad?

The upshot is that it’s best to avoid too many rapidly digested sugars and starches. However, concluding that all ‘carbs’ are bad is not justified. Many foods that contain sugars and starches are also sources of dietary fibre – an essential part of a healthy diet. On the other hand, ultra-processed foods with little or no dietary fibre and high content of sugar or starch should play no more than a small role in any healthy diet. That list includes sugary drinks, pies, pastries, cakes, biscuits, sugary cereals, many desserts, and sweet or savoury snack foods.

Take home message

The healthiest carbohydrate foods include:

  • Wholegrains – as in wholemeal or wholegrain breads, bulgur, freekah, farro, rolled oats, wholegrain barley, brown rice, teff, amaranth, buckwheat, millet or quinoa (not strictly a grain)
  • Legumes, including lentils, chick peas, black beans, Lima beans, soy beans, broad beans 
  • Fruit, as fruit, not juice
  • Nuts of all kinds
  • Seeds, including pepitas, sunflower, linseeds, sesame
  • Vegetables of all kinds.

Milk and yoghurt are also highly nutritious foods that contain lactose, a sugar that is not a problem for those who continue to produce the enzyme lactase after childhood.

Next month: I’ll discuss the importance of the diet, especially wholegrains, legumes, fruit, nuts, seeds and vegetables in the maintenance of a healthy microbiome.

Dr Rosemary Stanton OAM

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