Should I be choosing low glycaemic index carbs in my diet?
Following on from my article last month, I thought it might be useful to help provide some insight into the glycaemic index (also called GI), a tool that can be used to classify carbohydrate foods.
The glycaemic index is a score from 0 to 100 that reflects the rate at which a carbohydrate-containing food can be digested and absorbed, and the rate at which the sugars from that food then appear in our blood stream and increase our blood glucose levels. A low GI food has a GI value lower than 55, and a food with a GI value over 70 is classed as a high GI food. The term glycaemic refers to the impact that a food has on blood glucose levels specifically.
Lower GI foods are recommended to help manage blood glucose levels better in diabetes and gestational diabetes; to maintain a healthier body weight and to help lower blood cholesterol levels. The diagram (above right) illustrates the impact a high or a low GI food has on our blood glucose levels.There are some key things to remember about the glycaemic index:
Any food that does not contain carbohydrate will not have a GI value.
Because foods that do not contain carbohydrate cannot be tested for their GI value, they will not have a GI value; or might even be called low GI, which in my opinion is misleading marketing if there is not carbohydrate in that food in the first place. For example, bacon could be labelled low GI. However, it has no carbohydrate to be causing a glycaemic response in the first place. Calling it a low GI food would not make it a healthy choice, as it is high in saturated fat and consumption of processed meats is linked to bowel cancer. The NZ Cancer Society recommends eating very little, if any, processed meat. This can be relevant for any high-fat food being labelled low GI.
The glycaemic index values are typically calculated on a portion of food that contains 50g of carbohydrate.
In some cases, quite a lot of that carbohydrate-containing food needs to be consumed to achieve a 50g of carbohydrate dose, particulary for foods that have lower carbohydrate content. Thus, the portion of food used to test the glycaemic index can be more than one would realisitically eat in one sitting — the GI research participants can have quite a chore ahead of them to get through that much of the test food!
This can mean that an otherwise healthy food with a high glycaemic index value might thus be harshly judged. Watermelon is a great example. It has a relatively high glycaemic index value of 80 (out of a possible score of 100). However, to eat 50g of carbohydrate from watermelon, one would need to consume a whole kilogram of watermelon flesh. A more realistic serve size might be 1 cup of diced watermelon, which contains just 9g of carbohydrate. This size would cause minimal changes in blood glucose. As a fruit, watermelon is low calorie and packed with valuable vitamins, so it is still a great food to eat.
It can work the other way, too. For example, common sucrose or table sugar actually has a moderate glycaemic index value, but we still need to eat less of it for other reasons (it contributes a lot of calories to our diets as a population, with no other vitamins and minerals of benefit when added to processed foods).
All sorts of different food factors influence the glycaemic response to foods.
Food factors that slow the digestion of a carbohydrate-containing food can lower the glycaemic index value of that food, but that may not mean that that food is healthier for you, just because it has a low GI value.
The presence of fat with the carbohydrate can lower the glycaemic response to that food. However, just because the fat lowers the GI value, it doesn’t make that food a healthy option. Food processing typically increases the digestibility of carbohydrates in foods, particulary heat, moisture and pressure — increasing the GI of a food.
However, adding some food ingredients can lower the glycaemic response to that food. For example, if you were consuming bread with a salad, adding vinegar to the salad and bread meal can significantly blunt the increase in your blood glucose in response to the carbohydrate in the bread.
Consuming any carbohydrate food in the context of a mixed meal that includes meat and vegetables slows the digestion of that carbohydrate food, which can also blunt the increase in blood glucose. For example, eating mashed potato on its own can cause a fairly substantial increase in blood glucose, but eating that mashed potato in a mixed meal with some broccoli and carrot and chicken will lower the subsequent glucose response. This can be because the greater volume of food in your stomach slows the rate of digestion (your stomach gets quite busy), and the presence of fibre in the other vegetables also slows down the rate of food digestion.
As is often the case in nutrition, there are many factors we need to consider when deciding what foods we should incorporate more of into our diet. Some really good low GI foods we should all be including more of in our diet include wholegrain oats, wholemeal pasta and legumes such as chickpeas, beans and lentils. These foods are digested and absorbed more slowly and thus create a more gradual rise in blood glucose.
Exercise also plays an important role. The more you exercise, the better you are at clearing glucose out of your bloodstream. Exercise creates the demand for glucose from your muscles, so your muscles get better at taking up glucose out of the blood which can help protect other organs (such as your eyes) from the damage related to high blood glucose levels. Of course, exercise improves your health in myriad other ways, too.
So in summary, the glycaemic index is a useful indicator of how quickly your blood glucose levels (or blood sugar levels) rise after eating certain carbohydrate-containing foods, but still requires interpretation relating to the portion size of carbohydrate you eat. You may not need to exclude an otherwise healthy food from your diet (such as watermelon) just because it records a high glycaemic index value. All of us should be eating more wholegrain and low glycaemic index carbohydrate foods that come packed with other vitamins and minerals and fibre such as oats, legumes, and wholemeal pasta; ideally in a less processed form with minimal added saturated fat and sugar.
- Dr Kirsty Fairbairn is a health, wellness and sports dietitian at Invigorate Nutrition (www.invigoratenutrition.com), based at Eclipse Health, Wellness and Performance, Hanover St, Dunedin.
