Blood sugar control is the main aim of nutrition therapy for type II diabetes. Since all forms of carbohydrates are broken down into sugar many people advocate a low-carb diet to control diabetes instead of the traditional low-fat diet.
However, many studies show that in the long-term there is very little difference in blood sugar control when comparing a low-carb to a low-fat diet.
So, type II diabetes can consume a moderate amount of carbohydrates but quality is the most important factor, and this is where the glycaemic index comes in handy.
The Glycaemic Index (GI)
The glycaemic index was created by Dr David Jenkins as a tool to help diabetics choose carbohydrates. The GI classifies carbohydrates by the effect they have on blood sugar response.
This was introduced as an alternative to grouping carbs as complex or simple. Some complex carbohydrates such as potato and refined grains are rapidly absorbed and have a higher GI than table sugar (1).
The following are the three GI ratings:
- Low: 55 or less
- Medium: 56–69
- High: 70 or more
Foods that have a high-glycaemic index are rapidly absorbed and cause a large spike in blood sugar levels. Refined carbohydrates such as white rice and white bread tend to have a high-glycaemic index. Other examples of high-glycaemic foods are mashed potatoes, instant noodles, breakfast cereals, sweets, cakes and biscuits.
Foods that have a low-glycaemic index are absorbed more slowly and cause less of an increase in blood sugar. Examples include porridge made with rolled oats, carrots, broccoli, sweet potato, beans, quinoa, buckwheat and barely. Rye, sourdough and whole grains breads are also low-glycaemic.
Meats, fish, nuts, oils and herbs and spices can also be consumed as part of a low GI diet.
Factors That Affect Glycaemic Index
- Starch structure – starch accounts for most of the carbohydrates in various diets worldwide. Starch is made of two molecules called amylose and amylopectin. Amylose is difficult to digest and therefore absorption is slower. Starchy carbohydrates with a greater proportion of amylose have a higher GI (2).
- Physical form of food – The physical form of food can alter. Fruit in juice form has a higher GI than fruit in its whole form. Another example is of physical form affecting GI is mashing causing the GI of potato to increase (2).
- Sugar composition – The type of sugar also affects GI. Fructose which is mainly found is fruit has a GI of only 23, and lactose which is found in milk has a GI of 46. Glucose has a very high GI of 97 (2).
- Cooking method – Heat, moisture and grounding of foods makes carbohydrates more digestible. The longer a carbohydrate source is heated, moisturised and/or grounded then the higher then GI (2).
- Fruit ripeness – fruit contains resistant starch that breaks down into sugars as the fruit ripens. Therefore, overripe fruits have a higher GI than underripe fruits (3).
- Nutrient composition – Acidity of a meal vastly reduces GI through slowing down gastric emptying (4).
- Fibre – dietary fibre are a form of carbohydrate that cannot be digested or absorbed in the small intestine. Fibre slows down gastric emptying and therefore slows down glucose absorption, leading to a lower GI (4).
- Carbohydrate refining – The process of refining carbohydrates disrupt amylose molecules, which makes them more digestible and increases GI (2).
Health Benefits of Low Glycaemic Index
Blood Sugar Control
A systematic review and meta-analysis of randomised control trials reviewed all the studies that looked at the effects of GI on blood sugar control (5).
The review found that type II diabetics that followed a low-GI diet had greater reductions in HbA1c and fasting blood sugar levels compared to those that followed a higher-GI diet or control in type II diabetics. Also, compared with the American Diabetes Association (ADA) diet, the low-GI diet was as effective at lowering HbA1c levels using less medication (5). B
Another study compared the effects of a low-glycaemic index diet to a high-glycaemic index diet on indicators of inflammation and metabolic health in type II diabetics (6). The participants consumed either a high glycaemic or low glycaemic diet.
After the 30 days there was a greater reduction in body fat in the low-GI group despite similar levels of calorie intake. The high-GI diet led to increases in inflammation, which were not seen the low GI diet. There was a greater increase in fructosamine in the high-GI group. Fructosamine is a protein that has permanently combined with glucose similar to HbA1c, and can be used as an indicator of blood sugar control.
High post-meal sugar responses have shown to increase oxidative stress in type II diabetics (7). Therefore, a low-GI diet should prevent increased oxidative stress
Eating a low-GI diet has been shown to lower uric acid levels, while a lower carbohydrate diet increased uric acid concentrations (8).
This shows that eating low-glycaemic carbohydrates may reduce the risk of gout.
Pitfalls of low-GI diets
The GI is based on 50 g of carbohydrates of each food but there many foods in which 50 g of carbohydrates would not be consumed in one sitting.
For example, one apple contains only 13 g of carbohydrates, and it would require around 4 apples to consume 50 g of carbohydrates, which many people would not do.
Another negative aspect is that GI only focuses on individual foods.
However, meals typically consist of different foods that contain a mixture of carbohydrates, protein and fats. The co-ingestion of protein and fats alongside carbohydrates decreases the GI.
Glycaemic Load (GL) is another way to categorise foods based on its effect on blood sugar. Unlike the GI, the GL also takes into account the quantity of carbohydrates in a practical portion of food. GL is calculated by multiplying the grams of carbohydrates in a portion size of a food by its glycaemic index, then dividing by 100.
For example, a potato has a GI of 85 and contains 14 grams of carbohydrate, thus a potato has a GL of only 12. However, the GL does not take into account the non-carbohydrate components of meals. Therefore, it is still only beneficial for individual foods instead of meals.
High vs Low GI
Also, not all foods that are classed as high-GI are unhealthy e.g. watermelons. On the other hand, crisps have a lower GI than potatoes cooked without food.
However, crisps are more calorie-dense and are less nutritious.
The GI and load can be helpful for diabetics to understand which form of carbohydrates can help control blood sugar.
However, when it comes to deciding which foods to eat, it is important to choose whole and minimally processed foods such as whole grains, sweet potato, fruits and vegetables.
Highly processed foods such as cakes, biscuits, candy and sugar sweetened beverages are linked with an increased risk of diabetes (9) and should be limited.
There are healthy foods that have a high-GI e.g. watermelon and parsnips but can be part of healthy diet. Whereas some highly processed foods have a low-GI but are not nutritious. Therefore, strictly following a low-GI diet does not necessarily mean you will follow a healthy diet. However, a well-planned low-GI has been showed to have a beneficial effect on blood sugar control.
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- Pi-Sunyer FX. Glycemic index and disease. The American journal of clinical nutrition. 2002 Jul 1;76(1):290S-8S.
- Guevarra MT, Panlasigui LN. Blood glucose responses of diabetes mellitus type II patients to some local fruits. Asia Pacific Journal of Clinical Nutrition. 2000 Dec 29;9(4):303-8.
- Eleazu CO. The concept of low glycemic index and glycemic load foods as panacea for type 2 diabetes mellitus; prospects, challenges and solutions. African health sciences. 2016;16(2):468-79.
- Ojo O, Ojo OO, Adebowale F, Wang XH. The effect of dietary glycaemic Index on glycaemia in patients with Type 2 diabetes: A systematic review and meta-analysis of randomized controlled trials. Nutrients. 2018 Mar 19;10(3):373.
- Gomes JM, Fabrini SP, Alfenas RD. Low glycemic index diet reduces body fat and attenuates inflammatory and metabolic responses in patients with type 2 diabetes. Archives of endocrinology and metabolism. 2017 Mar;61(2):137-44.
- Monnier L, Mas E, Ginet C, Michel F, Villon L, Cristol JP, Colette C. Activation of oxidative stress by acute glucose fluctuations compared with sustained chronic hyperglycemia in patients with type 2 diabetes. Jama. 2006 Apr 12;295(14):1681-7.#
- Juraschek SP, McAdams‐Demarco M, Gelber AC, Sacks FM, Appel LJ, White KJ, Miller III ER. Effects of lowering glycemic index of dietary carbohydrate on plasma uric acid levels: the OmniCarb randomized clinical trial. Arthritis & Rheumatology. 2016 May;68(5):1281-9.
- Satija A, Bhupathiraju SN, Rimm EB, Spiegelman D, Chiuve SE, Borgi L, Willett WC, Manson JE, Sun Q, Hu FB. Plant-based dietary patterns and incidence of type 2 diabetes in US men and women: results from three prospective cohort studies. PLoS medicine. 2016 Jun 14;13(6):e1002039.