The low-glycemic (low-GI) diet is based on the concept of the glycemic index (GI).
Studies have shown that the low-GI diet may result in weight loss, reduce blood sugar levels and lower the risks of heart disease and type 2 diabetes.
However, the way it ranks foods has been criticized for being unreliable and failing to reflect their overall healthiness.
This article provides a detailed review of the low-GI diet, including what it is, how to follow it and its benefits and drawbacks.
What Is the Glycemic Index?
Carbohydrates are found in breads, cereals, fruits, vegetables and dairy products, and they are an essential part of a healthy diet.
When you eat any type of carbohydrate, your digestive system breaks it down into simple sugars that enter the bloodstream.
Not all carbohydrates are the same, as different types have unique effects on blood sugar.
The glycemic index (GI) is a measure that ranks foods according to their effect on your blood sugar levels. It was created in the early 1980s by Dr. David Jenkins, a Canadian professor (1).
The rates at which different foods raise blood sugar levels are ranked in comparison with the absorption of 50 grams of pure glucose, which is used as a reference food and has a GI value of 100.
The following are the three GI ratings:
- Low: 55 or less
- Medium: 56–69
- High: 70 or more
Foods with a low-GI value are the preferred choice, as they are slowly digested and absorbed, causing a slower and smaller rise in blood sugar levels.
On the other hand, foods with a high GI value should be limited since they are quickly digested and absorbed, resulting in a rapid rise and fall of blood sugar levels.
It is important to note that foods are only assigned a GI value if they contain carbohydrates. Hence, foods containing no carbs, such as beef, chicken, fish, eggs, herbs and spices, won’t be found on GI lists.
Factors That Affect the GI of a Food
A number of factors can influence the GI value of a food or meal, including:
- The type of sugar: It’s a misconception that all sugars have a high GI. The GI of sugar actually ranges from as low as 19 for fructose to up to 105 for maltose. Therefore, the GI of a food partly depends on the type of sugar it contains.
- Structure of the starch: Starch is a carbohydrate made up of two molecules, amylose and amylopectin. Amylose is difficult to digest, whereas amylopectin is easily digested. Foods with a higher amylose content will have a lower GI (2).
- How refined the carbohydrate is: Processing methods such as grinding and rolling disrupt amylose and amylopectin molecules, raising the GI. Generally speaking, the more processed a food is, the higher its GI (2).
- Nutrient composition: Both fat and acid slow down the rate at which a food is digested and absorbed, resulting in a lower GI. Adding fats or acids, such as avocado or lemon juice, will lower the GI of a meal (3, 4).
- Cooking method: Preparation and cooking techniques can change the GI too. Generally, the longer a food is cooked, the faster its sugars will be digested and absorbed, raising the GI.
- Ripeness: Unripe fruit contains complex carbohydrates that break down into sugars as the fruit ripens. The riper the fruit, the higher its GI. For example, an unripe banana has a GI of 30, whereas an overripe banana has a GI of 48 (5).
The Amount of Carbs Is Also Important
The rate at which foods raise blood sugar levels depends on three factors: the type of carb they contain, their nutrient composition and the amount you eat.
However, the GI is a relative measure that doesn’t take into account the amount of food eaten. It’s often criticized for this reason (1).
To solve this, the glycemic load (GL) rating was developed.
The GL is a measure of how a carbohydrate affects blood sugar levels, taking both the type (GI) and quantity (grams per serving) into account.
Like the GI, the GL has three classifications:
- Low: 10 or less
- Medium: 11–19
- High: 20 or more
The GI is still the most important factor to consider when following the low-GI diet. However, the Glycemic Index Foundation, an Australian organization raising awareness about the low-GI diet, recommends that people also monitor their GL.
It recommends that people aim to keep their total daily GL under 100.
You can use this database to find the GI and GL of common foods.
Otherwise, the easiest way to aim for a GL under 100 is to choose low-GI foods when possible and consume them in moderation.
Low-GI Diet and Diabetes
Diabetes is a complex disease that affects millions of people worldwide (6).
Those who have diabetes are unable to process sugars effectively, which can make it difficult to maintain healthy blood sugar levels.
However, good blood sugar control helps prevent and delay the onset of complications, including heart disease, stroke and damage to the nerves and kidneys (7, 8).
A number of studies suggest that low-GI diets are effective at reducing blood sugar levels in people with diabetes (9, 10, 11, 12).
One study in nearly 3,000 people with diabetes looked at the effects of low- and high-GI diets on participants’ levels of glycated hemoglobin (HbA1c). Levels of this molecule are an average measure of blood sugar levels over a three-month period (13).
The study revealed that HbA1c levels were 6–11% lower in those consuming the lowest-GI diets (GI 58–79), compared to those consuming the highest-GI diets (GI 86–112). In other words, the lower-GI diets were associated with lower blood sugar levels over the long term.
What’s more, a number of studies have reported that higher-GI diets may increase the risk of type 2 diabetes by 8–40% (14, 15, 16).
A systematic review of 24 studies reported that for every five GI points, the risk of developing type 2 diabetes increased by 8% (17).
The low-GI diet may also improve pregnancy outcomes in women with gestational diabetes mellitus (GDM), a form of diabetes that occurs during pregnancy.
A low-GI diet has been shown to reduce the risk of macrosomia by 73%. This is a condition in which newborns have a birth weight over 8 pounds and 13 ounces, and it’s associated with numerous short- and long-term complications for the mother and baby (18).
Studies have shown that the low-GI diet may also have other health benefits:
- Improved cholesterol levels: Low-GI diets have been shown to reduce total cholesterolby 9.6% and LDL cholesterol by 8.6%. LDL cholesterol is associated with an increased risk of heart disease and stroke (19, 20, 21, 22).
- May help you lose weight: Low-GI diets have helped healthy adults lose 1.5–4.2 pounds (0.7–1.9 kg) over 5–10 weeks. The availability of research on the effects of weight loss over the long term is limited (19, 23, 24).
- May reduce the risk of cancer: People who consume high-GI diets are more likely to develop certain types of cancer, including endometrial, colorectal and breast cancer, compared to people on low-GI diets (25, 26, 14).
- May reduce the risk of heart disease: A review of 37 studies found that people on high-GI diets were 25% more likely to develop heart disease than those on low-GI diets. Further evidence is required to confirm these associations (14, 27).
Foods to Eat on a Low-GI Diet
There’s no need to count calories or track your protein, fat or carbs on the low-GI diet.
Instead, a low-GI diet involves swapping high-GI foods for low-GI alternatives.
There are plenty of healthy and nutritious foods to choose from. You should base your diet on the following low-GI foods:
- Bread: Whole grain, multigrain, rye and sourdough varieties
- Breakfast cereals: Porridge made with rolled oats, bircher muesli and All-Bran
- Fruit: Such as apples, strawberries, apricots, peaches, plums, pears and kiwi
- Vegetables: Such as carrots, broccoli, cauliflower, celery, tomatoes and zucchini
- Starchy vegetables: Carisma and Nicola potato varieties, sweet potatoes with an orange flesh, corn, yams
- Legumes: Examples include lentils, chickpeas, baked beans, butter beans, kidney beans
- Pasta and noodles: Pasta, soba noodles, vermicelli noodles, rice noodles
- Rice: Basmati, Doongara, long-grain and brown rice
- Grains: Quinoa, barley, pearl couscous, buckwheat, freekeh, semolina
- Dairy: Milk, cheese, yogurt, custard, soy milk, almond milk
The following foods contain few or no carbohydrates and therefore do not have a GI value. These foods can be included as part of a low-GI diet:
- Meat: Including beef, chicken, pork, lamb and eggs
- Fish and seafood: Examples include salmon, trout, tuna, sardines and prawns
- Nuts: Such as almonds, cashews, pistachios, walnuts and macadamia nuts
- Fats and oils: Including olive oil, rice bran oil, butter and margarine
- Herbs and spices: Such as salt, pepper, garlic, basil and dill
To search for foods not found on this list, use this GI search tool.
Foods to Avoid on a Low-GI Diet
Nothing is strictly banned on the low-GI diet.
However, try to replace these high-GI foods with low-GI alternatives as much as possible.
- Bread: White bread, Turkish bread, bagels, naan bread, French baguettes, Lebanese bread
- Breakfast cereals: Instant oats, Rice Krispies, Corn Flakes, Cocoa Krispies, Froot Loops
- Starchy vegetables: Désirée and Red Pontiac potatoes, instant mashed potatoes
- Pasta and noodles: Corn pasta and instant noodles
- Rice: Jasmine, Arborio (used in risotto), Calrose and medium-grain white rice
- Dairy replacements: Rice milk and oat milk
- Fruit: Watermelon
- Savory snacks: Rice crackers, corn thins, rice cakes, pretzels, corn chips
- Cakes and biscuits: Scones, doughnuts, cupcakes, cookies, waffles, pikelets
- Extras: Jelly beans, licorice, Gatorade, Lucozade