Why A Food’s Glycemic Index and Glycemic Load Are Only Half The Story

By Vesanto Melina, MS, Registered Dietitian, www.nutrispeak.com


Glycemic index (GI) is a measure of the effect of 50 grams of carbohydrates from a specific food on blood sugar levels. Carbohydrates that are quickly digested release their sugars into the bloodstream rapidly have a high GI.

Glycemic load (GL) is calculated by multiplying the GI by the grams of carbohydrate in a serving of the food and dividing the total by 100. A low GL is between 0 and 10, a moderate GL is between 11 and 19, and a high GL is above 19.

Limitations of Glycemic Index and Glycemic Load

Although watermelon has a GI of 72, a “serving” of watermelon (meaning a half cup of the red section that is eaten) provides only 6 grams of carbohydrate and therefore has a GL of 4, which is low.

Note that this calculation is done using the weight of carbohydrates in a serving, not the weight of the serving. Of course people are quite likely to eat more than this at a time, but the GL will still be low.

The important point is that the total amount of carbohydrate in a food is just as important as its GI in determining its impact on blood sugar.

GI has sometimes been used to judge the healthfulness of foods. Unfortunately, it conveys nothing about a food’s total nutritional content, or any harmful contaminants, or products of oxidation that may be present.

Foods that contain little, if any, carbohydrate have a very low GI and a negligible Glycemic Load (GL).

For example, meat – even processed meat like Spam, or deep-fried pork – has a small impact on blood sugar (a low GI). However, it has the potential to significantly increase insulin resistance and to have adverse effects on blood glucose control over the long term.[1-2]

Unhealthy choices of carbohydrate-rich foods can result from over-reliance on their GI. The GI of potato chips is lower than that of baked potatoes because all that added fat does slows down the increase in blood sugar.

Other unhealthful snacks, such as candy bars, cupcakes, and ice cream, can also fall within the low-GI range. In contrast, plenty of nutritious, higher carbohydrate whole foods, such as some fruits, starchy vegetables (sweet potatoes), and whole grains, have a relatively high GI (like watermelon) or GL (like brown rice).

Generally foods aren’t eaten alone, but in combinations. The combination can have a profound effect on the meal’s overall glycemic impact.

For example, baked potatoes have a high GI and GL. However, when eaten with the skin and accompanied by legumes or vegetables – such as vegetarian chili, lentil loaf, or kale salad – a potato’s sugars are absorbed more gradually and the potato’s GI is blunted.

In addition, the body benefits from the many other nutrients, including vitamin C, that baked potatoes contain.

Although relying on a food’s GI and GL to make food choices has limitations, these indicators are helpful when appropriately used. For instance, compare the GI and GL of similar foods or foods of the same category:

Food Glycemic Index Glycemic Load
Rolled Oats 55 13
Instant Oatmeal 79 21
Cornflakes 81 20
Barley, cooked 28 12
Millet, cooked 71 26
“Original” Rice Milk 86 23
“Original” Soy Milk 34 5

Vegetarian GI and GL

Relative to the diets of nonvegetarians, vegetarian and vegan diets typically have a low GI and a low to moderate GL. This factor may contribute to the reduced risk of heart disease and type 2 diabetes in vegetarians, compared with non-vegetarians.

Foods Missing from GI Lists

Some foods are generally not included in GI lists because they lack carbohydrates (e.g. meat, poultry, and fish) or they don’t contain enough carbohydrates to make the GI test practical.

Non-starchy vegetables, such as leafy greens, broccoli, cauliflower, celery, peppers, and cucumbers, are good examples of the latter. To get 50 grams of carbohydrates from chopped broccoli, the test would require eating almost nine cups.

Factors Affecting Glycemic Index

  1. Type of monosaccharide present. Glucose has a much greater impact on blood glucose than does fructose. Agave (GI 13; GL 1) contains mainly fructose, but this doesn’t make it a more healthful choice.
  2. Type of starch present. The two principal and common starches in foods – amylose and amylopectin – are digested at very different rates. Amylopectin, which constitutes about 70 percent of the starch in foods, is rapidly absorbed into the bloodstream and thus has a higher GI; amylose is digested more slowly. Various strains of brown rice differ greatly in amylose and amylopectin content and thus GI can range from 50 to 87. Some varieties of brown rice (low in amylose) have higher GIs than some varieties of white rice (high in amylose).
  3. Amount and type of fiber present. Fiber generally reduces the GI of a meal, however foods rich in viscous fiber (i.e., beans and barley) reduce the meal’s overall GI to a greater extent than foods rich in nonviscous fiber, such as wheat bran. In addition, the GI of high fiber foods and generally lower than their refined counterpart. Thus a whole grain is likely to have a lower GI than the same grain, refined.
  4. Physical barrier. Beans and whole grains are surrounded by a coating of fiber that serves as a physical barrier to protect the seed. Because this barrier makes it more difficult for enzymes to digest the grain, these foods have a lower GI.
  5. Ripeness. As foods ripen, their starches turn into sugars, increasing their GI.
  6. Exposure to heat. Raw foods have a lower GI than the corresponding cooked foods. Cooking breaks down plant cell walls, increasing the rate at which its starches and sugars are absorbed by the body.
  7. Particle size. Small food particles have more surface area than the same amount of food in big pieces; this allows more rapid digestion and absorption. Thus intact whole grains have a much lower GI than ground grains, whole fruits have a lower GI than fruit sauces or juices, and mashed beans have a higher GI than whole beans.
  8. Density. Foods that contain less air have a lower GI than light and fluffy foods. Puffing grains also dramatically increases their GI. White bread has a higher GI than dense white pasta.
  9. Crystallinity. Raw starch is crystalline, with molecules that are organized in a sequence that repeats. Cooking disrupts this structure, making the starch more digestible and resulting in a higher GI. However, as the cooked starchy food cools, the starch recrystallizes to some extent, resulting in a lower GI. Thus red potatoes, cubed and boiled in their skin, have a GI of 89. Refrigerated overnight and eaten cold the next day, the same potatoes have a GI of 56.
  10. Acidity. Adding an acid, such as lemon juice or vinegar, to food reduces its GI. Even small amounts of vinegar (less than an ounce) have been shown to reduce GI by about 30 percent. Fermentation produces acid, yielding foods with a lower GI. Yogurt has a lower GI than milk, and sourdough bread has a lower GI than regular bread.


  1. [1] Davis B, Melina V. Becoming Vegan: Express Edition. Book Publishing Co, 2014.
  2. [2] Davis B, Melina V. Becoming Vegan: Comprehensive Edition – The Complete Reference to Plant Based Nutrition” Book Publishing Co, 2014.

15 Comment(s)

  1. What about the sugars in kombucha and kefir? I know the sugars are converted, but how much is left, is it still a large amount?

    Velvet | Reply

  2. Velvet — When you ferment something with sugars the bacteria will eat more and more of the sugars over time. The more fermentation time, the less sugar is left.

    I personally believe that GT’s Synergy Kombucha has no refined sugar left because it doesn’t spike my blood sugar. I have terrible reactions to even very, very tiny amounts of refined sugars, and I don’t have that reaction to their kombucha. Some brands add MORE sugar AFTER fermentation to make it sweeter for the consumer. I do not consume those brands.

    When I brew my own kombucha I make sure I wait a full two weeks, at least, to let the SCOBY convert all of the sugar I put in. (I use a half cup of sugar for a half gallon of black tea.)

    I’m not as familiar with kefir, since I don’t personally make it, but the same rule applies. Check to be sure that the brand you buy does not add MORE sugar AFTER fermentation!

    Raederle Phoenix | Reply

  3. Hi Velvet — for kombucha, the glycemic load or index will depend on the type of sugar used to feed the culture, e.g. cane sugar, molasses, honey, maple syrup, brown rice syrup, agave, stevia, xylitol, etc. The nutritional analysis of store-bought kombucha is one gram of sugar per 100g.

    Similarly for kefir, the sugar depends on the substrate, e.g. is sugar water, fruit juice, coconut water, goat milk or cow milk your base for the kefir? Clinicians report that coconut water is best for patients with digestive issues — flatulence, bloating, bacterial overgrowth, Candida, insufficient beneficial bacteria — and the sweet cravings that go with these digestive issues, as well as those with skin problems.

    Generally speaking, all ferments are good for our microbiome, but avoid the sugar-based ones if you’re diabetic.

    Trevor Justice | Reply

  4. From reading this article, is it correct to think that potato salad made with cold potatoes and a dressing containing vinegar would have a low to moderate GI? What might be the approximate ratio of how much vinegar is needed to how much potatoes to significantly reduce the GI?

    marlene | Reply

  5. Hi Marlene — potatoes have a fairly high GI no matter which way you slice it! Yes you can reduce GI by cooling them so the starch re-crystallizes slightly or by adding vinegar to slow down the starch absorption. But in reality, how much vinegar or lemon juice can you add to potato salad?

    My own view is that cooked potatoes will retain their moderate to very high GI (depending on the variety). Cooling and acidifying them will not make that big a difference. It’s when bread goes stale, that’s when the starch crystals have clearly relinked to each other. So long as the potatoes are soft — as they are even in cold potato salad — they’ve clearly not recrystallized much.

    Crystallized starch is hard. Think of a brown rice kernel. Breaking down the crystals make the starch soft. Think of cooked brown rice.

    The glycemic response is different from one person to another, and also in the same person from day to day, depending on blood glucose levels, insulin resistance, and other factors.

    Trevor Justice | Reply

  6. What is the GI of baked goods with coconut flour, almond flour or tapioca flour? I have an autoimmune thyroid condition and am supposed to stay away from beans and grains. Margaret

    Margaret | Reply

  7. Hi Margaret — GI is a measure of how quickly the available carbohydrates in a food impact your blood sugar level. The more water or fiber in a food, the less available are the carbs, and hence the slower their impact. Flours have the same fiber as the whole food, but of course no water. So logic dictates they’d impact your blood sugar quicker than the whole food.

    However, if the food itself is low GI, then the impact of the flour too is minimal. Almonds and coconut are both low GI (below 55) because they’re more protein and fat, than starch.

    Tapioca is made from a root known as cassava or yucca. Hence, like any root, it’ll be richer in carbs than in protein or fat, and will have a higher GI — it’s GI rating is 85.

    Trevor Justice | Reply

  8. Dear Trevor, thank you so much for this information. I appreciate all you are doing to help people, including myself. I am vegan, and do not eat processed and refined foods, or any meat kine. On rare occasions, I may eat an egg(white only). I would really like to put on weight, as the healthy meals I eat do not put on weight. Thank you for your response, and help. I am interested to get raw food recipes, and healthy cooked recipes, as well. I do purchase a few vitamins, and multivitamin.

    Abigail Taylor | Reply

  9. Hi Abigail — the number one problem with eating a fresh-produce vegan diet is that people don’t eat enough calories! You need to eat a lot more than you used to when you ate pizza, chips, meat and other high-calorie foods. Eat more “filler foods” that are higher in calories, such as raw fruit, nuts, seeds, avocado, and cooked grains and beans. Vegetables are very low in calories. Most people eat more caloric-dense foods like nuts, seeds & dried fruits at the start, then find themselves filling up on fresh fruit (rather than dried fruit or nuts) as their body cleanses.

    Strength-training exercises are a good way to gain muscle and weight. Muscle tissue weighs more than fat. As you add more lean muscle to your body, you’ll likely put on more weight. Begin by lifting weights and doing exercises that work major muscle groups, such as your chest, back, legs, arms, and abdomen.

    Trevor Justice | Reply

  10. Hi, why do I feel dizzy RIGHT AFTER eating /drinking ex a (green) smoothie (I ALWAYS have it when I have a smoothie with berries and cocomilk). Do we have to keep the fruit away from the fats or combine them? I eat a low fat diet. Maybe the 5 nuts a eat/day or the dressing with my salad is to much… I would like your opinion : fruit = mono meal or combining with fat (stabilize bloodsugar) or just answer “why I could feel dizzy right after (or even during!) a (slow drinking) smoothie.

    ilse | Reply

  11. Hi Ilse,

    > why do I feel dizzy RIGHT AFTER

    The dizziness could be any number of reasons based on your unique biochemistry. It’s more common after drinking a LOT of green juice, than it is after drinking green smoothie. With juice, it’s too much sugar hitting your bloodstream at once. But smoothie has fiber in it so this would slow down the rate at which the fruit sugar hits your blood. If it continues, try eating a smaller quantity of smoothie at a time, at intervals throughout the day, rather than a lot all at once. Let your body adjust to it slowly. You can keep the green smoothie refrigerated in a sealed opaque glass jar for up to 8 hours.

    > Do we have to keep the fruit away from the fats

    Again, it’s how robust your biochemistry is. As a general rule, it’s best to eat fruit and fat at separate times because fat is a sticky molecule that clumps together in your blood and blocks the insulin receptors on your cells. As a result, insulin can’t shuttle the fruit sugar into your cells. The sugar stays too long in your blood, which can lead to blood sugar problems and candida.

    > Maybe the 5 nuts a eat/day or the dressing with my salad is to much

    This is minimal fat! Fat can be 10% to 30% of your total calorie intake, depending on your unique biochemical needs. You may want to eat more Omega-3 seeds, such as flax, chia and hemp, rather than nuts which are monounsaturated and more acidic than seeds. A healthy balance of both is best.

    valarcher | Reply

  12. Hi Trevor – a request for the website listing I believe, 50 links to information on recordings on many topics on nutrition, one can download. I hope I am accurate in my request and that you know what I am referring to. You mentioned the site before the talk with the nutritionist. Jo S.
    Thank you

    Susan | Reply

  13. It sounds like you’re asking how to enroll in our Mastery Program. Just go here and click the button representing your diet: http://www.veghealth.com/4-button-lp/mastery-program.php

    Trevor Justice | Reply

  14. I eat fresh fruit with pumpkin or flax seeds and yogurt and a little honey in the morning generally with a cup of coffee with milk.
    Is this a good combo?

    Michele | Reply

  15. Hi Michele – if it works for your biochemistry and you feel no side-effects such as headache, then it’s a good combo for you. Normally one would have fat with leafy greens rather than fruit. The fat lines the insulin receptors so that the fruit sugars get stuck in your blood which can lead to proliferation of candida which feeds on the sugar. Fruit is best eaten half hour before a meal. But remember, everyone’s biochemistry is unique. I have no problem ending a meal with a banana, which gives me that sweet taste I crave after eating protein like a big plate of leafy greens.

    Val Archer | Reply

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