If you put the keywords glycemic load AND diet into Google Scholar for studies published in 2020 and 2021, you’ll get over 10,000 results. Some of those hits describe clinical trials comparing a low glycemic load (GL) diet with a high one. But what exactly do we mean by a diet with a low glycemic load? Can it mean two entirely different types of diet? The answer’s yes.
As originally defined in 1997 (1), glycemic load is the product of the glycemic index of a food and its carbohydrate content per serving. Because GI is the starting point, the food must contain enough carbohydrate in order for its GI to be measured in 25 g or 50 g carbohydrate portions, or ideally, at least 10 g of carbohydrate per serving (some highly concentrated sources of carbohydrate, like sweeteners, are exempt). This rule is set by the International Standards Organisation methodology for glycemic index testing (2).
This rule means that foods with a very low carbohydrate content don’t and can’t have a GI value and therefore a GL value. This includes meat, fish, shellfish, poultry, and dairy foods like cheese, cream and butter. So too, most nuts, avocados and vegetables like spinach and other leafy plant foods, capsicum, cucumbers, celery, etc… don’t have GI or GL values.
A diet with a low carbohydrate content is therefore not the same as a diet with a low glycemic load, i.e., one which is rich is low GI carbohydrates. Unfortunately, many foods with no or little carbohydrate such as sausages, salamis and bacon are high in saturated fats (thereby raising plasma LDL-cholesterol and the risk of cardiovascular disease). Others such as olive oil and avocadoes are high in desirable unsaturated fats (which help lower LDL-cholesterol, reducing the risk of cardiovascular disease).
Therefore, if foods are carefully chosen, a low carbohydrate diet could be a health-promoting diet, although it’s almost inevitable that it will contain more saturated fat than is currently recommended, in absolute amounts. The question is does this matter?
In the September 2021 edition of the American Journal of Clinical Nutrition, Cara Ebbeling and David Ludwig from Harvard compared 3 diets of varying carbohydrate (20, 40 and 60 percent of energy) and fat content – but with the same level of protein (20% of energy)(3). Saturated fats were approximately 1/3rd of total fats and all diets were high in fiber (25 – 35 g per 2000 calories (8,340 kilojoules)). Because diets were prepared in house, they could be fairly confident that the participants were eating exactly what was intended.
All 164 participants were required to lose around 10% of their body weight on a ‘run-in’ diet, before being randomised to one of the 3 diets for 20 weeks of weight loss maintenance. The GL of the 3 diets was 28, 80 and 135 g per 2000 calories (8,340 kilojoules).
Their findings may be surprising to some. They found that the low glycemic load diet, proportionately high in saturated fat, improved markers of insulin sensitivity and various blood lipids, without adverse effects on LDL cholesterol. These findings contrast with those of many other studies where LDL-cholesterol was increased by a low carbohydrate diet (typically high in saturated fats, but also typically low in unsaturated fats and dietary fibre) and by implication, the risk of cardiovascular disease. Interestingly, the intermediate glycemic load diet was associated with the greatest improvement in the inflammatory marker CRP, a possibly better indicator of cardiovascular risk in women than LDL-cholesterol.
However, Cara and David’s findings may not be generalisable because the participants were relatively healthy overweight adults who already had low LDL-cholesterol, and who had recently lost 9-12% of their body weight. The diets were also proportionately high in unsaturated fats and high in dietary fiber – unlike the typical North American diet.
This moderate glycemic load diet had 40% of energy from carbohydrate, close to the average level in many populations, including Australia, but an average dietary GI of 30, which is much lower than most populations averages, including Australia’s (4). This means that lowering glycemic load can be a simple process of swapping a high GI choice with a low GI choice, whether the food is rice, bread or breakfast cereal. This is easier than “cutting carbs” completely. You can find more information about low GI food swaps here.
On this moderate glycemic load diet, it is possible to include foods like legumes, pasta and yogurt that are high in carbohydrate and universally low GI, and associated with lower weight gain creep (5) and lower cardiovascular disease risk in longitudinal observational studies (6).
This more moderate glycemic load diet may also be more satiating, reducing appetite for sweet foods and the level of hunger hormones such as ghrelin that are associated with food cravings. This was found to be the case in the largest and longest randomised control trial of diet for the prevention of type 2 diabetes, the PREVIEW study (5).
In my mind, the take home message of studies to date is that a diet with a moderate glycemic load (and moderate amount of carbohydrate) based on low GI food choices is one of healthiest, easiest and most sustainable diets that we can recommend.
- Salmerón J, and colleagues. Dietary fiber, glycemic load, and risk of non-insulin-dependent diabetes mellitus in women. JAMA. 1997.
- ISO 26642:2010. Food products — Determination of the glycaemic index (GI) and recommendation for food classification.
- Ebbeling, C and colleagues. Effects of a low-carbohydrate diet on insulin-resistant dyslipoproteinemia-a randomized controlled feeding trial. AJCN. 2021.
- Kusnadi DTL, and colleagues. Changes in dietary glycemic index and glycemic load in Australian adults from 1995 to 2012. Am J Clin Nutr. 2017.
- Zhu, Z and colleagues. Dose-Dependent Associations of Dietary Glycemic Index, Glycemic Load, and Fiber With 3-Year Weight Loss Maintenance and Glycemic Status in a High-Risk Population: A Secondary Analysis of the Diabetes Prevention Study PREVIEW. Diabetes Care, 2021.
- Halton TL, and colleagues. Low-carbohydrate-diet score and the risk of coronary heart disease in women. N Engl J Med. 2006
Professor Jennie Brand-Miller holds a Personal Chair in Human Nutrition in the Charles Perkins Centre and the School of Life and Environmental Sciences, at the University of Sydney. She is recognised around the world for her work on carbohydrates and the glycemic index (or GI) of foods, with over 300 scientific publications. Her books about the glycemic index have been bestsellers and made the GI a household word.