The glycemic index (GI) is the best studied dietary factor in acne. To date, five studies have looked at the role of low GI/GL diet in acne. All have shown some degree of improvements both in acne severity (as measured by the number of pimples) and distal acne causes (mainly various hormones that affect acne).

I won’t go over the studies here as I don’t think these studies show what many people believe them to show. We’ll get back to these studies at the end of this section when we see what to make of all this.

The glycemic index (GI) is a numerical scale indicating how much and how fast a particular food raises your blood sugar levels. The higher the GI number, the more and faster the food increases blood sugar levels. A food with low to moderate GI only prompts a moderate rise in blood sugar levels and is thus better for the skin. A food with high GI may spike your blood sugar and the hormones linked to acne.

The glycemic load (GL) tries to correct some of the problems with the GI and also takes into account the amount of carbohydrates in a given food. The GL is calculated by multiplying the food’s GI with the amount of carbs. An example, watermelon has a high GI but few carbohydrates, so the overall GL is quite low. A whole wheat bread may have a low GI but had much more carbohydrates per serving, so its GL is much higher than for watermelon.

So it would make sense that eating foods with low GI/GL helps to reduce insulin resistance and lower many of the hormones linked to acne. Or so the theory goes.

However, scientists are starting to seriously question the value of choosing foods based on their GI/GL ratings.

This classification system [GI/GL] has been promulgated for use as a tool to guide food choices to reduce chronic disease risk. However, considerable controversy exists about the utility of supplementing current population-based dietary recommendations with specific guidance for the GI or GL value of foods.

Kristo, A. S., Matthan, N. R. & Lichtenstein, A. H. Effect of diets differing in glycemic index and glycemic load on cardiovascular risk factors: review of randomized controlled-feeding trials.Nutrients 5, 1071–80 (2013). https://www.ncbi.nlm.nih.gov/pubmed/23538939

In 2013 Austrian researchers published a study looking at the long-term effect of low vs. high GI diets. Their review only included studies that lasted for six months or more. They found:

• Low GI/GL diets reduced insulin resistance and insulin levels more than high GI/GL diets.
• Low GI/GL diets resulted in lower levels of systemic inflammation as compared to high GI/GL diets

If you’ve been following diet and health media, this is nothing new. Low GI and GL are touted as the solution to many health problems.

However, it turns out that GI and GL might not matter that much.

A review published in 2013 looked at all the studies where all the foods were given to the study participants. This is important because it allows the researchers to hone in on the effect the GI/GL has. In many studies, the participants are asked to make dietary changes that should lower the GI/GL, but it’s difficult to know how well the participants followed the guidelines. Furthermore, when people are asked to lower their dietary GI/GL, they often end up eating more protein and an overall healthier diet.

So studies where all the foods are given to the participants are considered as high quality – and more reliable.

This paper concluded that there’s no good evidence to show lowering the GI/GL has any effect on blood sugar, insulin, or systemic inflammation. Read that again, a paper that reviewed the highest quality studies concluded that the GI/GL doesn’t matter.

Similarly, a 2014 review on the effect of carbohydrate quality on inflammation showed no consistent effect. This paper looked at 13 low GI/GL vs. high GI/GL studies and how they affect systemic inflammation. 3 of the 13 studies showed a reduction in low GI/GL group vs. high GI/GL, 4 showed a possible effect, and 6 showed no difference between the low and high GI/GL groups.

How can three papers come to such different conclusions? I think there’s a simple explanation to this. Low GI/GL foods, in general, are healthier and less processed. What those studies probably show is the effect of eating more wholesome and less processed food. They are confusing correlation with causation. Because studies that control the macronutrient ratio, caloric content, and the degree of processing show dietary GI/GL itself doesn’t matter that much.

I’m not saying it’s completely useless concept. A study published in 2013 showed that lowering dietary GI can be somewhat helpful in reducing acne-related hormones. The researchers asked 21 women with PCOS to complete two dietary phases:

• Normal diet that maintained current weight
• Low GI diet with the same amount of calories as during the normal diet phase

During the low GI diet phase, the women were asked to substitute high GI foods (like white bread) for low GI foods (such as multi-grain bread). After 12 weeks on the low GI diet, there were moderate improvements in insulin levels. Insulin sensitivity improved by 19% and fasting insulin levels dropped by 14%. This, despite the fact that the participants didn’t lose any weight and ate the same amount of calories. Also, the dietary GI dropped only 11%, so the women didn’t make drastic changes. The biggest seems to be 18% reduction in sugar intake.

Overall, this study showed that reducing dietary GI (and sugar intake) does reduce acne-related hormones.

Take-home message and recommendations

Let’s circle back to the studies that showed low GI/GL diets improved acne. None of them adequately controlled the macronutrient ratio between different study groups. It was always that the high GI group not only ate carbs with higher GI but they also ate more carbohydrates overall. The low GI diets tended to be somewhat higher in protein. Finally, the high GI groups often ate foods that were more processed.

As such, these studies don’t show that the GI/GL is linked to acne. These studies show, eating fewer and healthier carbohydrates reduces acne.

Given this, I don’t think it makes sense to pay too much attention to the GI/GL ratings. Rather, focus on eating real food. Minimally processed food. And don’t avoid real, whole foods just because it has a high GI.

I may sound like a broken record, but that’s because there’s no point to overcomplicate this diet thing.