Wednesday, April 22, 2015

Do Slower-digesting Carbohydrates Make Us Feel More Full?

One of the most common pieces of advice in the health-nutrition world is that we should focus our carbohydrate intake on slowly-digesting carbohydrates, because they make us feel more full than rapidly-digesting carbohydrates.  Rapidly-digesting carbohydrates, such as potatoes, stand accused of causing us to overeat, resulting in obesity, diabetes, and many other chronic ailments.  Is this true?

This concept is usually discussed in terms of the glycemic index.  Foods with a high glycemic index cause blood glucose to rise more than foods with a low glycemic index, and the former are usually digested more rapidly.  This measure was originally developed to help diabetics control their blood sugar levels, and it may indeed be useful for that purpose.  However, more recently it has been suggested that non-diabetics should focus on low-glycemic foods as well.

A number of single-meal studies do suggest that low-glycemic carbohydrates provide more prolonged satiety than high-glycemic carbohydrates (1, 2, 3, 4, 5, 6, 7).  Yet these studies are often difficult to interpret, because the test foods frequently differ in many ways besides glycemic index (e.g., fiber content, calorie density).  Also, a few studies have either failed to replicate the finding, or found that high-glycemic carbohydrates are actually more filling (8, 9, 10, 11, 12).

Many observational studies have shown that people who eat lower-glycemic carbohydrates tend to fare better over time.  However, again these studies are difficult to interpret, because the most common high-glycemic foods are generally highly processed, high-reward, and low in fiber.  Adding to my skepticism, nearly all longer-term (10+ weeks) randomized, controlled trials have found that the glycemic index of the diet makes little or no difference in calorie intake, body weight, or metabolic health (13, 14, 15, 16, 17, 18, 19, 20).  This is consistent with the fact that many traditional cultures have remained lean and healthy eating diets that are extraordinarily rich in high-to-medium-glycemic carbohydrate such as cassava, yams, taro, sweet potatoes, millet, rice, palm starch, and potatoes (Western Diseases. Trowell and Burkitt. 1981)

The single-meal studies that have been conducted to date have usually only focused on one food, or a few foods, which makes it difficult to generalize the findings.  What I really want to know is this: when people eat a variety of typical foods, is there any correlation between the food's glycemic index and how full that person feels?  In other words, can we increase satiety by favoring lower-glycemic foods over higher-glycemic foods in the context of a typical diet?

I finally found a study that answers this question.  It was published in 1996.

The study

Susanna Holt and colleagues recruited healthy college students, mostly lean but including a few that were overweight (21).  The research team fed the volunteers 240-Calorie portions of 38 common foods individually, and measured satiety, glucose, and insulin levels every 15 minutes for two hours after each test meal.  Test foods included many of the most common foods people eat in their everyday lives, for example, cereal, potatoes, fish, meat, fruit, nuts, pastries, rice, popcorn, and bread.

The results

I'll cut right to the chase: there was no association whatsoever between the glycemic impact of a food and the satiety it provided.

Potatoes (plain) were a remarkable outlier: they were the most satiating food of all, despite a high glycemic index.


This result shows that among a wide variety of common foods, the glycemic index doesn't predict which ones will be more filling than others, per unit calorie.  Other papers based on the exact same data set show that a food's protein content, calorie density, fiber content, and palatability are all important predictors of satiety (22), so we know this experiment was adequately designed to detect effects on satiety.

One important limitation is that the study only lasted two hours.  It's possible that differences would have emerged if the volunteers had been studied for three or four hours.

In the end, I'm not convinced that non-diabetic people benefit much from the glycemic index concept.  As always, if you find that low-glycemic foods help you control your appetite, then by all means keep eating them.  Yet for most of us, the glycemic index seems like an unnecessary complication.


Tim said...


I applaud your truly scientific-minded willingness to repeatedly question some of the favorite dogmas held by the low-carb and paleo communities, particularly given your own past involvement in those communities as a blogger and the prospect of alienating some of your former followers and earning angry reponses!

While I agree that the the study at hand shows that the GI is probably not very relevant to predict the immediate satiety response to meal, I think it should be added that it was not designed to test the effect of the overall glycemic index of a diet on long-term weight gain and other health outcomes, including effects on gene expression which may or may not be related to insulin signaling. Several observational studies have found the dietary GI to be independently associated with T2DB or even all-cause mortality:

Bhupathiraju et al. Glycemic index, glycemic load, and risk of type 2 diabetes: results from 3 large US cohorts and an updated meta-analysis. Am J Clin Nutr. 2014 Jul;100(1):218-32.
Castro-Quezada et al. A high dietary glycemic index increases total mortality in a Mediterranean population at high cardiovascular risk. PLoS One. 2014 Sep 24;9(9):e107968.

However, one should not forget to mention that other studies, notably in Asian cohorts with white rice being the major source of high GI carbohydrates, did not show such an association (or even an inverse association):

Nagata et al. Dietary glycaemic index and glycaemic load in relation to all-cause and cause-specific mortality in a Japanese community: the Takayama study. Br J Nutr. 2014 Dec;112(12):2010-7.

I think those disparate result show once again the importance of the overal dietary pattern in determining the health effects of particularly components or aspects thereof. Low-carb advocates like to ignore the fact that animal protein is strongly insulinogenic - more so than most high-GI carbohydrates and that the combination of animal protein with high-GI carbohydrates - a hallmark of Western dietary patterns - elicits an even stronger insulin response. See this video and the sources section on the website.

In my opinion, this interaction seems to offer a nice hypothesis to explain the fact that both low-carb and high-carb plant-based diets have shown sucess in treating or preventing T2DB beyond the effects of weight loss. Both provide a way out of the "macronutrient swampland" as Denise Minger coined it in her recent AHS talk.

Tim said...

Oh, and don't get me wrong: of course the interactions occuring in those dietary patters are much more complex than just the synergistic effects protein and carbohydrates have on the insulin response. Fats play an important role (in general, saturated fats tend to increase insulin resistance, while milk fat seems to have a redeeming quality) as well as micronutrients and phytochemicals.

In fact the Mediterranean diet - as well as other ancestral diets - shows that it is indeed possible to safely navigate the "macronutrient swampland". This is a cultural achievement - in contrast to those modern and extreme dietary approaches which are as "sciency" as primitive.

Caloric moderation, a moderate intake of animal food, as well as an abundance of whole, unrefined plant foods rich in phytonutrients seem to be the major, unifying determinants of such ancestral diets bestowing people with low risks of chronic diseases.

RLL said...

When all of this came out some years ago diabetics thought it might help. In theory if you have diminishing insulin production if it takes longer to digest the carbs maybe the pancreas could keep up with food becoming glucose in the blood stream.

In the real world (of diabetic blogs) many of us tried it. It didn't help. The variations of what else I eat with those carbs, how fast or slow my digestive tract works, how fast injected insulin absorbs and is 'denatured' all overwhelm alleged glycemic indexes.

jojo said...

Hi Stephan,
I have enjoyed reading your blog over the years and you have revealed some interesting new ideas.
There is something I think you have overlooked though.
Foods with a high glycemic index probably don't cause initial weight gain, however -- once a person has become overweight, leptin resistant and metabolically dysregulated. I believe that people lose their tolerance for carbohydrates and can only lose weight comfortably by choosing lower glycemic index foods or reducing carbohydrates. I describe on my blog, a theory of how, in a leptin resistant state,(of lower innate immunity)carbohydrates interact with intestinal bacteria to create inflammation and further LR and weight dysregulation.
I don't believe the satiety index has much value for those seeking to lose weight, as acute satiety is not nearly as important as long term satiety.
The foods which have the highest satiety scores are the most insulinemic foods.
For individuals with leptin resistance and thus higher secretions of insulin; these highly insulinemic foods can be high enough to create rebound ghrelin and appetite, which 'persist for the rest of the day'. I present a recent study on my blog which evidences this rebound ghrelin effect following hyperinsulinemia.
Speaking from experience; I used the satiety index as a guide for my food choices a couple of years ago and it was the most 'fattening' food strategy I have ever tried. I know a lot of people who have found the glycemic index helpful in reducing appetite. Again, I explain on my blog why I think the glycemic index is helpful for people who are trying to lose weight, as I believe it helps to reduce inflammation and Leptin resistance.
My blog is - if your interest has been piqued

Jasmin Johend said...

This last paragraph spot on Tim with tb or two of butter/evoo added to plants for satiety. As a LC'er battling to keep 100lbs weight off I was always hungry after eating a high protein meat based diet and high BS readings. It was only when I cut down the meat and added fats to plants that I was able to finally be satiated with less and beautiful post prand BS results.

Reijo said...

I agree with your overall conclusions, GI is not very useful in determining food intake or satiety. Yet another example would be fructose which has very low GI vs glucose but is still equally or less filling (perhaps due to incomplete absorption and smaller rise in blood glucose levels).

Rye seems to pose very nice "second meal effect", ie. it reduces food intake still during the second or third meal after consumption of a rye bread meal. Explanation might be rather slow colonic fermentation of indigestable carbohydrates rather than GI value itself. Many, but not all, rye breads are rather high in GI despite high fiber content. GI is very complex and elusive concept.

Laura said...

Just read another discussion of resistant starch and I wonder whether it might not be a better predictor of satiety. It was a concept I was not familiar with. I attempted to use the GI some years ago and found it not particularly useful in predicting the response my body would have to any food.

Kathleen Jade said...

Hi Stephan,

The glycemic index has largely been replaced by the glycemic load. Glycemic load seems to be much more useful than the glycemic index in terms of understanding a food's affect on satiety. Can you comment?

Kathleen Jade, ND

Michael Parish said...

Do I buy most of the satiety index? The answer is no. Why? I'll use potatoes as an example. Holt tells me that potatoes are twice as filling as beef/meat. If you spend anytime on the McDougall support board you'll find their starch based diet with lots of potatoes has them eating every three hours. The average consumption is two 11 ounce baked potatoes (580 calories) per meal. I spent a year on the plan and can tell you from my perspective 11 ounces of meat loaf (593 calories) fills me up for five to six hours while the same number of calories from baked potatoes barely gets me past three hours. I spent my entire day either preparing food, eating, or sitting on the toilet from all that fiber. There just is no way that potatoes are anywhere near as filling as the same number of calories from beef, pork or chicken. Holt needs to rethink the Satiety Index. (yes I know her study was a two hour period, but anyone who has eaten 11 ounces of meatloaf at a sitting knows it feels like a brick is sitting in your stomach and why I max out at six ounces per meal)