That title is half a fiber joke and half pointing to the need for a Nutrition Fact Check website (similar to the politics version). Fiber also happens to be a great way to slow down digestion, to reduce the surge of blood sugar following a higher carbohydrate meal and maintain satiety for longer (1).
I see/hear a lot of things about Glycemic Index and many of them are not factual, misguided or simply untrue. The most common myth I've heard lately is that all whole grains have the same Glycemic Index value as processed grains (it varies a lot, some do , some don't - see below). Besides the fact that one should be using the Glycemic Load and ditching the Glycemic Index, it's hard to substantiate those rumors - I don't like to call out web sites, simply type into google "whole wheat same GI as white wheat" if you want to see the numerous websites passing around this myth.If you would like a reliable source regarding general Glycemic Indices, try this Harvard site - even if your food is not on there, you can get an idea of what the glycemic effects will be. You can see the differences on their chart between different types of whole grain products and their processed equivalents - and that a difference does, in fact, exist. But how do you interpret those differences?
There is going to be variability in the glycemic response of different whole grain products, even before they get to the supermarket - the amylose:amylopectin ratio (2) plays a key role in digestion of starches - amylose tends to be more resistant. The types of fibers naturally occurring also matter (3). Then, whatever your grain product is has to be processed in some way (wheat kernel does not come in the shape of pasta). The grain has to be pulverized, and the processors can choose to remove the germ and the bran if they want ( making it a refined grain). This pulverization increases the surface area of the molecules that digestive enzymes can act on. This is why you might hear different values for a whole grain bread vs a whole grain pasta, etc - I have still yet to see a credible source, however, show that there isn't still a decent impact from leaving the fibers. This is part of the reason why some health movements have become more focused on so wheat berries or quinoa over a pulverized and shaped grain - they retain their intact structures and have less surface area for enzyme activity.
Other factors also come into play: how long are you cooking/baking the grain for? what is the water activity of that specific grain item, and consequent level of gelatinization of the starch? This is before we even get to the laboratory methods for testing foods - the accuracy of measurements and the protocol for doing so can differ based on lab testing, hence the variability in values reported (Powell, AJCN, 2002). There's also the issue of individual genetic response to blood glucose; as I point out in my post on amylase, this a potential component of variability in glycemic response.
It seems to me like people think the glycemic index is a measure of health, because consistently elevated levels of blood glucose are related to a number of different diseases (5,6,7,8). However, we can't confuse this with a postprandial change in blood glucose that rises and falls within a few hours. The glycemic index was originally created to help diabetics in controlling their blood sugar, not as a marker of health. Diabetes causes elevated blood sugar, but that doesn't mean elevated blood sugar causes diabetes.Regardless of the variability in the blood glucose response to food, the Glycemic Index is actually a poor measure of postprandial blood glucose because it doesn't take into account the serving size of that food typically consumed/carbohydrates per serving. It's simply 25,50 or100g of a specific carbohydrate's ability to raise blood glucose relative to pure glucose. If you follow by the glycemic index, whole wheat bread will look more unhealthy than table sugar, or watermelon will be worse than a coca-cola. Flashing red alarms should be going off in your head at this point regarding the GI's ability to predict the healthfulness of a food. This is part of the reason the Glycemic Load was developed - suddenly watermelon is absolved of it's indices' sins. If someone is talking about the Glycemic Index, I don't wanna say discredit their opinion on nutrition but definitely don't rely on that source entirely - it's a rather narrow perspective to take on health.
Even the glycemic load is only a moderately robust measure of healthfulness of a food item - if you look at low GL foods, they tend to be things we think of as being healthy e.g. whole grains, beans, nuts, vegetables, etc. But that doesn't make the GL a perfect measure of health. Again, read up on that Harvard link's page or the Powell paper, the measurement of the glycemic index/load are based solely off that one food's ability to raise blood glucose in isolation. The glycemic index of a food drastically reduces in the presence of other foods - particularly, fat, fiber and protein. These are the foundations of satiety - though not all glycemic index studies suggest reduced subjective measures of satiety from high GI meals (10). Take this study in particular (7) - the authors looked at the dose-dependent response of eating almonds alongside whitebread - it drops pretty drastically. This is where the glycemic index fails as a parameter of health - I don't think white bread is suddenly something you should be eating a lot of, as long as you have almonds paired with them. As with anything in nutrition, you need to look at the dosage, and the context of the whole diet. I harp on this a lot, but you can't look at nutrition in isolation - GI, saturated fat, grains, isolated vitamins - the list goes on; looking at aspects of metabolism/nutrition in isolation doesn't work well because they rarely work in isolation. Find me evidence that says a cup of white rice per day is going to have negative outcomes when the rest of your diet is nutritionally adequate (high in fiber, hitting the DRI's for vitamins/minerals, falling within the AMDR's). If you fancy having the occasional refined grain, it's not going to kill you, or make you diabetic, or give you cardiovascular disease.
But aren't high GI foods making you fat? The most recent review of 23 clinical trials of low GI/GL diets found very mixed evidence (11). Most of the evidence found non-significant trends towards weight loss, suggesting something else about these diets that leads to greater weight loss. My guess? Low GI/GL can mean higher protein, which has been shown to be effective for increasing satiety and losing weight (calories still count) (12, 13, 14). For my take on higher protein diets, see here. Be careful when extrapolating data from studies using the GI for weight loss in obese people to mean that everyone should never consume foods that are high GI.
UPDATE: since i've written this, more studies finding null effects have come out. See these (15, 16, 17, 18). A 2008 workshop that I stumbled onto found that the effects of Glycemic Index diets appear to be attributable independently to their fiber content (19). However, that won't stop Harvard epidemiologists from pushing that GI matters with their dietary recall data (20).
There's good reason to be concerned with controlling your long term blood glucose levels (9), if you have impaired glucose metabolism - and ensuring you get adequate amounts of protein, fats and fiber will ensure that you'll avoid problematic blood glucose spikes (10) - but relying on the Glycemic Index is narrow, relatively uninformative for the non-diabetic individual (arguably still not very informative for diabetics), and not doing anything to help nutritional sciences. There's a huge difference between demonizing someone who has their sugary breakfast cereals alongside some skim milk and oj in the morning vs. a piece of whole grain toast with nut butter, a piece of whole fruit and milk.
1. http://www.ncbi.nlm.nih.gov/pubmed/22564018
2. http://www.ncbi.nlm.nih.gov/pubmed/9430599
3. http://www.ncbi.nlm.nih.gov/pubmed/17151593
4. Powell et al, International table of glycemic index and glycemic load values: 2002
5. http://ajcn.nutrition.org/content/early/2013/01/30/ajcn.112.041467.abstract
6. http://content.onlinejacc.org/article.aspx?articleid=1138298
7. http://www.ncbi.nlm.nih.gov/pubmed/?term=almonds+glycemic+index+white+bread
8. http://www.ncbi.nlm.nih.gov/pubmed/16864756
9. http://aje.oxfordjournals.org/content/161/10/995.full#ref-2
10. http://www.ncbi.nlm.nih.gov/pubmed/22688548
11. http://www.ncbi.nlm.nih.gov/pubmed/21280171
12.http://www.ncbi.nlm.nih.gov/pubmed/11838888
13. http://www.ncbi.nlm.nih.gov/pubmed/20847729
14. http://ajcn.nutrition.org/content/82/1/41.abstract
15. http://jama.jamanetwork.com/article.aspx?articleid=2040224
16. http://www.nature.com/ijo/journal/v32/n1/abs/0803717a.html
17. http://www.ncbi.nlm.nih.gov/pubmed/17413101
18. http://ajcn.nutrition.org/content/100/1/27.abstract
19. http://www.ncbi.nlm.nih.gov/pubmed/18175760
20. http://ajcn.nutrition.org/content/early/2014/04/30/ajcn.113.079533
Comments
Post a Comment