The Glycemic Index ~ It Was Supposed to Be About Carbs!
As I have quite a few tangential thoughts going on, in search of a cohesive theme here, I've decided I'd just throw up a few short posts, and perhaps come back and put them together at some later date.
The Glycemic Index is credited to Canadian researcher David Jenkins, and originated, near as I can tell, with this paper: Glycemic index of foods: a physiological basis for carbohydrate exchange.
INTRODUCTION: Recent work has suggested that the carbohydrate exchange lists that have regulated the diets of many diabetics for over three decades may not reflect the physiological effect of foods. Such factors as food form, dietary fiber, and the nature of the carbohydrate have been shown to have a marked influence on the postprandial glycemia and allowances cannot be made for these in lists which take into account only the available carbohydrate content of foods. Currently, very good blood glucose control has been advocated for diabetics to reduce the incidence of long term complications. We have, therefore, fed a range of commonly eaten foods to healthy volunteers so that physiological data on the blood glucose response in man could be obtained to supplement tables based solely on chemical analysis.
DISCUSSION: The results demonstrate great inequality in the extent to which different carbohydrate sources raise the blood glucose and indicate that simple carbohydrate exchanges based on chemical analysis do not predict the physiological response.I'm going to bullet-point and break up the discussion with some paraphrasing:
- Differences Within Classes of Foods: Cereals ranged from 72% for wholemeal bread while wholemeal spaghetti was 42%. Root vegetables ranged from 97% for parsnips to 48% for sweet potatoes.
- No Significant Relationship with Fiber: "Surprisingly". May have been due to the fact that wheat fiber has little impact on glucose, little difference seen between wholemeal and white counterparts.
- Legumes: Legumes "were remarkable in how little they raised the blood glucose." Mean GI for legumes about half that for grains.
- Sugar: Sugar content was not related to blood glucose response even though absorption may have been more rapid. This is presumably due to the very small rise (20%) produced by fructose and reflected in the response to sucrose.
- Fat & Protein: Both fat and protein showed a significant negative correlation with glycemic index. Fat delays gastric emptying, protein stimulates insulin secretion. "However, it is not clear whether these actions or a direct effect of fat and protein in reducing the digestibility of food were responsible for the negative correlation." It may not be as simple as just fat or protein content.
- Starch: Research has shown that starch type matters.
- Preparation: Cooking method and time may lead to some GI variations.
THE GLYCEMIC INDEX WAS ABOUT DIGESTION/ABSORPTION RATE OF DIETARY CARBOHYDRATE, AND THE RESULTING BLOOD GLUCOSE PROFILE.
I capitalized and bolded that to shout at you for a reason. The GI was not intended to be a diet gimmick. It was intended to help diabetics better control their blood sugar. The GI was standardized to a 50 g carbohydrate dose, and fat and/or protein were only considered for their impact if they went along with the carb in a particular food/preparation.
Whatever the utility of the GI, and I do believe that for diabetics there is some utility vs. straight exchanges, it has become hopelessly clouded by diet gimmickry. Furthermore, the progression of Type 2 diabetes and varying hormonal responses in those we would call "prediabetic", makes the GI somewhat of a crapshoot for these people in managing blood glucose levels. The GI was never about the blood sugar roller coaster. Likewise, it was never about some advantage for normal (non-diabetic) people to having some sort of "stable" blood sugar vs. "spikes".
So where I'm going with this is two-fold. First, I want to discuss the anomaly that is the hypoglycemic blood sugar "crash" and its relevance to real life eating. Secondly, I want to stress that a lower "glycemic" response to protein and/or fat is NOT in keeping with the spirit of the glycemic index. Too often "low GI" is used to describe a meal that is simply low carb (or lower in carb) -- essentially a low glycemic load (GL). Whether or not GL is important for health is a separate issue, but GI comparisons need to be macronutrient matched. Jenkins et.al. *did it right* in this short, small study (6 normal men, 2 weeks per diet arm): Metabolic effects of a low-glycemic-index diet. I've chopped together the dietary composition and glucose responses.
As you can see, the diets were well matched to control well for JUST the glycemic effect. Now look at the three post-meal glucose profiles. Perhaps glucose dipped between meals in the regions "split" out, but I would note the following:
- Pre-meal glucose is identical between diets for each meal
- Post-meal glucose is slightly more for high GI at breakfast, but returns to baseline in the same time frame
- Post-meal glucose is virtually identical between high and low GI for the lunch meal
- Post-meal glucose profiles differ most significantly after the evening meal and if anything, you have extended hyperglycemia with the high GI meal.
The plots don't extend past 90 minutes, so some "crash" is not ruled out, but there seems to be no indication of these hypoglycemic dips, and there would be a longer way to go after the evening meal. What exactly the evening results mean I'm not sure. I'm reminded of a study where folks lost more weight eating their carbs at night. Relevance? Don't know.
Let's contrast this study with one that is often cited in GI literature and in low carb advocacy circles: High Glycemic Index Foods, Overeating, and Obesity. It has become clear that it is America's GI Man (as I've dubbed him) who is most responsible for trying to tie the glycemic index to obesity. It is his schtick. I've talked about this study before, in A Matter of Control. It is an example of a poorly controlled study. This study impressed J. Stanton so much he wrote:
This article could easily be subtitled “The Study That Tells You Everything You Need To Know About Insulin, Blood Sugar, Carbohydrates, Satiety, And Obesity”. Yes, I admit to a degree of hyperbole—but this study is so well instrumented and controlled, and its results so informative, that I believe it’s important for everyone to read it.
If only that were true. The more I look at the details of that study, the more troubling it becomes. The study is bad enough, but how it is used, by Ludwig himself and others, is downright maddening. Case in point, in his definitive 2002 paper, The glycemic index: physiological mechanisms relating to obesity, diabetes, and cardiovascular disease, while discussing satiety and GI, Ludwig wrote:
For example, obese children were given high-glycemic index instant oatmeal or low-glycemic index steel-cut oats with identical energy and macronutrient content at breakfast and lunch, and ad libitum energy consumption was monitored throughout the afternoon. Energy intake was 53% higher after the high- compared with the low-glycemic index meals.
WOW. Sounds ominous! No wonder things are so bad!! But in contrast to Jenkins' study, all of the GI meals were not well matched. Furthermore, the conditions were rather more than a bit artificial. They took twelve, almost 16 year old obese boys, averaging 234 lbs, and did the following:
- The night before they were checked into the hospital and fed a "standard low GI meal" of 18.5% of RMR around 6 pm and given a 5% RMR fruit and cheese snack around 10 pm. This amounts to 386 + 104 = 490 calories.
Let's stop right there. This meal was not a usual meal for these kids, it was high protein (30%), moderate carb (40%), "low" fat (30%). Why? Wash-outs and run-ins are supposed to be something "usual". You're giving these boys less than one-quarter of their baseline RMR for dinner and snack? This is around 15% of their TDEE. For dinner! Absurd.
- On Test Day morning ~ 7 am, they get a test meal, consisting of, again, 18.5% of RMR calories (1.63 MJ = 386 Calories).
Sorry, but there is unforgivable confounding in these meals. The low-GI meal is not a carbohydrate meal. This meal contains almost twice the protein and 50% more fat than the other two meals. This is a comparison of glycemic load masquerading as glycemic index, and even if it were GL, that should standardize total carb content of the meal. According to the researchers, however:
This meal (a vegetable omelet and fruit) contained more protein and fat and less carbohydrate than did the high-GI meal and was designed to increase the range of GI in the study beyond that which could be achieved by manipulating only food structure and carbohydrate type (as with the high- and medium-GI meals)
In other words, beyond which any choice of carbohydrates could reasonably produce in the real world. They had to include this ridiculously mis-matched meal to make their point. To be fair, Ludwig wasn't referring to this meal in his review article, he was actually comparing the mid to high-GI.
The hormone and hunger responses were reported only after the breakfast meal. So keep in mind that these 230 lb teenage boys, after having a measly 490 cals for dinner the night before, were fed an equally measly 386 calorie breakfast some 13 hours later (oh wait, they did have about 100 of the night time calories at around 10 pm). The "low-GI" meal contained roughly 30 grams of protein -- not a bad amount -- while the oatmeal meals contained roughly half that much. So with the unfair matchup as it was, here are the results:
Apparently all went "hypoglycemic" by 4 hours after the meal, with the high-GI meal producing about a 10 point greater depression in glucose levels. Unfortunately, baseline levels aren't given so one has no way of knowing if these levels were anywhere near true hypo. Look at that hunger rating. We see a significant drop in hunger from the higher protein (assuming time 0 is right after eating). I would note that for all meals, insulin is back to baseline at 4 hours at which time glucose levels off. But comparing the two oatmeal meals, there was no difference in insulin, lower glucose and no difference in hunger in that "crash" phase where refined foods are supposed to be at their most dastardly.
- At noon, they were given lunch, the same test meal as breakfast, another 386 calories. The two oatmeal groups are now up to only 30 g protein for the day, while the "low GI" group has had 60 g.
- They were then encouraged to engage in relatively quiet activities and ask for an ad libitum meal plate when they were "very hungry". Time to request and amount eaten were recorded.
- The ad libitum platters are described as a mix of foods but there is no indication of the overall macronutrient ratio, nor is there any indication what types of foods were consumed. That might have been interesting.
Is it any surprise to anyone that all of the boys ate a lot following this semi-starvation? (Within 5 hours after noon). Ad libitum intake ranged from 765 to 1385 calories! The high protein group took longer to get hungry. Who knew? Perhaps the take away from a study like this is that if you want to restrict calories in obese teenage boys, keep their protein intake relatively normal. Other than this, one can try to read more into this study than is there ... clearly Ludwig has. Neither oatmeal meal was a normal meal. The mid-GI was sweetened with fructose (so much for 30 g of that lighting up the reward centers and causing uncontrolled eating!). The high-GI with dextrose (glucose) AND lactase treated milk. Since they are so dastardly, why not test potatoes. Something that a kid, or anyone for that matter, *might* eat as a meal.
By now, everyone should know where Ludwig is coming from:
A potential adverse consequence of the decrease observed in mean fat intake in recent years is a concomitant increase in dietary GI. A reduction of dietary fat tends to cause a compensatory increase in sugar and starch intake.
Too bad this is NOT what he tested in his study. If you want to test this, keep protein constant at around 15-7% (as it has been at for decades in the American diet) and have four groups of all combos between high vs. low GI, and "high" vs. "low" fat (which would necessarily be lower or higher in carb). I use the quotes because you will have to compare 40% to 30% fat, because that is a stretch of what may have happened. Assessing hunger? Try using normal caloric levels for the meals as well. Just a thought!
It may seem like I'm going on a bit of a tangent here, but this is in sync with the evolution of the whole carb-insulin hypothesis (or what I call TWICHOO). It seems the advocates are retreating to the fall-back position of refined carbohydrates as culprit. Which in some ways, per Brand-Miller's work, is how the GI journey began in terms of obesity and type 2 diabetes.
While everyone else seemingly moves on, a "low GI" diet will again be tested in groundbreaking research funded by NuSI! This time protein will be kept constant at 20% while fat:carb will range from 65:15 to 40:40 to 20:60. Too bad they won't match the food quality between diets to just look at the macros. Too bad their Low GI diet is misnomered. No, despite the fact that LF is to emphasize veggies and whole grains and limit sugar, the LGI will replace some refined carb (how much will there be?) with legumes and fruits (it's not that any low fat diet ever was heavy on legumes, and/or fruit, right?) As such, where there could at least be some information gleaned from the middle group, it will be confounded from the get-go ... which makes one wonder why NuSI chose Ludwig's group for their most expensive ($14 million) study. Oh ... and don't look now, but they are primarily looking at energy expenditure -- aka the elusive metabolic advantage. It is strange that this was the outcome of Ludwig's 2006 devised study published in 2012, when we are still hearing about satiety and blood sugar swings and insulin trapping energy. Make up your minds?