The glycemic index, the DASH diet, and heart health - what matters?

Background about carbohydrates, insulin, and glucagon

Sugars, starches, and fibers make up the three main categories of carbohydrates. The human body digests and absorbs sugars and starches but fiber passes through the gastrointestinal tract and gets excreted in the feces. Glucose is a simple form of carbohydrate. Glucose is used by the cells and is needed by the body. Blood sugar, also referred to as blood glucose, reflects the amount of glucose present in the bloodstream.

Insulin and glucagon are hormones secreted by the pancreas. After a meal that contains carbohydrates, blood glucose levels are higher than at other times. In response to this, the pancreatic beta cells secrete insulin. Insulin signals to cells that glucose is available. Cells pick up glucose and this lowers the level of blood glucose (sugar) in the bloodstream. Once the cells have taken-up the glucose that they need, liver cells and muscle cells can store some of the excess glucose as glycogen. The muscle cells will use glucose stored in glycogen to meet their own metabolic needs. The liver, however, is unique because it sends glucose from glycogen stores out to the entire body when more glucose is needed. As time passes between meals and overnight, blood glucose levels begin to drop. In response, pancreatic alpha cells secrete glucagon (glucose gone – glucagon). This signals to the liver that blood glucose levels have dropped and that more glucose is needed throughout the body. Liver glycogen stores are broken down and glucose is released into the blood. Together, insulin and glucagon are hormones that help keep blood glucose levels within a fairly steady range.

By C. Muessig (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

The body has a limited capacity to store carbohydrates as glycogen. Once the glycogen storage limit has been reached, excess carbohydrates are stored as fat in fat cells.

The glycemic index and glucose load

The glycemic index (GI) is a scale that attempts to quantify how quickly a given carbohydrate causes blood glucose levels to rise. The scale ranges from low to high (0 to 100) and foods are categorized as being low, medium or high on the GI scale. What does that mean? That is a good question. Low scores tend to represent foods that take longer to digest. The benefit of foods that take longer to digest is that these types of foods cause blood glucose levels to rise more slowly. As a result, insulin releases from the pancreas into the bloodstream more gradually and glucose moves from the blood into the cells at a steadier pace. Theoretically, this reduces dramatic highs and lows in blood sugar.

By darwin Bell from San Francisco, USA (beet it  Uploaded by SunOfErat) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

The GI was developed for diabetics, so the potential therapeutic benefit of identifying foods that may reduce spikes in blood sugar is attractive; it can help diabetics make food choices to support their health. The GI was not intended for use by the general population.

The glycemic index of a food item is measured in healthy participants who consume 50 grams of digestible carbohydrates of the food item. The blood glucose response of the participants is measured over the course of 2 hours. Each participant's response is compared to her response to the same amount of carbohydrates in either pure glucose (preferred) or, sometimes, white bread. Here is a link to the website where the GI database is maintained at the Sydney University GI Research Services in Sydney, Australia Glycemic Index - University of Sydney.

One limitation of the GI is that it measures 50 grams of digestible carbohydrates. The term “digestible” is used because some parts of carbohydrates, like fiber, are not digestible. In 50 grams of beets, there will be about 13 grams of digestible carbohydrates per serving. This means it would take about 3.5 to 4 servings of beets (beetroot in the database) to actually consume 50 grams of digestible carbohydrates. Beets are considered a medium GI food because the score is 64. That score, however, is not really reflective of a portion size someone would eat.

	Glycemic Index	Glucose Load
Low	1-55	1-10
Medium	56-69	11-19
High	70 and higher	20 and higher

The glucose load (GL) was developed to address disconnects between GI, how much a person may actually consume in a serving of food and how GI and carb intake might impact blood glucose. Here is the calculation: GL = (GI x the amount of carbs per serving)/100. I love beets so here we go back to the beets with their GI of 64 with 13 grams of digestible carbs. GL = (64 x 13 g carbs per serving)/100 = 8.3. What does this mean? A GL scale has been developed. Based on the GL scale, a GL of 8.3 puts beets in the “low” GL category. That makes more sense because beets are very high in fiber which will slow their digestion. In summary, GI of beets is 64 (medium GI food) but 8 (low GL). I have not even addressed the fact that it is unlikely most people (well, other than me) would just eat beets without any other food.

There are other limitations to GI and GL 

• GI is measured in healthy participants on otherwise empty stomachs.

• We eat carbohydrates with other food components including protein, fat, and other carbohydrates. Food combinations changes the calculated GI of a carbohydrate. Protein and fat can lower the GI of a food. Coupling high GI carbs with low GI carbs can lower the GI of higher GI carbs.

•  Food processing as well as the method and length of cooking affect the GI of a food. Longer cooking time usually breaks down carbohydrates so actual GIs may be different than the GI value found in the index because of cooking.

• Acidity can lower the GI of foods.

The effect of food on blood glucose levels is complicated if you have decided to follow a diet plan based on GI scores and/or GL scores. The science is unclear whether or not it helps facilitate weight loss, reduce hunger or reduce the risk of cardiovascular disease. 

Resources:

• Mayo Clinic - Glycemic Index
•  Association of Nutrition and Dietetics - Glycemic Index
• Clinical Diabetes: Three Rs of Glycemic Index: Recommendations, Research, and Real World
• Linus Pauling Institute: Glycemic Index and Glycemic Load

What is the DASH diet?

Dietary Approaches to Stop Hypertension (DASH) is an eating plan that emphasizes whole grains, vegetables and fruits, lean proteins (like lean cuts of meat, poultry, fish, legumes), low-fat and fat-free dairy, and nuts and seeds. The eating plan is rich in fiber as well as calcium, magnesium, and potassium. Diets high in calcium, magnesium, and potassium have been associated with reducing blood pressure in people with hypertension and improving lipid profiles. According to the National Heart, Lung, and Blood Institute, the DASH diet reduces the risk of cardiovascular disease because of its positive effects on blood pressure and lipid profiles.

The DASH diet recommends choosing unsaturated fats and oils like olive oil or canola oil. The DASH approach to eating is high in fiber, low in fat and cholesterol, and low in sodium. This is not only potentially beneficial for blood pressure but also is a reasonable and healthful approach to eating, weight reduction, and/or weight management.

   Overview of DASH eating plan:

• Mayo Clinic - Overview of DASH Diet
• National Heart, Lung, and Blood Institute - DASH Diet
• Family Doctor.org - DASH Diet

   Sample menu:

• Mayo Clinic -DASH Diet Sample Menu

   Tips for eating out:

• Mayo Clinic - DASH Diet - Eating Out

   Other Resources:

•    National Heart, Lung, and Blood Institute (a PDF)

The glycemic index and heart health in the news

A flurry of news articles started popping up about the glycemic index, heart health, and how low GI foods do not protect against cardiovascular disease in the middle of December 2014. The journalists picked up the story from a recently published paper that appeared in the Journal of the American Medical Association (JAMA). Here are a couple of headlines from that time:

• Time (Alice Park): Here's What Low-Carb Diets do to Your Heart (12/16/2014)
•  Renal & Urology News: Low-Glycemic Index Diet does not Improve CV Risk Factors (12/16/2014)

What the researchers did:

The researchers investigated whether the glycemic index coupled with either a high carbohydrate or low carbohydrate diet improved risk factors of heart disease (Sacks et al., 2014). To do this, the research group conducted a randomized controlled trial that involved a total of 163 participants, although not all participants completed the entire study (which is not uncommon). This was a dietary study that had four diet groups and ideally all participants consumed all four diets. Each of the four dietary interventions was based on guidelines from the DASH diet and the Optimal Macronutrient Intake to Prevent Heart Disease. There was a 2-week “wash-out” period from the time a study participant finished one diet and started another. During the active intervention phases of the study, the researchers provided all of the food. During the wash-out period, people ate their own diet (self-determined).

	High Carbohydrate (58% of calories)	Low Carbohydrate (40% of calories
High GI (≥ 65)	5 weeks on, 2 weeks off	5 weeks on, 2 weeks off
Low GI (≤ 45)	5 weeks, 2 weeks off	5 weeks, 2 weeks off

Since this was not a weight loss study and the investigative team wanted to look specifically at the relationship between the diet and markers of cardiovascular health, each participant’s diet was designed to provide the right amount of calories to maintain a person’s current body weight during each phase of the study. All of the participants were at least 30 years old, were overweight or obese and were pre- or stage 1 hypertensive but not taking any medication for it. People were ineligible to participate if they had diseases, conditions, or were taking medication known to alter the outcomes of the study (such as lipid lowering medication, existing diabetes, heart disease, etc.).

Main outcomes of the study (the results):

The researchers were interested in 5 main outcomes: insulin sensitivity, LDL cholesterol, HDL cholesterol, triglycerides, and systolic blood pressure. When insulin sensitivity or HDL decreases, the risk for heart disease increases. When LDL cholesterol, triglycerides, and/or systolic blood pressure increases so does the risk for heart disease.

The results of their research revealed that when you eat a healthy diet like that which is outlined by the DASH plan with the optimal macronutrient intake guidelines for heart health, there may not be any added benefit to selecting carbohydrates based on their GI score. The researchers did find the diet had some marginal effects on blood parameters, but the clinical relevance of these effects is not clear:

  If you are eating a healthy diet (and are not diabetic) choosing foods based on their GI score does not appear to improve insulin sensitivity.

o   Insulin sensitivity was not affected by low GI or high GI

o   They did find that a high-carb + low GI diet for 5 weeks resulted in slightly poorer insulin sensitivity in participants compared to the response after 5-weeks of high-carb + high GI diet. While the difference was statistically significant, the mean values between the two groups were similar and the standard deviations were quite large. Only time will tell if the effect of this type of diet is replicable and clinically relevant.

  If you eat a healthy diet, GI score of carbohydrates does not affect cholesterol.

o   HDL was not changed in any groups.

o   After 5 weeks of eating the high-carb + low GI diet, participants had significantly higher LDL cholesterol than when they consumed any of the other diets. While the difference is significant for research purposes, it remains to be seen as to whether or not the difference is clinically relevant.

  If your diet is heart-healthy, eating a low-carb diet may improve triglycerides.

o   Low-carb diets, regardless of the GI score, resulted in lower triglycerides.

Overall, the results of this study indicate that in people without known metabolic disease, selecting foods based on their GI score or even the GL does not dramatically impact any of the risk factors of heart disease that they tested.

Sacks, F. M., Carey, V. J., Anderson, C. A., Miller, E. R., 3rd, Copeland, T., Charleston, J.,. .. Appel, L. J. (2014). Effects of high vs low glycemic index of dietary carbohydrate on cardiovascular disease risk factors and insulin sensitivity: the OmniCarb randomized clinical trial. JAMA, 312(23), 2531-2541. doi: 10.1001/jama.2014.16658

Final thoughts

As a nutrition professor, I was obligated to address the glycemic index and load in nutrition courses. Students asked about these topics and textbooks include GI and GL. Meal planning based on the GI or GL is unnecessarily complicated. The health benefits that may be associated with using GI and GL are more likely associated with other components of foods in the lower and medium GI and GL categories: high fiber, inclusive nutrient composition, and phytochemicals (plant chemicals). At the end of the day, eating a healthful diet based on GI and GL rankings is quite similar to following a DASH or, for some people, even a Mediterranean style eating plan.

If you decide to incorporate the DASH eating plan, or any new plan, you may find it easier to make changes gradually, especially when it comes to fiber. Adding too much fiber all at once can (will) cause gastrointestinal distress. You can minimize that potential discomfort by incorporating fiber into your diet over a period time (which varies based on your current eating style and your gastrointestinal tract). By doing this over a few weeks, your body will have time to adjust. It is also important to make sure you get enough fluid. Fluid is important to keep non-digestible waste moving through the colon. If there is not enough fluid, then the feces becomes hard and difficult to pass. I am sure you get the picture.

With any kind of dietary adjustment, it is very helpful to plan out meals and snacks. Snacks are okay and need to be accommodated and accounted for in the overall dietary scheme and caloric count for the day based on your specific needs and goals. One approach is to make a single substitution at a meal and/or snack time. Enjoy an apple for a snack to introduce more fruit and fiber. At dinner, decrease how much meat is on the plate and increase the amount of vegetables or whole grains to incorporate more vegetables and fiber. For dessert, try a cup of fat-free or low-fat yogurt (refrigerated or frozen) with berries as a substituted for a sugary dessert.

There is no right or wrong way to adopt a healthy change. It may take weeks to incorporate a new eating plan, whether it is the DASH diet or something else, into your life. Some people are most successful when they introduce change in a deliberate and planned fashion over time. Other people adopt change all at once and like to dive in head first. If that dive in head first has been initially successful for you but then later you have drifted, you may be interested in exploring a different approach.