For some individuals, however, a sudden increase in fiber intake may cause abdominal discomfort, cramping, bloating, gas, and diarrhea.
It is even possible to become constipated when consuming a lot of fiber without enough fluids.
In the most severe cases, it’s possible to develop an intestinal blockage.
This can happen if the fiber becomes a hard, dry mass in the intestine, blocking the passage of food.
The best way to prevent the adverse effects of a high-fiber diet is to increase your intake gradually (over several weeks) and boost your fluid intake along with it.
Another concern is that very high fiber intakes can reduce the absorption of minerals. While research has shown a reduction in the absorption of some minerals, the impact tends to be fairly small. Also, it’s not clear if fiber itself is the culprit or if phytates and oxalates, which can bind to minerals, bear more of the responsibility.
Although this is a valid concern, these minerals can be liberated, at least partly, during fermentation in the large intestine. Short-chain fatty acids (also products of fermentation) help to facilitate their absorption from the large intestine.
In addition, when compared to refined foods, high-fiber whole foods generally provide enough extra minerals to compensate for any losses incurred. Regardless, it is advised to limit concentrated fiber, such as wheat bran, and to minimize the use of fiber supplements when eating a plant-based diet. The best balance of healthful fiber and nutrients comes naturally with a varied whole-foods, plant-based diet.
REDUCE THE GLYCEMIC LOAD (GL) OF YOUR DIET
T he glycemic index (GI) is a measure of how carbohydrates impact blood sugar levels.
Carbohydrates with a high GI are more quickly digested, absorbed, and metabolized, causing a rapid and dramatic rise in blood glucose.
Foods with a high GI usually trigger an exaggerated insulin response, adversely affecting long-term blood glucose control, increasing triglycerides, and reducing protective HDL cholesterol.
Carbohydrates with a low GI are more slowly digested, absorbed, and metabolized, causing a lower and more gradual rise in blood glucose.
Foods with a low GI may positively affect insulin response, triglycerides, and HDL cholesterol levels (see figure 5.2 , opposite page).
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Replacing high GI foods with low GI foods improves blood sugar control, reduces a hsCRP (a measure of inflammation), and significantly reduces the risk of developing type 2 diabetes.
To determine the glycemic index of a food, several people eat a sample of the food providing 50 grams of carbohydrate.
For each study participant, changes in blood glucose are monitored over time (usually two hours).
The values from all of the participants are averaged to obtain the glycemic index of the food.
The GI uses a scale of 0 to 100, with higher values given to foods that cause the most rapid rise in blood sugar.
Pure glucose serves as a reference point and is given a GI of 100.
White bread has a glycemic index of 75 relative to glucose, which means that the blood sugar response to the carbohydrate in white bread is 75 percent of the blood sugar response to the pure glucose.
By comparison, barley has a glycemic index of 28 relative to glucose.
The glycemic index tells us how a serving of food containing 50 grams of carbohydrates affects our blood sugar.
However, we rarely consume exactly 50 grams of carbohydrate from any food.
Therefore a more practical tool called the glycemic load (GL) was created so the glycemic impact of a food could be estimated based on the carbohydrate in a typical serving size (based on the USDA food database).
GL is calculated by multiplying the glycemic index by the grams of carbohydrate provided in a portion of the food and dividing the total by 100.
The formula for calculating GL is as follows
FIGURE 5.2 Blood glucose curve of high and low GI foods.
Note: The amount of carbohydrate in two test foods containing 50 grams of carbohydrates.
Glycemic Load (GL) = GI x CHO content per portion of food 100
So, for example, the GL of brown rice with a GI of 66 and 52 grams of carbohydrate per a 1-cup (250-ml) serving would be: 66 x 52 ÷ 100 = 34.
The critical point about GL is that serving size matters.
For example, if you ate ½ cup (125 ml) of the brown rice (GI = 66; 26 g carbohydrate), the GL would be 17 (66 x 26 ÷ 100 = 17).
If you ate 2 cups (500 ml) of brown rice (GI = 66; 104 g carbohydrate), the GL would be 68 (66 x 104 ÷ 100)!
Both the GI and GL are rated as being low, medium, or high, according to their impact on blood glucose.
A GI of 55 or less is low, 56–69 is medium, and 70 or more is high.
A GL of 10 or less is low, 11–19 is medium, and 20 or more is high.
Foods that have a high glycemic index do not always have a high glycemic load.
For example, watermelon has a glycemic index of 72; however, a 4-ounce (120-g) serving of watermelon provides only 6 grams of carbohydrate.
You would need to eat over eight servings (2 lbs/960 g) of watermelon to get the 50 grams of carbohydrates needed to determine its glycemic index.
A 4-ounce (120-g) serving of watermelon has a glycemic load of 4 (72 x 6 ÷ 100), which is low.
Table 5.7 ( pages 99 – 100 ) provides the glycemic index and glycemic load of several common foods.
The impact of decreasing the GL of the diet is measurable and proportionate. One study reported that the risk of developing diabetes is increased by 45 percent for every 100-gram increment of GL. (To get the total GL for the day, add up the GL of each food consumed).
To put this into perspective, 100 grams of GL is equal to 100 grams of carbohydrate from food with a GI of 100, 200 grams of carbohydrate from food with a GI of 50, and 300 grams of carbohydrate from food with a GI of 33. The lesson here is that the glycemic impact from a small serving of food with a high GI can be remarkably similar to a much more generous serving of a food with a low GI.
The GI of various foods is not as predictable as one might expect. For example, sucrose (white table sugar) has a glycemic index of 68—lower than that of white bread, which has a glycemic index of 75, or whole wheat bread, which has a glycemic index of 74.
How can it be that bread, a complex carbohydrate, has a glycemic index that is higher than table sugar, a simple carbohydrate?
The reason is that table sugar is half fructose and half glucose, while bread is all glucose.
Glucose goes directly into the bloodstream and has the greatest impact on blood sugar.
Fructose is metabolized quite differently, and its impact on blood sugar is about one-fifth that of glucose.
So while bread may be more slowly digested and absorbed than sucrose, the glucose in bread causes a greater rise in blood glucose than the combination of glucose and fructose present in table sugar.
There are several key factors influencing the glycemic index of foods
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TYPE OF SUGAR PRESENT.
Glucose has a much greater impact on blood glucose than do fructose or galactose (a sugar in milk).
When comparing the glycemic index of various types of natural sweeteners, the differences are determined by the relative amount of fructose contained in the sugars.
Sugars with a lower GI contain more fructose.
This does not make them more healthful, as was shown in the discussion of fructose on page 76 .
Although sugar alcohols aren’t technically sugars, their GI ranges from zero for erythritol to 36 for maltitol; other sugar alcohols have a GI of 5–12.
TYPE OF STARCH PRESENT.
The two principal starches in foods—amylopectin and amylose—are digested at very different rates.
Amylopectin is a highly branched chain of glucose molecules that are rapidly absorbed into the bloodstream.
Amylose, a more compact, straight chain of glucose molecules is broken down relatively slowly.
Foods that are higher in amylose have lower GIs.
If you look at very detailed GI charts, you will notice that the GI of different types of rice varies wildly, with those low in amylose and high in amylopectin having a much higher GI.
AMOUNT AND TYPE OF FIBER PRESENT.
Fiber resists the work of digestive enzymes, so it slows the digestion and absorption of sugars from foods, reducing GI.
Foods rich in soluble, viscous fiber reduce the glycemic index to a greater extent than foods rich in insoluble, nonviscous fiber.
Beans and barley are good examples of foods rich in viscous fiber.
Wheat bran is a good example of a food rich in nonviscous fiber.
Refined foods that have had most or all of their fiber removed have a higher GI, as they are more rapidly absorbed.
PHYSICAL BARRIER AROUND THE FOOD. Beans and whole grains are surrounded by a fibrous coating that serves as a physical barrier to protect the seed. This barrier makes it more difficult for enzymes to break the food down, reducing the GI.
RIPENESS OF THE FOOD. As foods ripen, starches turn into sugars that are more rapidly digested, increasing their GI. For example, a banana that is slightly green or underripe has a GI of about 30, while an overripe banana has a GI of about 50.
EXPOSURE TO HEAT. Raw foods have a lower GI than the same foods cooked. Cooking increases GI because it breaks down the plant’s cell walls, increasing the rate at which its starches and sugars are absorbed. Lightly steaming vegetables to the tender-crisp stage will result in a lower GI than very well-cooked vegetables.
PARTICLE SIZE. Whenever particle size is reduced, the surface area of the food is increased, and it is more rapidly digested and absorbed. Thus, intact whole grains (such as wheat berries and barley) have a much lower GI than ground grains (flours), and whole fruits have a lower GI than fruit sauces or juices.
DENSITY OF THE FOOD. Dense foods containing little air have a lower GI than foods that are light and fluffy. Even though white flour is the main ingredient in white bread and white pasta, the bread has a much higher GI than the pasta because it is light and fluffy and quickly broken down. Puffed cereal grains also have a much higher GI than cooked grains.
CRYSTALLINITY OF THE FOOD. When a starch is raw, it is crystalline; its molecules are organized in a sequence that repeats. When it is cooked, this order is lost; it becomes more easily broken down and digested, and the GI increases. However, when the food cools, it recrystallizes, reducing its GI. For example, red potatoes cubed and boiled in their skin have a GI of 89. When the potatoes are stored overnight in the refrigerator and eaten cold the next day, the GI drops to 56.
ACIDITY. The addition of vinegar, lemon, or lime, ideally near the beginning of your meal (perhaps on a salad), can reduce the glycemic impact of the meal. Even 2–3 teaspoons (10–15 ml) is often enough for an effect. Fermentation also produces acid, reducing the GI. Yogurt has a lower GI than fluid milk, and sourdough bread has a lower GI than conventional bread.
Another important consideration is that people tend to eat foods in combination.
For example, baked potatoes have a very high GI and GL, so some health authorities suggest they be avoided.
Potatoes, like rice or other starchy staples, are high-carbohydrate foods that when eaten alone are rapidly absorbed into the bloodstream.
However, if a baked potato is eaten with a black bean–peanut sauce and broccoli, or a lentil loaf and kale salad, the absorption of the sugars in the potato will be much more gradual.
Potatoes (with skin) are high in fiber, cholesterol-free, very low in fat and sodium, and good sources of potassium, manganese, vitamin C, and vitamin B 6 .
The point is that these whole plant foods with concentrated carbohydrates can be included in a healthy diet when they’re consumed as part of a high-fiber, whole-foods, plant-based diet and portion sizes are controlled.