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Nutrition and Metabolism


What are carbohydrates?

Carbohydrates are sugars and long chain molecules made of sugars. They are the primary sources of energy fuel for human body. Although energy also comes from fats and proteins, carbohydrates provide the first source of energy supply to the whole body in the form of glucose, a basic building block for energy producing carbohydrates. Only when blood glucose decrease to a certain level, for example during fasting or exercise, will human body mobilizes the energy storage in fat and proteins.

There are two major groups of carbohydrates in dietary: simple carbohydrates and complex carbohydrates.  These two groups have different nutritional values. Simple carbohydrates are sugars such as glucose, fructose, sucrose, and lactose. Complex carbohydrates are made of long chains of simple sugars.  The most common complex dietary carbohydrate is starch. Dietary fibers are also complex carbohydrates.  Dietary fibers do not provide energy. They are normally considered as another class of macronutrients due to their distinct nutrition values.

What is the function of carbohydrate?

Carbohydrates are the primary energy sources for human body.  All carbohydrates from foods are eventually turned into glucose in human body. Glucose is then transported in blood to other parts of human body for energy production. When the glucose is not needed immediately, it will be stored in a person's liver and muscles as glycogen. The glycogen reserves can be turned quickly into glucose for energy when needed. When people eat too much carbohydrate, the glycogen reserves may become saturated. The excess energy from the carbohydrates will be converted to fat for long term energy storage, leading to weight gain.

What are the “bad” carbs and “good” carbs?

In diet, “bad” carbs refer to the foods containing too much simple carbohydrates. These foods include sugary drinks and refined carbohydrates (with added sugars) such as white bread, cakes, and cookies. Simple carbs are digested more quickly and raise blood glucose level faster. As a result, you will experience sugar spikes and crashes, dubbed “blood sugar roller coaster” by many nutritionists. Sugar spikes trigger the acute insulin response to lower the excess blood glucose as fast as it can by turning it into storage fat, leading to weight gain AND the "sugar crash”. Sugar crash causes fatigue and mood swings.  More importantly, it makes you go hungry sooner and craving for more foods. This will eventually lead to over eating and more weight gain. 

“Good" carbs refer to foods containing more complex carbohydrates such as sweet potatoes, oatmeal and popcorn. Complex carbs take longer to break down, therefore raising blood glucose level slower and more gradually.

“Bad” carbs normally have a high glycemic index while “good” carbs have lower glycemic index.

What is glycemic index?

Glycemic index (GI) indicates how fast glucose is released from the food to your blood.  A higher GI food releases glucose into your blood faster and the energy supply lasts shorter.

Using glucose as a reference of GI 100, foods with GI less than 55 are considered low GI; foods with GI between 55 and 70 are considered intermediate GI; and foods with GI more than 70 are considered as high GI.

In general, simple carbohydrates have higher GI and should be limited from your diets while complex carbohydrates have lower GI and should be the main form of carbohydrates in your diets.  However, exceptions do exist. For example, the GI of watermelon is 76. But it contains less than 8% of carbohydrate and a cup of diced watermelon only provide 46 calories.  So it is unlikely to be over consumed.  On the other hand, granulated sugars have an intermediate GI of 58, but contain a whopping 387 calories per ½ cup serving.  Therefore, it should be limited in your diets. Table 1 shows the GI numbers of some common foods.

Table 1. The GI (glycemic index) of some common foods.

GI Groups Foods GI Carb Calories per Serving Serving size
High Glucose 100 240 1/2 cup (60 g)
Baked potato 85 120 1 small (5 oz)
Sports drink, Gatorade 78 60 1 cup
Waffles, Aunt Jemima 76 52 1 piece
Cereal, Total, General Mills 76 88 1 oz
Rice, long grain, white 76 160 1 cup (158 g)
Doughnut, wheat, deep-fried 75 80 1 small (50 g)
Cracker, Graham wafers 74 72 1 cup
Bread, white 73 56 1 slice
Bagel, white 72 140 1 piece
Intermediate Sugars, granulated 58 387 1/2 cup (100 g)
Low Orange 42 44 1 piece
Tomato soup 38 68 1 cup
Apple 34 64 1 piece
Skim milk 32 52 1 cup
Lima beans 32 120 1/2 cup
Dried apricots 31 82 1/4 cup
Lentils 29 76 1/2 cup
Kidney beans 28 100 1/2 cup
Garbanzo beans 28 120 1/2 cup
Fruit yogurt, reduced fat 27 96 3/4 cup

How much carbohydrate should I eat?

The Institute of Medicine of the National Academies recommends that a 45% to 65% of the total energy should come from carbohydrates to maintain a balanced diet. Too less dietary carbohydrate is normally associated with bad breath, constipation, and in severe cases kidney failure, kidney stones and gout due to increased protein and fat intake. Too much dietary carbohydrates risk malnutrition due to decreased protein and fat intake.

Not all the carbohydrates in foods contribute to energy production.  Therefore, the concept “glycemic load” is introduced to reflect the energy contribution of carbohydrates more accurately.

What is glycemic load?

Glycemic load (GL) is a number that estimates how much the food will raise a person's blood glucose level after eating it. One unit of GL approximately equals to the effect of consuming one gram of glucose. High GL foods provide you more carbohydrate intake while low GL foods provide less.  For one serving of a food, a GL greater than 20 is considered high, a GL of 11-19 is considered medium, and a GL of 10 or less is considered low. Table 2 shows some common high GL foods.

Table 2. Some common foods with high GL.

Foods GL per Serving Carb Calories per Serving Serving size
Spaghetti, plain, cooked 48 176 3/4 cup
Rice, white, cooked 30 160 1 cup
Potato, Russet, baked 26 120 1 small (5 oz)
Bagel, white, frozen 25 140 1 bagel
Raisins 25 180 1/4 cup
Rice, brown, cooked 23 152 3/4 cup
Pizza, plain baked dough 22 108 1 slice (100 g)
Cereal, Cream of Wheat, instant, Nabisco 22 120 1 oz
Oatmeal 17 104 1 cup
Blueberry muffin 17 116 1 muffin
Power bar, chocolate 17 104 1 bar

What genetic makeup influence dietary responses to carbohydrate?

Many genes involved in energy metabolism affect how you body reacts to different kinds and different amounts of carbohydrates.  For example, variants of the Apo E gene determine if you should eat carbohydrates more or less. As described in Apo E and Cholesterol Management, the APOE4 variant has a higher rate of LDL formation so carriers are prone to high LDL cholesterol level.  In contrast, the APOE2 variant has a lower rate of LDL formation and the carriers normally have lower LDL cholesterol level. Due to the different in LDL formation rate, APOE4 carriers are better fitted with high-carb, low-fat diets while APOE2 carriers are better served by low-carb, high-fat diets. APOE4 variant is quite common in human population, ranging from 10 to 30% occurrence among ethnic groups. APOE2 variant is distributed from 2 to 13% depending on the ethnic groups.  In addition, people carrying gene variants that predispose them to obesity or type 2 diabetes should also be alert about carbohydrate they are eating.  For these populations, avoiding high GI food is particularly important.

Cereal, POST, Honeycomb
Per 100 g (g) 87
Per Serving (g) 28
Serving Size 1 NLEA serving (32 g)
Oatmeal, QUAKER, Instant, Regular
Per 100 g (g) 67
Per Serving (g) 27
Serving Size 1 packet (41 g)
Rice, Long grain
Per 100 g (g) 77
Per Serving (g) 143
Serving Size 1 cup (185 g)
Spaghetti, uncooked
Per 100 g (g) 75
Per Serving (g) 43
Serving Size 2 oz (57 g)
Dried fruits
Per 100 g (g) 79
Per Serving (g) 23
Serving Size 1 oz (28 g)
Palm dates
Per 100 g (g) 75
Per Serving (g) 110
Serving Size 1 cup (147 g)
Muffin, corn
Per 100 g (g) 51
Per Serving (g) 58
Serving Size 1 medium (113 g)
Bagel, wheat
Per 100 g (g) 49
Per Serving (g) 48
Serving Size 1 bagel (98 g)
Bread, rye
Per 100 g (g) 48
Per Serving (g) 12
Serving Size 1 slice (25 g)
Beans, Lima beans
Per 100 g (g) 63
Per Serving (g) 113
Serving Size 1 cup (178 g)
Potato, baked
Per 100 g (g) 22
Per Serving (g) 34
Serving Size 1 potato (156 g)

How to improve your dietary carbohydrates?

First you are recommended to use the GB Food Calorie and Nutrition Calculator to get the most accurate estimate of your current carbohydrate intake. Depending on your genetic background and your health status, you may want to maintain, increase, or decrease your carbohydrate intake.

In general, you should limit the amount of high GI food and increase the amount of low GI food for a healthy blood glucose and constant energy supply.  However, some high GI foods such as watermelon and pumpkins that have high GI but intermediate or low GL.  These foods can provide many beneficial nutrients and serve as balanced food sources.

When choosing low GI foods, or the ‘good carbs”, you should be looking for foods that are made from whole grains. Whole grains have not only complex carbs but also fibers. Other “good” carb sources include cereal (especially the ones with fortified dietary fibers), unprocessed grains (barley, oatmeal or quinoa), beans, brown rice, fruits and vegetables.

Finally, you should always count the calories and avoiding over eating.  Extra energy, no matter if it is from whole grain foods or from pure sugar, will be converted to fat and stored in your body!

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