Carbohydrate Energy is Quickly
Processed in the Body
Once monosaccharides are absorbed, where do they go?
As mentioned, monosaccharides (glucose, fructose, and galactose) are absorbed into the body by crossing the wall of the small intestine and entering circulation via a special blood vessel called the portal vein. As the portal vein carries blood from the digestive tract directly to the liver, the liver gets the first shot at the absorbed monosaccharides. The liver is able to pull most of the galactose and fructose from our blood as well as a respectable portion of the glucose. However, much of the glucose continues past our liver and enters the general circulation where other tissue will have a shot at it. This increases the concentration of glucose in the blood from a normal or “fasting” level of 70 to 100 mg to 140 mg of glucose per 100 mL of blood or higher.
How does our body respond to the rise in blood glucose?
The concentration of glucose in the blood is very tightly regulated. When the level of circulating glucose climbs above the normal fasting level, the pancreas releases the hormone insulin. Insulin will interact with receptors on muscle cells and fat cells and promote the movement of glucose into these cells. Because skeletal muscle and fat cells together tend to make up more than half of our total body mass, the net effect is a fairly rapid lowering of the glucose concentration. Insulin increases the movement of glucose in these cells by increasing the number of glucose transport proteins on their plasma membranes. As the level of glucose returns to the normal fasting level the pancreas responds by releasing less insulin into circulation.
All cells in our body will continuously take glucose from our blood throughout the day to help meet their need for energy. However, after a meal, the liver, muscle, and fat cells will take a lot more glucose out of the blood than they immediately need. This allows blood glucose levels to return to a normal fasting concentration.
What does our body do with the glucose from a meal?
Insulin directs muscle, fat and the liver to use glucose, fructose and galactose as the primary fuel. This allows for a lot of carbohydrate entering the body from a meal to be used for energy as quickly as possible. In addition, insulin directs muscle and liver, and to a lesser extent other tissue to store extra carbohydrate as glycogen. Glycogen is composed of large branching links of glucose and is very similar to plant starch. However, only so much glycogen can be made since glycogen is meant to be a short term not a long term energy reserve.
How much glycogen is in our body?
Our liver can generate up to 6 to 8 percent of its weight as glycogen for about 75 to100 grams total. Meanwhile, only about 1 percent of the weight of skeletal muscle cells is attributable to glycogen. Since the total amount of skeletal muscle in our body far exceeds our liver, muscle will contribute much more to our total glycogen stores. Skeletal muscle may contain about 250 to 400 grams, which is about four-fifths of our total glycogen stores. Since carbohydrate provides 4 calories per gram the potential energy from glycogen is typically 1400 to 2000 calories, not very much. As you may expect, people with more muscle resulting from exercise training will have more body glycogen due to increased muscle mass. In addition, their muscle will adapt to hold double and even triple the amount of glycogen it can store. Interestingly, even though carbohydrates contribute approximately one-half of the energy in our diet, our body composition is not reflective. That’s because only 1 percent or less of our body weight is comprised of carbohydrate. This means that carbohydrate is stored with limitations, most of which is in our liver and skeletal muscle as glycogen. Other tissues, such as fat cells and the heart, contain a little glycogen as well; however, the contribution to our total body glycogen stores is very small. Since glycogen stores are relatively small there must be another means of storing the excessive energy from diet derived carbohydrate.
Can carbohydrate from our diet become body fat?
Since the potential to store carbohydrate as glycogen is somewhat limited, we need another means of storing excessive diet carbohydrate energy. As our liver and skeletal muscle is busy making glycogen, our liver and fat tissue will also begin to convert some of the extra glucose to fat. The fat that is made in our fat cells is stored within those cells. Meanwhile, the fat that is made in the liver is transported in the blood to fat cells and to a lesser degree other tissue such as muscle, breast tissue, etc.
Interestingly, scientists have determined that our ability to convert excessive carbohydrate to fat might not be as efficient under normal conditions as we once thought. This helps explain why excessive carbohydrate energy consumption can lead to chronic elevated blood glucose levels. It now seems that consuming excessive carbohydrate diet can support an increase in body fat by decreasing our use of fat as a daily energy source. So more of the fat in the diet ends up depositing and staying in adipose tissue (body fat). Eating too many calories that contain a lot of carbohydrate increases the influence of insulin throughout the day. Insulin will in turn increase carbohydrate use as a primary fuel as well as hinder fat breakdown and use as fuel. This situation tends to happen more when people eat too many calories and have type 2 diabetes (or prediabetes).
Can eating a low carbohydrate diet make us fatter?
Eating too much energy makes us fat, not too much of any one energy nutrient such as carbohydrate. Without question eating a high carbohydrate diet in conjunction with eating excessive energy will certainly support weight (fat) gain, so too will excessive fat and/or protein.
One of the reasons that carbohydrates have been bashed as of late is because of the effects of insulin upon stored fat. Insulin hinders the release of fat from adipose tissue. Therefore many dieters believe that carbohydrates, or more specifically insulin, are working against them. However, this function of insulin is very important in the normal scheme of things. By design, insulin keeps the fat tissue from breaking down and releasing fat during and for a couple hours after a meal. At this time absorbed food energy nutrients are circulating in our blood so there would be no need to break down our fat stores. Insulin will also promote the formation of fat from excess diet energy. So, the combination of decreased fat breakdown and increased fat production may lead people to believe that insulin makes them fat.
Before we dismiss the notion that insulin is working against people in their quest to lose body fat we should recognize that many people have elevated insulin and glucose levels during fasting. More times than not this occurs in people who have a higher level of body fat and low levels of activity. Thus eating a higher carbohydrate diet may indeed work against them to some degree. And, eating a lower carbohydrate diet would allow for more fat to be used for energy. However the biggest considerations here are going to be 1) how many calories are being consumed, 2) how much/type of daily activity and. 3) ability to metabolize the glucose. For most people with elevated blood glucose, controlling calories, being more active and improving body composition will lead to significant improvements.