If you want to take good care of your car to last you at least 239,000 miles (that is the distance be- tween the earth and the moon) you should know some basics about the car. Similarly, if you want to stay healthy, a few bits of knowledge about how our body uses nutrition will go a long way. In the next few paragraphs we will learn a few things about nutrition and our body.
It is better to understand how blood glucose is intimately influenced by different organs, so that you can understand why a small change in blood glucose can begin to affect different bodily functions. The topic is quite complex, and scientists are still trying to figure out how the system works, but we will try to simplify it. Once you learn these, you will become an expert in managing your own glucose and health.
Macronutrients and how they are interconnected with each other.
As you may remember, the major ingredients in most of our food are carbohydrates, protein and fat. You may wonder whether our body can convert fat to protein, protein to sugar or sugar to fat. The simple answer is not so easy. But it does happen on a daily basis. A good chunk of sugar or carb we ingest are converted to fat and are stored. Some sugar can be technically converted to amino acids (basic building blocks of proteins), but our body cannot make all the amino acids it needs. So the answer is we cannot make all the proteins from carb. Similarly, it is not easy to make protein from fat.
The route is too circuitous and our body cannot make all the building blocks of proteins (amino
acids). Some amino acids and some breakdown products of fat can be converted back to glucose as an energy source. While these may sound too confusing, it may help if we learn a little about how
different organs in our body use, store and sometimes interconvert these macronutrients.
The major organs of our body that consume, store or affect blood glucose are the liver, muscle, fat, kidney, and the brain.
The liver: accounts for only 2-4% of your body weight. Yet, it is the most important organ in glucose regulation. It is the first stop where most nutrients and other molecules absorbed from our gut are first received and sorted. The liver absorbs nearly two thirds of all glucose and almost all other car- bohydrates that are absorbed in the intestine. It uses glucose for at least three major purposes, producing energy to power itself, storing glucose into glycogen, after the glycogen store is filled up, storing glucose as fatty acids or fat, and making anti-oxidants used in producing other building blocks of life. Liver can also make glucose and release them into the blood. In fact, it is perhaps the only organ that can produce enough glucose and supply to brain and other organs under normal condition.
The Brain: is the constant user of glucose. Your brain needs around 100-120 grams of glucose every day to function properly. It does not store any glucose, protein or fat as stored fuel, so the brain has to constantly depend on your blood glucose for its energy needs. It also has a special mechanism for absorbing and using glucose. Although the mechanism is complicated, the key point is the brain requires the blood glucose level to be above ~50mg/100ml. If the blood glucose goes below this level, the brain cells cannot absorb glucose properly and can immediately starve. Since brain cells use glucose to make enough energy to make neurotransmitters or brain chemicals that make our brain function properly, when our blood sugar level drops, we feel dizzy and we can pass out. This is important to remember when we discuss hypoglycemia in later chapters. There is emerging evidence that the brain can use some ketone bodies or breakdown products of fat as an energy source, but for most normal day-to-day purposes the brain needs glucose. The best exam- ple to illustrate this is when someone goes dangerously hypoglycemic, the person is given a glu- cose drink – never a ketone drink to bring the patient back to normal.
The Heart: is very special. Heart actually prefers fat as fuel. At any given time the heart uses some- where between 60% to 90% energy from fat. Heart also does not have much room for any energy storage, so it has to constantly soak up glucose, fat and amino acids from the blood. (But too much fat in the blood can make the heart sick. We will get to it later.)
Fat tissues: play an important role in glucose regulation. To successfully store fat in non-toxic form, fat tissues need a little bit of glucose. The fat cells break the glucose to smaller fragment and then use those small sugar bits – like a key ring – to hang on to three fatty acid molecules making up what is known as triglycerides. In the fasted state, fat cells can release the stored fat which travels to the liver to be used for energy production. A special type of fat, called brown fat can also burn some fat to produce heat to warm up our body.
Muscle: is the most important organ for regulating glucose. Right after we eat, for the next 2-4 hours, our muscles can soak up a large amount of glucose and store them as glycogen. When we use our muscle, particularly when we do moderate to intense exercise, it breaks down glycogen and use glucose as an energy source, while the resting muscle prefers fat as an energy source. Although muscle can store a good chunk of carb as glycogen, it lacks a key enzyme that can release stored glucose to circulation. Therefore, muscle almost acts as a one way street for ab- sorbing glucose. This is important to remember. Because the more muscle you have and the more active you are, you are more likely to soak up glucose and burn them directly. In other words, even though our brain can use 120 g of sugar everyday, there is no evidence that thinking hard can make our brain work faster and use more than usual amount of blood sugar. Similarly, the glycogen stored in liver is used primarily for use by the rest of the body. Fat cells do not absorb too much glucose nor can they store them. Skeletal muscles are also site where a large about of proteins can be stored. After many hours of fasting, some of these muscle proteins are broken down and released into the blood to be absorbed by liver to make glucose.
What is the role of kidney? Our kidneys filter our blood almost 60 times every day. During the pro- cess, the kidneys reabsorb glucose and necessary electrolytes from our blood. For these functions, kidneys need a good amount of energy. Although it weighs only 0.5% of our body weight, kidneys can consume as much as 10% of the oxygen we take in. During extreme starvation, kidneys can produce as much as 50% of our blood sugar.
The take home messages are
Every organ in our body has some preference for glucose or fat as energy source. That’s why eating a balanced diet is to feed each organ its preferred food.
Protein is rarely used as energy source in the fed state, but can be broken down to supply glucose in fasted state.
Our body stores a good amount of energy as glycogen, fat and protein so that we can easily go through several hours or even a couple of days without food.