Every time we cat, we set off the amazingly complicated series of events that make up our metabolic processes: all the chemical reactions that enable us to use food. We use food for energy; we use it for repairing old cells and for making new ones. Thanks to our metabolism, a tuna salad sandwich can fuel a walk in the park or find its way to a double chin. In healthy people, metabolic processes function automatically, without our thinking about them. But if you have diabetes, you need to understand how the food you eat affects your blood sugar levels. People diagnosed with diabetes should be immediately referred to a nutritionist to learn the basics, so they can manage their disease.
Everything we cat must be broken down into components that can circulate in the bloodstream. That's the job of the digestive system. The three major types of nutrients in food are fat, protein, and carbohydrates.
When fat is metabolized, it's turned into fatty acids and lipids that are released into the blood. If you're concerned about excess weight, you might not realize that fat is actually essential to good health-but it is. We need dietary fat to manufacture cells and some of the chemicals our body requires, including certain hormones and digestive acids.
The protein in our food is made up of long chains of amino acids. As food is metabolized, the chains are broken apart. These "links" of amino acids are sometimes called the building blocks of life. Our bodies reassemble them into new proteins, which are made into muscles, hormones, and other chemicals required for our functioning.
Carbohydrates-sugars and starches-are essential, too. To enter the bloodstream, starches and sugars must be broken down to glucose, a simple form of sugar. Our body extracts glucose not only from sweets such as candy and cake, but also from starchy foods such as grains and beans. Glucose is the body's main fuel. Without glucose, we couldn't move and we couldn't think. Our cells, particularly our brain cells, require a constant supply simply to maintain life.
Glucose can reach certain cells directly-the cells of the nervous system (including the brain), the red blood cells, and the cells that line our blood vessels. But most of our cells can't use glucose without insulin. In a healthy person, insulin and glucose match their moves like graceful dancers. As soon as we eat, the glucose level rises in our bloodstream, censors in the pancreas detect the change, and the pancreas releases a corresponding amount of insulin from special cells, called beta cells, in the islets of Langerhans. The insulin goes to receptors on cells throughout the body. Receptors are like docking stations: when insulin docks in its receptors, responses are triggered inside the cells that allow glucose to enter them. As glucose moves into our cells, the glucose level in our bloodstream drops. The pancreas senses the change and stops releasing insulin.
These mechanisms keep blood sugar levels remarkably steady. That's vitally important, because our body can't function properly if blood sugar is too low or too high. In the morning, after an overnight fast in which we've consumed no food at all, our blood sugar level normally doesn't fall below 70 milligrams per deciliter (mg/dl)-enough to ensure a steady supply of fuel to our brain and essential organs. And even after a large meal, our blood sugar concentration normally doesn't exceed 140 mg/dl, so we're protected against the harmful effects of excess sugar in the blood.
Our bodies are designed to maintain stability-or, as we say in medicine, homeostasis. Another important mechanism for this is our reserve fuel supplies. Thanks to these stores, we don't have to worry about running out of glucose if we skip a meal on a busy day or exercise for an hour or two. Indeed, most of us could survive for weeks or even months without food. The body's short-term storehouse-which might be tapped if we miss lunch or join a volleyball game and need extra energy-is the liver. The liver stockpiles glucose in the form of glycogen, which is made up of glucose molecules strung together. When our blood sugar level becomes low, the pancreas releases a hormone called glucagon, which signals the liver to discharge some of its glycogen stores. Glucagon is made in the islets of Langerhans of the pancreas, in the cells right next to the beta cells, which manufacture insulin. These cells communicate with one another, helping our blood sugar levels to remain stable. As insulin production rises-which happens when there's abundant glucose in the blood-glucagon production falls, because there's no need to tap the liver's glycogen stores.
We have another great fuel reserve: our body fat. When a person consumes more calories than his or her body can burn, insulin helps trigger metabolic processes that turn the excess food into fat. The average man or woman of normal weight has enough body fat to serve as fuel for a month or so. Of course, many people accumulate much more because they chronically eat more than their body requires. Stored fat is consumed as fuel when caloric intake is insufficient for our needs-for example, when we go on a diet or become ill and can't eat. In a dire situation, including starvation or following an unhealthy diet, the body can also use muscle tissue for fuel. But under normal circumstances, the glycogen reserves in our liver, along with our body fat, can provide all the backup energy we need.