Carbohydrates, fats, and proteins are the 3 major macronutrients that the human body uses to create energy.
It is easy to forget today that food is fuel, not just fun!
Examples of different types of carbs.
A carbohydrate, like it sounds, is made up of carbon atoms, hydrogen atoms, and oxygen atoms. In their simplest form, a carbohydrate is what we know as sugar, but they are found in a variety of forms and chemical complexities.
Simple sugars, or monosaccharides, are carbohydrates such as glucose and fructose. Fructose is found in fruits and juices naturally, but both glucose and fructose are commonly added to products to enhance flavor. Disaccharides, larger molecules, are carbohydrates such as sucrose, or table sugar, and lactose, the natural sugars found in milk products.
Complex carbohydrates include starches, cellulose, and fiber. We find these in products such as breads, pastas, oats, corn, potatoes, vegetables, the pulp and skin of fruits, beans, legumes – basically, anything that grows and didn’t come from an animal!
The simpler the carbohydrate, the faster it can be broken down, and absorbed into the bloodstream for usage, or storage, by our body’s cells. Fats and proteins have the same fate, but are used by the body in different ways.
Although carbohydrates have received a great deal of negative attention in recent years, carbs have been, and will continue to be, an important source of energy, as well as essential vitamins and minerals for the human body.
Not just providing energy, carbohydrates may provide iron, calcium, vitamin C, folic acid, potassium, vitamin B12, magnesium and dietary fiber, which is a very complex carbohydrate considered to be essential for proper bowel function and cholesterol control.
However, the over-processing of carbohydrates (think white breads, pastas, candy) and the addition of simple sugars (think corn syrup) to foods that otherwise did not contain them is the true driving force behind the health problems associated with excess carbohydrate intake.
For example, to create fruit juice, the fructose is extracted, but the fibrous portion of the fruit is removed, and in most cases, more sugar is added.
Carbohydrates are clearly not created equal!
How does our body use carbohydrates (or any food we eat)?
Essential to understanding the magic that is a carbohydrate is the process by which our bodies create energy, or Adenosine Tri Phosphate (ATP). Just like gasoline for a car, all of the cells in our body use carbohydrates, fats, and (less-so) proteins to generate molecules of ATP.
A carbohydrate provides approximately 4 calories per weighed gram of carbohydrate. For example, if a product states that it contains 20 grams of carbohydrates, it provides 80 calories to the body. A calorie is simply the ability of a molecule to elevate the temperature in the body for a chemical reaction (in this case, ATP synthesis) to occur. Thus, the more calories that are in a food, the more energy that the food can potentially create.
Our bodies generate ATP directly from 2 molecules: glucose and fatty acids. Some cells of our body only use glucose for energy, but most use a combination of fatty acids and glucose depending on the tissue and demands at that time. For example, the brain and other neural tissue rely exclusively on glucose for ATP generation, while muscle tissue uses a combination.
At resting conditions, such as sleeping, our bodies prefer to use fatty acids, since they contain almost double the calories that carbohydrates do, and have storage depots (i.e. body fat) throughout the body. However, it takes more time and more complex chemical processes to create ATP from fatty acids, and thus, under more moderate and strenuous conditions, energy usage will shift to glucose, a fast track to ATP.
Amazingly, since all the basic backbones of carbohydrates, fatty acids and amino acids (the components of protein) are the same, the body can convert amino acids to glucose, and fatty acids to a molecule known as a ketone, which acts very similar to glucose. Thus, people can survive on very low carbohydrate or no carbohydrate diets!
Macronutrients are not only broken down to generate energy, but to contribute to the building of all the structures and products of the body. Amino acids make up almost all structures in the body and are, unless under extreme conditions, not used to generate ATP.
Fats play a dual role – they provide energy and are vital to the synthesis of all the walls and membranes of cells, cholesterol, hormones, and nerves. Although glucose is a major energy source for many cells, nothing in our body is “made” directly of carbohydrates.
Thus, there are no “essential” levels of required carbohydrates for health, but simply recommendations. These vary dramatically from organization to organization, diet to diet, and guru to guru.
What happens to a carbohydrate when we eat one, and how does this differ by type?
The fate of carbohydrates depends on the type of carbohydrate, how fast the carbohydrate is broken down, and how much glucose freed from that breakdown is then used or stored by our body’s cells.
Simple carbohydrates provide our body with quickly usable energy. Since our brain functions on glucose alone, several mechanisms are in place to keep blood glucose levels at an appropriate, healthy range.
If you’ve ever experienced a bout of low blood sugar, you know how shaky, confused, irritable, and weak you can feel. The body is no fool!
You probably also experienced an immediate relief from eating a very simple carbohydrate such as a cracker or piece of candy. That is how fast the mouth sends messages to the brain that all is well! The brain can quickly absorb and use glucose.
However, not all of our body’s cells can readily absorb glucose, and require some help getting into the cells. This help comes from the hormone insulin, created by the pancreas.
When food arrives, insulin is released according to the rate at which that food causes blood glucose levels to rise. The simpler the carbohydrate, and/or the greater the amount of carbohydrate, the faster and larger the rise in blood glucose may be.
In turn, the greater and/or faster the rise in blood glucose levels, the greater the insulin response need to move glucose into the cells for usage or storage. This concept is also known as the Glycemic Index, a measure of how fast and how large of a blood glucose spike is caused by a food or meal, although the GI of any food will change based on the amount that’s consumed, and what other foods are consumed simultaneously.
Ok great, but what does this have to do with body fat?
Although carbohydrates do not contribute much to the components of the body, we do have some small storage depots of glucose in a form known as glycogen. These little warehouses are found in the liver and muscle tissue, which is convenient for working skeletal muscle! Surrounding these glycogen molecules is also water, which we will touch on later when discussing low carbohydrate diets and water fluctuations.
When we take in carbohydrates that are not immediately needed for energy, the body will link together glucose molecules into glycogen, and store it.
However, if these storages are at capacity at any given time, the body will convert glucose to fatty acids for storage in fat tissue.
“Aha! I knew it! Carbs make you fat, especially at night when you’re resting!”
That is a gross oversimplification of the process I just mentioned.
We all know a number of very lean individuals with high carbohydrate diets, many of which do little physical activity. Thus, there are many more factors at play.
There is an association between diets with moderate to high consumption of simple carbohydrates and obesity, or eating late at night and obesity, but these are not 1:1 relationships.
It is important to understand that energy balance is a 24/7, constant process, with times of storage and times of pulling fatty acids and glycogen from storage for energy. Thus, losing body fat requires an imbalance to occur, where less energy is being taken in than required by the body. There are many moving parts and processes that contribute to how much ATP each of us require, and how our individual bodies respond to the various energy sources.
In part 2, we will look at the history of the carbohydrate craze, the evidence for or against carbohydrate restriction, and what this may mean for your lifestyle! Keep reading, and feel free to ask any questions!
Krista Rompolski holds a Ph.D in Exercise Physiology from the University of Pittsburgh and is an NSCA Certified Personal Trainer.
She is Assistant Professor at Drexel University’s College of Nursing and Health Professions in Philadelphia, PA where she teaches Pathophysiology, Anatomy and Physiology and courses in clinical research.
Her interests include obesity and its associated co-mordbidities, physical activity promotion, nutrition and body image dysmorphia.