Fats, proteins, carbohydrates, vitamins, minerals – I just finished teaching my students about the digestive system. Each year, I preface this unit by saying that it’s an easy system to learn because everyone has a sense for how it works. I say this optimistically. And I’m met with skeptical stares by those who’ve looked ahead and looks of relief by those so far behind that they haven’t a clue what they’re in for. But most people do have a sense for how digestion works, don’t they?
Preferring to do my grocery shopping in the last half hour before close, I made a late run to the store last night. I grabbed milk, half a dozen eggs and some coffee and was on my way to the cashier down the cereal aisle when I witnessed an age-old struggle – a woman frowning, holding two boxes of granola. I watched. She compared labels for ages. Do the carbohydrates and protein in one offset the fats, saturated or not, in the other? What about sodium and potassium? I almost asked her if she knew what any of those components that she so carefully examined are and why your body needs them. Do you know?
I asked my students this question at the beginning of this unit and the only answer on which they all came to consensus is that we need food for energy. It’s likely the same answer they’d have given as children. But by university, after countless high school science classes, I’m surprised there’s no deeper understanding of something as fundamental as how we use the food we eat.
Energy is part of it but not the whole story. We are made of billions of cells – an unfathomably large number and our cells use each component of our food for different purposes. Each cell is an extraordinarily complex factory, performing countless chemical reactions every second, building some structures and breaking others down. And yes, some of these reactions make energy to power other reactions. For these reactions to continue, our cells need a steady supply of reactants. Our food supplies these reactants. The carbohydrates, fats, and proteins that we eat are mostly large molecules composed of individual units, like Lego pieces attached together. Since the molecules are too large for our cells to use as is, a variety of enzymes in the stomach and small intestine break these molecules down into their individual units or Lego pieces. These units are transferred to the bloodstream which then delivers them to every cell in the body. And so our cells receive their reactants.
For example, the protein in the milk and eggs that I bought last night are complex structures whose constituent units are called amino acids. When I put the milk on my cereal or eat a scrambled egg (an all too common breakfast, lunch, and dinner for me), the proteins are broken down. Specialized transporter proteins transport the individual amino acids from my small intestine into my blood where they’re then delivered to my cells. My cells then use these amino acids to build proteins of their own. And cells make thousands of different proteins, each integral to the proper functioning of the cell and hence the proper functioning of the body. In fact, your DNA is really just a recipe book for proteins, they’re responsible for virtually everything your cells do.
And that’s just the proteins. The fats, carbohydrates, sodium, potassium, etc each have their own functions. And all are essential to your cells’ vitality. Maybe it’s a physiologist’s bias but surely grade school science classes should cover something as basic as why we eat. We all have bodies yet are taught very little about how they work. For my students, after coming to terms with the convoluted names of enzymes, I saw flickers of dawning realization that each component is used for different processes, that they have even learned some of these processes, and finally, that ‘energy’ is a cop-out of an answer. As for the grocery store lady, judging by her look of consternation, she might still be standing holding her two boxes of granola, locked in indecision. Then again, decision-making is another process altogether and a story for another day.