This article was originally published on January 23, 2012 and reviewed and updated by the CurioCity team in September 2017.

Why do you have a sweet tooth?

Why do you sometimes get a sugar high?

Why can't my diabetic grandmother have sugar in her coffee?

The answers to all these questions started at the very beginning of life on earth, before diseases or the ability to taste even existed.

Like a dollar at the shopping mall, glucose is a unit of currency. Glucose is a sugar, and it’s the basic unit of currency for life. At the mall, you can exchange dollars for food, clothes, games and many other things. In your body, glucose can be exchanged for energy — energy that is used to make heat, to move muscles, and to sustain all of the chemical reactions that keep you alive.

Tiny organelles called mitochondria work inside cells to convert glucose into ATP, which is the universal body energy source.

Did you know? Mitochondria have their own special DNA and proteins. Scientists think that when life began, mitochondria were separate organisms that were eventually engulfed by larger cells. This arrangement worked for both the mitochondria and the larger cell, and so it continued for billions of years!  Mitochondria are now the main energy powerhouses of cells and use glucose as their fuel.

Glucose is also exchangeable for other important materials in the body. For instance, your body converts glucose to fat or glycogen for storage. You can burn that fat by exercising, as the fat gets converted back to glucose (and then energy) to move your muscles.

You can even convert proteins and amino acids into glucose, starting with your muscles. This is why athletes and bodybuilders have such large appetites — if they don't keep eating, their muscles start to eat themselves! (Of course, if they keep eating the same and aren't exercising, they begin to store glucose as fat in their bellies).

Why did life use glucose instead of other sugars or substances? No one truly knows. It may be because it tends not to slows down other processes within cells. That said, your body tightly controls the concentration of glucose in your blood.

Diabetes is a disease that prevents bodies from adjusting glucose concentrations. There are two types of diabetes. Type I is due to an autoimmune attack on the pancreas and occurs in children. Type II is due to a failure of the body to regulate blood glucose levels and occurs mainly in older adults. Type II diabetes is one of the most common diseases in the world.

Humans have evolved to be deeply reliant on glucose levels, to the point where glucose levels can affect mood.Ever yell at someone, and then after you have some food, things seem better? Low glucose can make you very anxious. On the other hand, a sugar high from candy, cakes and sodas is caused by a high glucose load on the brain (and is met by a post-sugar crash, usually half an hour later).

Did you know? Simple sugars, like those found in sweet treats, release their glucose very rapidly. Complex sugars are converted into glucose more slowly and are less stressful for your body. These sugars are found in breads, pastas, and cereals.

This answer was researched and written by Michael Jain. Michael is a medical student at McGill University in Montreal and did his Ph.D. research in cell biology. He has learned to control his irritability with small, healthy snacks.


Glycogen is stored, unused glucose in your body.

ATP is short for adenosine triphosphate.Your body uses ATP as energy for your cells.

Autoimmune disease is a disease that attacks your own immune system

Learn more!

Biology4Kids (2017)

Carbohydrates and Sugar (2017)

ATP & Respiration: Crash Course Biology #7 (2012)


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