Every organ in the body has an important, specific role. However, what some organs do to help the body function isn’t always immediately obvious. You will have learned what key organs such as the heart, the brain, and skeletal muscles do. The heart pumps blood around the body, the brain thinks and coordinates movement, and our skeletal muscles contract to help us move. However, the importance of white adipose tissue in regulating body weight and appetite has only come to light in the past 20 years, and the very existence of brown adipose tissue in adult humans was only confirmed in the past decade.
What is “adipose tissue,” what do the different types do, and why are they important? Let’s start by thinking about where the energy for heart pumping, brain thinking, and muscle movement come from.
Sources of biological energy
The immediate source of energy for biological processes such as muscle contraction comes from adenosine triphosphate (ATP). ATP is made by cells from macromolecules such as sugars and fats by cellular respiration. These sugars and fats come from the food that we eat. Cellular respiration releases the chemical energy stored in these molecules so that the cell can use this energy for biological processes (see Activity 2 for more details). So we can think how our body handles energy as an energy conversion process, for example for muscle contraction:
Sugars and fats (chemical energy) -> ATP (chemical energy) -> muscle contraction (kinetic energy)
White adipose tissue (or, white fat)
From the flow of the diagram above, it is clear that our bodies need to take energy in as food to fuel biological processes that use up energy, such as movement. The law of conservation of energy states that “energy cannot be created nor destroyed.” If energy cannot be created nor destroyed, what happens when we take in more energy (eat more food) than we use up?
1. What happens to extra food energy that we don’t immediately use?
Energy balance relates changes in body weight to how much energy we take in as food and how much energy we use up in biological process.
Read the section entitled “Explain the Relationship of Activity Levels…”.
In neutral energy balance, body weight remains the same; in positive energy balance, body weight increases; in negative energy balance, body weight decreases.
2. Using the concept of energy balance, can you explain why a person who does a lot of intense exercise needs to eat more than a person who does no exercise to maintain the same body weight?
3. What role do you think white adipose tissue plays in a state of positive energy balance? In a state of negative energy balance?
Brown adipose tissue (or, brown fat)
Brown adipose tissue is similar to white adipose tissue in that it is an organ that stores fat. However, what a brown fat does with that fat is very different to what white fat does.
4. What is brown adipose tissue’s unique role in the body?
5. Why do you think a warm-blooded mammal might need such an organ, especially in cold environments?
6. How does white fat also help regulate body temperature?
7. Why do you think brown adipose tissue is more important to a smaller mammal compared to a larger mammal?