Cedars-Sinai Blog
Q&A: Metabolism
Metabolism is the process of converting food into work energy or stored energy.
Metabolism gets a lot of credit, especially when it comes to weight loss.
We hear about diets that can keep us thin by boosting our metabolism and we complain about slow metabolism causing weight gain, but most of us would struggle to explain how either works.
"As you get older, if you follow the same diet you always have, your body will store more fuel as fat."
To help us understand metabolism, we spoke with Dr. Roberta Gottlieb, director of Molecular Cardiobiology, who studies metabolic processes and their effects on the body at the Cedars-Sinai Gottlieb Laboratory.
What exactly is metabolism?
Dr. Gottlieb: Metabolism is the process of converting fuel that you eat or drink into one of two things: work energy or stored energy.
What's work energy?
Dr. Gottlieb: The energy currency that's responsible for work energy is ATP (adenosine triphosphate). ATP releases energy that does the work of muscle contraction, or thinking, or seeing, or any other actions that our body does on a daily basis.
Our muscles, for example, are a big site of ATP consumption.
Because ATP is fragile, you can't store energy in the form of ATP. You store energy in other ways.
If ATP handles the work energy, then how does the body store energy?
Dr. Gottlieb: There are two ways: one is to store energy as starch, which is basically sugar that can be used when the body needs an extra source of fuel.
The other way you store energy is as fat.
Our bodies have limited ways of storing energy as starch, so storing energy as fat is easier and more efficient. This is part of the reason why people gain weight.
People often claim they have a slow metabolism. Is it a real thing?
Dr. Gottlieb: Yes, it can be true. Slow metabolism is when our energy needs decrease, and the body gets more attuned to storing fats—this happens especially as we age.
Is that why it's so hard to lose weight as you get older?
Dr. Gottlieb: Well, the body gets really good at storing energy as we age, and storing energy as fat is really efficient.
So, as you get older, you require fewer calories to meet your daily needs—partly because we get less active and our muscle mass decreases, so there's less muscle tissue burning ATP.
A 20-year-old might require about 2,000 calories a day, whereas for women after menopause or men in their 60s, daily calorie needs drop below 1,500 and sometimes below 1,200 calories.
As you get older, if you follow the same diet you always have, your body will store more of that fuel as fat.
Is there any scientific evidence that certain diets, such as eating 6 smaller meals a day, help speed up metabolism?
Dr. Gottlieb: If you start eating at 7 am and you keep eating until 11 at night—even 6 smaller meals—you'll never give your body the chance to tap into its energy reserves.
What I do think is metabolically beneficial is the concept of intermittent fasting.
What's intermittent fasting?
Dr. Gottlieb: Several studies suggest that limiting your daily eating time to 8 hours and fasting for the remaining 16 will tap into your stored energy reserves.
When we look at a 16-hour fast at the cell level, something interesting happens: The body digs into its fat storage.
The other thing that happens when individual cells go into fasting mode is a process called autophagy, which is a kind of cellular housekeeping. Cells will look to get rid of the oldest, rattiest things inside the cell. They take the crummiest mitochondria and send them off to be used as fuel.
Damaged mitochondria create free radicals (which cause inflammation and DNA damage), so you want to get rid of those. When we live in this life of constant calories and constant food availability, our bodies never get to do this cellular housekeeping.
