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Does Nature Want Us to be Fat?

CU Anschutz expert discusses how we evolved to overeat and what we can do about it

minute read

Written by Kiley Carroll on April 28, 2022
What You Need To Know

Richard Johnson, MD, says we all have a “survival switch” that protects against starvation, but now it’s permanently stuck in the “on” position.

For years, Richard Johnson, MD, professor of renal medicine in the Division of Renal Diseases and Hypertension at the University of Colorado School of Medicine, has investigated the impact of sugar, especially fructose, on the human body and how we process it. He recently published “Nature Wants Us to be Fat,” a book outlining why evolution has programmed us to overeat on the promise that we will lose the weight during lean times. 

But what happens when it’s no longer feast or famine – just feast? And what does Johnson mean when he says we all have a “survival switch” that protects against starvation, but it’s now stuck in the `on’ position? We talked with him recently to find out.

Podcast Episode: How Nature Compels Us to Overeat 


Richard Johnson, MD, digs into the mechanics of how we gain weight. In discussing biology versus behavior, the process may be more out of our control than we think. Listen to his podcast episode of CU Anschutz 360 here.

Q&A Header

Let’s start with the rather provocative title of your new book, “Nature Wants Us to be Fat.” What do you mean?

Animals normally regulate their weight within a tight range, but in preparation for situations where food will not be available, such as a deep winter or when migrating long distances, animals purposely gain weight so that they have enough fat to survive the period of food shortage. This change from tightly regulating their weight to suddenly being hungry, foraging for food and putting on fat, is very similar to the activation of a switch, and is biologically driven. It can be viewed as a process that nature wants to help the animal survive during times when food is not available. So, in a sense, nature wants us to be fat when a period of famine or food shortage is on the horizon.

And now, at least for humans, you say that switch is stuck in the “on” position. How?

Our studies suggest that the switch is activated by a specific nutrient, which is fructose. Normally nutrients are used to generate energy, which is adenosine triphosphate (ATP), and only a little of the calories are used to store energy as fat. However, fructose reduces the amount of ATP being made, such that much more of the calories are turned into stored energy (or fat). Thus, fructose tends to create a low-energy state with low levels of ATP.  This is one of the reasons why we stay hungry.

You talk about sweeping changes humans have undergone for thousands of years – feast and famine, the need to store up food during lean times. When did all of this start to change?

Famine can still occur, but for most of us, our culture is so successful that we have open access to food practically all the time. Thus, foods like sugar are easily available. Indeed, there has been a parallel rise in obesity and diabetes that tracks with the increased intake of sugar (sucrose, which contains fructose) and high-fructose corn syrup. One can say the epidemic began around 1890 to 1900 but really accelerated with the introduction of high-fructose corn syrup beginning in the 1980s.

You point out that we essentially no longer feel full, so we don’t know when to stop eating. Can you explain this?

One of the reasons is the development of leptin resistance. Leptin is a hormone produced by our fat cells. Normally it tells our brain to tell us when to stop eating by creating a sensation of fullness. However, many overweight and obese people do not respond to leptin. When this occurs, it is called leptin resistance, and it means that you no longer get that signal of fullness when you are eating. Fructose intake in animals is a major cause of leptin resistance. Thus, chronic fructose intake can make a person very hungry.

You have spent years investigating the often-detrimental effects of fructose on the human body. Why is it so bad, and are there any upsides?

Fructose was meant to be a savior for animals at risk for starvation. So there is a good side. The problem is that we are eating way too much: like 15% to 20% of our diet is from table sugar or high-fructose corn syrup, of which half or so is fructose. Because we are eating so much, it is causing obesity, diabetes, high blood pressure and other conditions.

What do we have in common with hibernating bears, sperm whales and the world’s fattest bird?

All of these animals gain fat to help them survive periods of food shortage. We have unwittingly activated this switch by eating a lot of fructose — and so we are also getting fat and getting metabolic syndrome. However, unlike the bears and sperm whales, we do not need the excess fat to survive. Indeed, we do not hibernate, but just keep on activating the switch and gaining weight.

If we are programmed by nature to overeat, how can we possibly overcome it?

We have to know how to turn off the switch and also how to improve the energy factories in our blood (mitochondria). This includes avoiding foods that activate the switch, drinking plenty of water, reducing salt and exercising in zone 2 to maximize the repair and growth of our mitochondria.

(Zone 2 exercise is lower-intensity, longer duration workouts at about 60% to 70% of maximum heart rate. Example: jogging while talking for 40 or 50 minutes.)

What practical steps can we take to maintain a healthy weight?

Drink six to eight glasses of water daily, take 500 mg vitamin C, avoid sugary beverages, reduce sugar and salt, eat a natural fruit but not multiple fruits at a time, avoid beer, drink green tea and coffee, and eat dark chocolate!

Topics: Research, Faculty

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Richard Johnson, MD