Fruit, Fructose, and Fat
So what’s the real deal with fruit? We’ve spent our whole lives hearing that fruit is so good for us. Like many things in food and nutrition there is a heck of a lot more to the story. We are going to spend the next few minutes trying to get to the bottom of this. Let me first start by painting an evolutionary picture. Our metabolism is the by-product of millions of years of dealing with the selective pressures of nature. We must recognize that we were not always able to visit the grocery store or fast food joint anytime we were hungry. In fact, we spent a great deal of our lives hungry! Side note: I would argue this is probably better than spending the majority of our lives “sick”. Anyway, our system had to adapt to the conditions or we never would have made the advances we have. The irony is that since we have become “higher” and “smarter” than nature, we actually have disrupted a system that worked perfectly well. Let’s take fruit for instance. We can go get a piece of fruit that is only in season half way across the globe at any point in time. We can also get as much of it as we want. We rarely stop to ask ourselves if this is a good idea – and why would we? We have been lead to believe that eating fruit is perfectly fine. But is it a good idea? Let’s cover the science and then we can answer that question a little better.
Let’s transport back 50,000 years ago to a small tribe of hunter gatherers – in America if that makes you feel better. The summer is coming to a close and fall is approaching. You have been fed pretty well to this point on the local game and the occasional root vegetable. You are a bit excited because the fruit trees have really started to sprout their delicious fruit. What you may not take into consideration is that these fruits are also out for survival. They evolved to have bright vibrant colors (apples, oranges, grapes, bananas, etc) and to be sweet. The color better attracts animals and the sweetness keeps them coming back for more. This is the main reason why having a “sweet tooth” is addicting. For the fruit, their vibrant color and sweet flesh is a sure fire way to get someone else to distribute their seeds. This is the fruits way of reproducing. Grains have a much different approach. Grains are simply seeds – we eat the actual seed! Wheat is a seed! Corn kernels are seeds! Beans are seeds! How could it reproduce if we destroy its reproductive organ!? A grain’s reproductive strategy is to be ugly and bitter. Bitter is usually nature’s way of saying, “I’m toxic, don’t eat me”. Maybe we’ll get into grain Lechtins and Phytates someday…
Anyway, back to our fruit. So, it wants to be eaten so that the seeds can be dispersed. What happened to the fruit that wasn’t as colorful or as sweet? It did’t have any advantage so its traits died off. Fruit adapted to its environment and we in turn adapted to fruit. How did we do this? Metabolically speaking we developed the ability to gain weight fast from consuming fruit. This makes perfect sense if you think about it. Winter is coming and history has shown that food is very scarce this time of year, so if we can develop a way to gain weight prior to winter, we will have a greater chance of survival. This will make more sense if we look at it mechanistically.
Fructose is the major component of fruit. The interesting thing about fructose vs. other monosaccharides such as glucose or galactose is that it can only be metabolized by the liver. This was one of our adaptations to gain weight via fruit consumption. With the liver being the only route to fructose metabolism, when even slightly overloaded it will start to produce triglycerides and accumulate body fat. The other adaptation is actually really interesting. Fructose has been shown to induce leptin resistance. In short, leptin is sort of like the master hormone signaling to the hypothalamus the current energy balance in the body. Leptin is created by adipose tissue (fat cells) and circulates through the blood stream to communicate to the hypothalamus (in the brain) to let the body know it has plenty of energy (and you feel full and satisfied). Likewise, if you haven’t eaten in a long time you lose fat resulting in a decrease in leptin production (your body has an increased need for energy and you get hungry). So, just like with insulin resistance, we can become resistant to leptin. This is a good thing if you’re trying to gain a few pounds for the winter, but a bad thing if you still want to look good naked 😉 So, this ability of fructose to induce leptin resistance was beneficial 50,000 years ago and is probably what helped us to survive through ice ages. Then, as we went through winter with no fruit (or really any carbs at all) we gained our leptin sensitivity back and lost the accumulated fat the liver had stocked up all fall. The problem now: WE LIVE LIKE IT IS A PERPETUAL STATE OF SUMMER/FALL. Not only do we get a lot of fructose from fruit in the winter, but every bit of sugar you eat is 50% fructose. That’s right – sucrose is a 50/50 blend of glucose and fructose. So that added sugar in bread, condiments, pastries, soda, juice, etc is an even bigger hit of fructose.
In reality, some people can eat fruit and get away with it. But does this describe you?
- A lean individual who limits other intakes of sugar and carbs
- Someone who has a lechtin free diet (grains/beans)
- Someone who gets plenty of sleep (lack of will induce leptin resistance)
- Someone who is not worried about this “last 10 pounds”
Any nutritionist will argue that fruit is good for you because of various vitamins/minerals, fiber, and polyphenols. I agree. However, these can be gotten from other sources such as quality meats and vegetables. Stick to berries as they pack the most beneficial punch with a low fructose load. http://thepaleodiet.com/wp-content/uploads/2011/04/Fruits-Sugars.html
Sorry for the length…