Naturally sweet, colorful, and delicious—fruits are the only parts of plants that are specifically designed to be eaten. For those who can tolerate carbohydrate, fruits are the healthiest sources on the planet, but what if you are carbohydrate-sensitive and need to avoid fruit? Don’t you need all those vitamins and antioxidants? What are antioxidants, anyway?
What is fruit?
Sugar alcohols (sorbitol, mannitol, xylitol, etc)
Sugars (fructose, glucose, sucrose)
We believe fruits are good for us because they contain fiber, vitamins, and antioxidants. While there is no evidence that humans require fruit in order to be healthy, of all the plant foods you can eat, fruits are the least likely to cause trouble. That’s because fruits are the only parts of the plant that are specifically designed to be eaten.
Since plants need animals like us to eat their fruits, most chemicals in fruits, including fruit antioxidants, are not designed to harm us (vegetables are not so innocent). Most fruits (with the notable exception of a wide variety of poisonous berries) are quite safe to eat whole
What all fruits have in common are seeds. This means that some foods we think of as vegetables—cucumbers, tomatoes, squashes—are actually fruits.
Seeds can be large (peach pit) or small (apple seeds). Each seed contains a precious plant embryo (baby plant), which needs to find a fertile spot and grow into an adult plant. Since plants can’t move, they need you (or other creatures) to eat their fruits in order to disperse their seeds.
When you stop to pick an apple from a tree, you will probably eat the fruit and toss the core, with its seeds inside, onto the ground, hopefully somewhere not too close to the tree. This helps the apple tree spread its seeds a little farther away from its trunk, and increases the chances of finding fertile ground; therefore the tree may reproduce more successfully. If a bear snacks on an apple, he will probably eat the whole apple, core and all, in which case the seeds get to take a free ride through the bear’s intestines. By the time the seeds exit the bear, the bear will probably not only be very far away from the tree, but will also deposit the seeds onto the ground complete with a supply of natural fertilizer, as a parting gift.
So, ultimately, the fruit is there to serve the reproductive needs of the plant, and creatures like us are just unsuspecting taxicabs for their seeds.
Plants have evolved over hundreds of millions of years to use animals for their own purposes, and they know what they are doing. Not only have they made most fruits sweet, delicious, and easy to pick, they have also included in the flesh of the fruit sugar alcohols, like sorbitol, mannitol, and xylitol. If you have ever eaten “sugar-free” chocolates or candies that contain sugar alcohols, you know that they can have a laxative effect. This is no accident. Fruits contain sugar alcohols to speed the seed along its route through your body. This is also why prunes, which are high in the sugar alcohol sorbitol, are famous for relieving constipation. Plants want you to transport their seeds, they do not want you to digest and destroy them. The longer it takes for a seed to travel through an animal’s gastrointestinal tract, the higher the chance that it will be damaged in the process, so the plant would like its seeds to move through you as quickly as possible.
Plants know we have a sweet tooth, so they include sugars in their fruits. Fruits contain three types of simple sugar: fructose (fruit sugar), glucose (the same sugar we have in our bloodstream), and sucrose (table sugar—which is made from sugar cane and beets). Humans evolved from a fruit-eating primate ancestor, and most human populations have eaten fruit ever since, so we are probably very well-adapted for handling the amount of sugars found in whole fruits. Unfortunately, many of us have developed difficulty processing sugars, probably because we have been eating far too much refined carbohydrate as part of the Western diet for too many years of our lives.
There is a huge difference between eating whole fruits and eating concentrated and added sugars in processed foods and drinks. Fruits contain lots of fiber and water, so they are very filling and satisfying; it’s difficult to overeat fruit without feeling uncomfortable. However, it’s all too easy to drink a liter of fruit juice, eat a pint of ice cream, or polish off a king-sized candy bar. Most of us have not adapted very well to a high refined carbohydrate diet, since it is a relatively new trend in human history.
It is well established that diets high in sugars are unhealthy (see my carbohydrates page), but if you choose to include some sugar in your diet, the healthiest sources of sugar in my opinion are whole fruits—not fruit smoothies, not fruit sauces, not fruit juices, not even dried fruits—these are all much higher in sugar than whole fruits and too easy to overeat.
If you have trouble processing carbohydrate or if you are trying to lose weight, you would be wise to choose low-sugar fruits (such as berries), significantly limit fruit intake, or even avoid fruit entirely.
If you have IBS-D you may want to limit fruits, especially those highest in sugar alcohols.
Soluble fiber is a type of carbohydrate that gives fruit its shape. It is the scaffolding of the fruit. Its only purpose is to hold the fruit together.
Since phyto means plant, phytochemicals are just plant chemicals. You’ve probably heard a lot about “phytochemicals” or “phytonutrients” or “phytoestrogens”. We are told that colorful fruits and vegetables contain lots of phytonutrients, and that they are beneficial for our health because they are antioxidants.
What are antioxidants, anyway?
Antioxidants protect living cells from damaging “oxidation.” Oxidative damage is caused by things like radiation (sun rays), and the chemical reactions that naturally take place inside of living cells as part of everyday metabolism. These types of events have the power to remove electrons from stable molecules, thereby generating "reactive oxygen species” (ROS) or “free radicals.” Free radicals are very reactive and unstable, because they are missing an electron (a negatively-charged particle). Molecules like to have a perfect balance of positive and negative charges at all times. So, these free radicals, in their mad search for an electron, will rip off electrons from vulnerable molecules in their path until they are happily balanced again.
So, the original free radicals are now stable (phew), but now we have two new problems:
First problem: when an electron is torn away from a molecule, the molecule can break apart or stop functioning properly.
Second problem: these neighboring molecules are now missing electrons and are now unstable themselves…and so they go looking for electrons…and a chain reaction can occur. If this chain reaction is not stopped, the cell could be damaged beyond repair.
This is why antioxidants are so important. All living things need antioxidants to protect them from the oxidative dangers of daily living. Luckily, Mother Nature provided us with plenty of our own human antioxidants within our bodies, many of which are quite different from plant antioxidants. These include things like uric acid and cholesterol.
Unfortunately, it’s not that simple. Just because a chemical behaves like an antioxidant in a plant, or in a test tube, does not mean that it will behave the same way in our bodies. Many research studies show that plant antioxidants are poorly absorbed by our bodies, changed by our bodies into completely different compounds, or rapidly eliminated:
“Unlike the traditional vitamins, phytochemicals as dietary components are not essential for short-term well-being, and whereas the body has specific mechanisms for the accumulation and retention of vitamins, in contrast, phytochemicals are treated as non-nutrient xenobiotics [foreign substances that shouldn’t be there] and metabolised so as to eliminate them efficiently.” [Crozier 2009]
However, when scientists extract chemicals from fruits and give them to people in unnaturally high doses, they can be harmful. Nature intended us to eat the whole fruit, not purified concentrates of fruit chemicals.
The majority of fruit phytochemicals that you have heard about fall into two categories: the polyphenols and the carotenoids.
Polyphenols (aka flavonoids)
Polyphenols are colorful plant chemicals that share a similar chemical structure. Most of them are just pigments, not toxins, so the polyphenol family is relatively mild-mannered, at least at concentrations found in nature. While you may have heard that these substances have magical anti-inflammatory properties and that they can “boost” your immune system, the truth is that the vast majority of scientific studies have been done in vitro (test tube conditions) and have used doses far too high to be found in nature. Even if polyphenols do have magical healing powers, we are unlikely to benefit from them, because they are poorly absorbed, and transformed by our small intestine, liver, and colon, into completely different substances. Below is a list of the most commonly-encountered fruit polyphenols:
These are blue, red, and purple pigments that give fruits like blueberries, cherries, raspberries, strawberries, blackberries, and concord grapes their beautiful colors. They also serve as natural plant sunscreens.
Based on experiments in test tube conditions (in vitro) and in laboratory animals, scientists think anthocyanins may have anti-cancer, anti-inflammatory, and antioxidant properties, but there are no useful human studies yet that can tell us whether or not this is true.
Even if they were miracle molecules, eating bushels of berries would not help you, because less than 0.1% (one in a thousand!) of the anthocyanins you eat make it into your bloodstream, and those that make it disappear within a few hours.
This is the most common polyphenol pigment in the diet; many fruits and vegetables contain at least some quercetin. Capers, cranberries, black currants, apples, grapes, blueberries, and apricots are the fruits that contain the most quercetin.
Of all the polyphenols, quercetin is the easiest to absorb; about 20% of what you eat will make it into your bloodstream, but it only lasts a few hours.
There have been some human studies that suggest possible anti-inflammatory and anti-oxidant effects for quercetin. However, the doses that are used in these studies are impossible to get from food. There are currently no clinical trials of quercetin as a cancer treatment drug. [Read my post "Foods that Cause Hypothyroidism" to learn how quercetin can interfere with thyroid function.]
Resveratrol is found in a variety of fruits, including blueberries and cranberries, and also in peanuts, but grape skins contain the most resveratrol, and are the most famous example. For a more in-depth look at the proposed health benefits of the resveratrol in red wine, read my Psychology Today post: "Can Red Wine Reduce Your Risk for Alzheimer's?"
Resveratrol is not a pigment (pure resveratrol is white). Plants use resveratrol to prevent fungus from infecting their fruits, therefore it is a fungicide.
These are more common in vegetables than fruits, but some fruits, like grapes, persimmons and blueberries, do contain tannins, as do apple skins and pear skins. Tannins are bitter and have an astringent quality (they make your mouth feel dry).
Tannins are tannish-white in color but are not pigments; they are defensive chemicals designed to protect plants from harsh weather and all kinds of predators.
Tannins are destroyed in the drying process, so dried fruits (like raisins) do not contain tannins. Cooking reduces the amount of tannins in foods.
Tannins are hard to absorb; most tannins make it all the way to the colon (towards the very end of the digestive tract) without being absorbed, and by that time, most of them have been changed into different molecules by the digestive process.
Tannins have the ability to bind to proteins, which is what makes them good for tanning leather (this is how they got their name). However, this same power to bind proteins means that tannins can bind to proteins in our digestive tract. Since our digestive enzymes are proteins, tannins can interfere with proper digestion of foods, especially proteins.
Tannins also interfere with our ability to absorb “non-heme iron”, which is the form of iron found in plant foods. They do not interfere with our ability to absorb “heme” iron, which comes from animal foods.
Some tannins behave like antioxidants under test tube conditions (in vitro) but this hasn’t been proven in living systems (in vivo). Some tannins have antibiotic (bacteria-killing) and antiviral properties. A few animal studies suggest that tannins may be helpful in cases of diabetes and heart disease, but there have been no clinical studies yet. Some in vitro studies suggest possible anti-cancer activity, but there have been no clinical studies in humans.
In my post "Cranberries for UTI Prevention: Crimson Crusader or Juicy Gossip?" I provide a more detailed analysis of where the science stands in relation to the claim that the tannins in cranberries can prevent urinary tract infections.
In laboratory studies (in vitro), tannins have been shown to interfere with the activity of an important enzyme called Topoisomerase II. This enzyme is required for DNA to function properly. This makes sense because plants use tannins to protect themselves against predators. However there are no studies available to prove or disprove this activity in humans (in vivo).
Carotenoids are red/orange/yellow pigments that come in various forms, the most famous one being beta-carotene. Most carotenoids come from vegetables such as carrots and sweet potatoes, and in fruits that we think of as vegetables (such as pumpkins and squashes—see my post: "Are Pumpkins Cancer-Squashing Superfoods?"). However, there is one carotenoid that is famous for being found in fruits—lycopene.
Lycopene is a bright red pigment found in watermelon, guava, pink grapefruit, papaya, pomegranate and tomatoes. Unlike many other carotenoids, lycopene possesses no Vitamin A activity.
We are just lousy at absorbing carotenoids, especially lycopene, from raw foods. Only about 1% of the lycopene from a raw carrot makes it into your bloodstream! This is because the tough cellulose (fiber) of the plant’s cell walls is in the way. Cooking helps to free the lycopene from the food, but it also can reduce the amount of lycopene in the food. Eating fat with lycopene helps absorption. When eaten with some fat, people absorb an average of 5 mg of the lycopene from cooked foods, regardless of how much of the food they eat.
So, is lycopene good for you? Scientists would have us believe that it can help protect us from heart disease and cancer. However:
Studies in humans looking at lycopene and its effects on heart disease have produced conflicting results.
Most of the studies looking at the potential for lycopene to reduce cancer risk have been negative. The FDA reviewed the evidence and concluded that:
"there was no credible evidence to support qualified health claims for tomatoes or tomato-based foods and a reduced risk for lung, colorectal, breast, cervical, or endometrial cancer. FDA further concluded that there was no credible evidence to support qualified health claims for lycopene, as a food ingredient, component of food, or as a dietary supplement, and a reduced risk of any of these cancers… there was very limited credible evidence for qualified health claims for tomatoes and/or tomato sauce and a reduced risk for prostate, gastric, ovarian, and pancreatic cancers provided that the qualified health claims were appropriately worded so as to not mislead consumers.” [Kavanaugh 2007]
These innocent chemicals are mainly found lurking deep inside the rugged pits of certain fruits, such as apricots, peaches, cherries, mangoes, and plums. These types of seeds are virtually indestructible without tools, and it’s a good thing we can’t chew them open. When these seeds are damaged, the nontoxic glycosides mix with an activating enzyme and poof—you’ve got cyanide. Other foods that can generate cyanide include: bitter almonds, marzipan, bamboo shoots, cassava root (tapioca), lima beans, sorghum, apple seeds and pear seeds. Luckily, proper processing of these foods by grinding, boiling and soaking can remove the cyanide and make them safe to eat.
The human body can detoxify tiny quantities of cyanide, but at higher doses, cyanide can interfere with iodine within your thyroid gland and cause goiter or hypothyroidism. At higher doses still, cyanide can suffocate your mitochondria (your cells’ energy generators), which can be fatal.
Glycoalkaloids are produced by nightshades—eggplants, tomatoes, peppers, white potatoes, tobacco, and goji berries. People think of most of these as vegetables, even though most of them are fruits because they have seeds. These compounds will be explored in the vegetables section of the site.
The bottom line about fruit
- Fruits, though delicious, are not required in the human diet.
- The specialized plant chemicals found in most edible fruits are far less likely to bother most people than those found in vegetables.
- There is no clear evidence that the naturally-occurring phytochemical antioxidants in fruits are beneficial to human health. In fact, some of these chemicals, particularly in concentrated forms, are potentially harmful.
- Due to sugar and sugar alcohol content, many people may need to limit how much fruit they eat. However, if you do choose to include some sugar in your diet, whole fruits are the safest sources of simple sugars available in nature.
- If you are at a healthy weight, do not have IBS-D, and do not have diabetes or any other carbohydrate-related health conditions, you are likely to be able to safely enjoy fruit.
Cornell University Department of Animal Science. Plants poisonous to livestock.
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