Sometimes after sharing a delicious green smoothie recipe, I will get a comment or two from people who are concerned about the sugar content of the fruit that I used.
Their concerns are twofold:
1) They have heard that fructose causes weight gain, and fruits contain fructose, therefore fruits must cause weight gain,
2) Fruits are “high in sugar”, and sugar is bad for your metabolism and will derail weight loss – or increase the risk for diabetes.
And then I get people who refer to fruit as “sugary junk food”, or they just call it a “sugar bomb”, making absolutely no distinction between a piece of vitamin-rich, fiber-rich fresh fruit and a frosted cupcake.
These concerns are not isolated, and I often find myself having to defend fruit as a healthy food (yes, really!). Of course, if you have diabetes, then you have a legitimate need to monitor your carbohydrate intake for each meal (try these green smoothies submitted by readers with type-2 diabetes).
However, if you do not have diabetes, but you are concerned about the sugar content of fruit, I’ve written this post just for you. It contains the latest research on the subject of fructose, weight gain, metabolism, and how that relates to sweet fruit (and green smoothies) in the diet.
Are sweet fruits like bananas, mangoes and grapes good for you? Or are they metabolic (and weight loss) nightmares? Is it possible to eat too much sweet fruit? If we are encouraged to cut back on sugar, should we also cut back on bananas and grapes? Is sugar really toxic?
This is a very long post as I will cover a lot of sciency stuff about sugar, nutrition, weight loss and metabolism, so use these links to jump to the the sections most relevant to what you are looking for. Or read the bullet point summary below the links:
More than 20 peer-reviewed, scientific journals were consulted for this article. Additional sources are linked within the article that point to articles written by medical doctors and scientist. Most of the cited research is from 2009 or later.
What it all says is this:
- Sugars in fruits and added sugars in processed foods behave very differently in your body. Fruits contain vitamins, minerals, antioxidants and fiber which mitigate the effects of the sugars, while processed foods with added sugars are unhealthy.
- Dietary sugar intake guidelines (including those from the World Health Organization, American Heart Association, and the Institute of Medicine) do not pertain to naturally occurring sugars in fruits and other whole foods. Bananas, mangoes and grapes are, in fact, health food, not sugar bombs.
- There are ZERO legitimate, science-based dietary guidelines that call for a reduction or avoidance of sweet fruit in the diet due to sugar content for those who want to lose weight or reduce their risk for diabetes. In fact, the majority of studies show that increased fruit consumption correlates with a reduced risk of obesity and metabolic disorder.
- Fruits (even extreme fruit-based diets) do not cause weight gain, and every health study has shown that the higher the fruit intake, the lower the average body weight. Anecdotal evidence has shown this as well.
- Fructose has not been shown to directly cause weight gain. It may only cause weight gain because it contributes to excess calorie consumption, particularly when consumed in a sweetened beverage (but not from fresh fruits).
- The sugars in fruits do not cause, nor do they increase, ones risk for diabetes. In fact, lots of published health studies show that instances of diabetes is lower in populations with the highest fruit (and leafy green) intake.
- Green smoothies still have their fiber, vitamins, minerals and antioxidants intact and therefore do not pose additional health risks due to the naturally occurring sugars in the fruit.
Is Sugar Toxic?
Dr. Robert Lustig has made a name for himself on the Internet after he uploaded a video to YouTube downplaying the role of calories in the diet and making a case for why sugar is evil – especially fructose. Naturally, this video has made a lot of people ask me about the safety of sugar in fruit, even though Dr. Lustig himself says that consuming fructose in fruit is not the same thing as getting it from soft drinks.
Of course, a quick Google search turns up lots of rebuttals to Dr. Lustig’s statements on the “toxicity” of sugar. The International Academy of Cardiovascular Sciences published a concise overview of the criticism of Dr. Lustig’s statements on their website. For a more detailed rebuttal, read Dr. David Katz’s well-written article in Huffington Post that injects some reason into Dr. Lustig’s alarmism. Then there is nutrition expert Alan Aragon’s blog post exposing “the bitter truth about fructose alarmism”.
It is also interesting to note that our bodies require glucose (a sugar, also found in fruits) in order to feed our cells. Without glucose, you’d die. So to say that “sugar is toxic” is taking a very narrow view, and not a particularly honest one.
Of course, nobody is defending high fructose corn syrup as a health food. Nobody is saying that it is okay to eat all of the sugar you want. The purpose of this article is to shed light on where “fructose alarmism” on the Internet deviates from science, and why the naturally-occurring sugars in fruits are nothing to worry about.
What’s So Bad About Fructose?
For years, health studies have implicated sugar sweetened beverages (called SSBs) as a leading cause of weight gain in adults and children. One of the most commonly used and studied sweeteners in these studies is high fructose corn syrup (HFCS), which contains one of the same sugars that are naturally found in fruits – fructose.
Several studies have found that high-fructose diets (but not high-fruit diets) can cause leptin resistance in rats. 1,2 Leptin is a hormone that suppresses appetite after you have eaten a sufficient amount of food (ghrelin, the “hunger hormone” works in the opposite way, giving your body the signal to eat).
However, rat studies have yielded inconsistent results 3, and a study published in September 2014 in the journal PLOS One found that three months of feeding a high-fructose diet to mice failed to induce excessive weight gain or leptin resistance in mice. 4 Rodent studies also may not accurately portray how fructose is metabolized in humans. 5
In humans, leptin resistance is commonly associated with obesity. Leptin resistance means that even though your body might produce higher levels of leptin, you are unable to detect satiety, meaning that you cannot properly distinguish when you are full, so you continue to overeat. 6
An article in the Journal Nutrition in 2009 found that “fructose stimulates insulin secretion less than does glucose and glucose-containing carbohydrates. Because insulin increases leptin release, lower circulating insulin and leptin after fructose ingestion might inhibit appetite less than consumption of other carbohydrates and lead to increased energy intake. However, there is no convincing experimental evidence that dietary fructose actually does increase energy intake.” The authors of the report go on to say that “Concern about fructose should not extend to the naturally occurring fructose in fruits and vegetables. These are healthy foods that provide only a modest amount of fructose in most people’s diets.” 7
When it comes to sugar in the diet, context is important.
Added Sugars vs. Naturally-Occurring Sugars in Fruits – Is There A Difference?
It is important to differentiate between naturally-occurring sugars in whole foods, such as a banana or a mango, and added sugars (sweeteners) that are added to processed foods such as candy, soft drinks, baked goods, or concentrated into sweeteners such as high-fructose corn syrup (HFCS) or agave “nectar” syrup. While these sugars are identical on a molecular level – (the fructose in HFCS is the same fructose that is in a piece of fruit), the sugar that you ingest from drinking a soft drink and the sugar that you ingest from eating a banana behave very differently in your body.
A sweet fruit, such as a banana or an apple, contains vitamins, minerals, antioxidants, protein and fiber, as well as a mixture of fructose, glucose and sucrose. The fiber in fresh fruits slow the release of sugar into your system. The vitamins, minerals and antioxidants provide necessary nutrients.
A sweetened beverage or soft drink, on the other hand, does not contain fiber or any other nutrients, so the sugar hits your blood stream rapidly, causing a sugar spike, and subsequent crash. Fruits, and green smoothies, do not do this because they have fiber and other nutrients. There is hardly any nutritional value of a cookie, a chocolate bar or a can of soda. These foods provide empty calories, in that they give you a ton of sugar and/or fat, but no nutrients that your body needs.
Dietary Guidelines For Sugar Intake Do Not Include The Sugars In Fresh Fruits
A lot of people mistakenly think that established dietary sugar intake guidelines pertain to all sources of dietary sugars, including the naturally-occurring sugars found in fruits and vegetables. However, this is simply not the case.
All major sugar intake guidelines published by health organizations are for added sugars only, and do not pertain to fresh fruits. The World Health Organization (WHO), which has recently proposed slashing added (or “free”) sugar intake guidelines from 10% of total calories to 5% or less, does not include fresh produce in their sugar intake guidelines.
Similarly, the American Heart Association released updated sugar intake guidelines in 2009, tightening recommended limits for added sugars in the diet. 8 As for naturally occurring sugars, Rachel K. Johnson, PhD, MPH, RD, and one of the scientists who authored the AHA guidelines was quoted in an article in Prevention Magazine stating that: “Naturally occurring sugars in fruits, vegetables, low-fat dairy and whole grains don’t need to be avoided, and make up part of a healthy diet.”
The National Institute of Medicine published their recommendations on sugar intake in 2002. In the report, they also make a clear distinction between naturally-occurring (intrinsic) and added sugars, concluding that “sugar intake can be limited my minimizing the intake of added sugars and consuming naturally occurring sugars present in nutrient-rich milk, dairy products, and fruits.” The report also makes an evidence-based, yet still controversial, recommendation of limiting added sugars to no more than 25 percent of total calories consumed for adults.
In an article published in the Journal of the American Medical Association (JAMA), Dr. David S. Ludwig examines the health effects of fructose in the diet and concludes that while “excessive intake of refined sugar plays a significant role in the epidemics of obesity and related diseases”, “fructose in its primary natural form (whole fruit) is not associated with adverse effects up to the limits of human consumption.” 9
There are no legitimate, science-based dietary guidelines that call for a reduction or avoidance of sweet fruit in the diet. In fact, the overwhelming majority of published, peer-reviewed research on weight gain and metabolism suggests that people who consume more fresh fruits and vegetables have a lower incidence of obesity and diabetes.
Fruits Do Not Cause Weight Gain
Apples, banana, grapes or mangoes are not even on the radar for causes of the so-called obesity epidemic in the United States and other developed countries. Every single health study looking at fructose and its effect on weight gain has used isolated fructose, usually in a sweetened beverage. These beverages, which include sweetened lemonade, iced tea, and especially soft drinks, provide calories and sugar to the diet, but do not provide any other nutritional benefit.
I have combed through hundreds of scientific journal articles while researching articles and recipes for Incredible Smoothies. One thing I consistently notice is an overall pattern in the scientific literature. People who consumed more fruits and vegetables typically showed a lower risk of obesity, and for developing chronic illness, including metabolic disorders such as diabetes.
In fact, a 12-year study published in the International Journal of Obesity (2004) found that “participants tended to gain weight with aging, but those with the largest increase in fruit and vegetable intake had a 24% of lower risk of becoming obese compared with those who had the largest decrease in intake after adjustment for age, physical activity, smoking, total energy intake, and other lifestyle variables”. 10
Yet another study published in June 2006 in the Journal of the Academy of Nutrition and Dietetics also showed that normal-weight adults consume more fiber and fruit than obese adults. 11
The only published study I found relating to unrestricted fruit consumption and body weight is this one from 1971 in the South African Medical Journal. A very small sample of subjects were used, just 17 adults. Over a period of 24 weeks, they ate a diet that was primarily fruit. About 20 servings of fruit each day provided approximately 200 grams of total fructose. (For context, a medium banana contains less than 6 grams of fructose – less than half of its total sugar content). After the 24 week period, researchers found that the study participants did not gain weight, nor did they have negatively impacted insulin and lipid levels”. 12
Lots of anecdotal evidence for a negative correlation between fruit consumption and weight gain can be found online at many of the websites and blogs that promote fruitarian diets (not a diet I promote). In my experience, fruitarians tend to have lower body weights, and exhibit low body fat, even though 80+ percent of their calories comes from sweet fruits.
I have first-hand experience with this during periods when I dabbled in the raw vegan movement. I followed the 80-10-10 diet promoted by Dr. Douglas Graham, consuming 80% of my calories from sweet fruits. I would eat up to 9 bananas per day, and that was just half of my fruit calories for the day (about 950 calories). During this period, I dropped weight and weighed the lowest I ever have in my life. My husband, who did the trial with me, also dropped weight and he had to add an extra 500 calorie of sweet fruit per day just to maintain his lowest weight he’s ever been as an adult. (He consumed at least 2500 calories per day, about 1800-2000 of which was from sweet fruits).
During this time, we ordered our own blood tests and found that our fasting glucose remained within the normal range. So did our triglycerides. In fact, all of our health markers were spot on within the normal range, even after several months of eating this way.
While these examples are extreme, and I do not follow, nor do I recommend following fruitarian diets, our personal experience confirms what scientists are finding out about sugar, fructose and weight gain. Basically, sugar is not the sole cause of weight gain, and sugar, in and of itself, does not possess magic weight gaining abilities over other foods.
Sugar (Fructose) Is Not The Sole Cause Of Weight Gain
Fructose itself is not the sole contributor to weight gain. It might not even be a major contributor to bulging waistlines. Rather, it is the over-consumption of calories that lead to weight gain, regardless of whether those calories come from sugar (including isolated fructose), or fat.
In a 2012 article published in the Annals of Internal Medicine, researchers found that fructose “does not seem to cause weight gain when it is substituted for other carbohydrates in diets providing similar calories. Free fructose at high doses that provided excess calories modestly increased body weight, an effect that may be due to the extra calories rather than the fructose.” 13
A September 2014 article written by Dr. Gareth Leng and published in Experimental Physiology came to a similar conclusion, noting that “tempting though it is to blame fast food manufacturers, it is likely that many factors contribute to the increased prevalence of obesity”. 14
Yet another article published in Nutrition Research Reviews in June 2014, found that most studies on the health effects of fructose used isolated fructose, rather than fructose as it is typically ingested in the diet (combined with glucose and other carbohydrates). The author goes on to say that: “Based on a thorough review of the literature, we demonstrate that fructose, as commonly consumed in mixed carbohydrate sources, does not exert specific metabolic effects that can account for an increase in body weight.” While the author acknowledges that “the available evidence indicates that the consumption of sugar-sweetened beverages is associated with body-weight gain”, that “energy over-consumption is much more important to consider in terms of the obesity epidemic”. 15
The reason why fructose (isolated, concentrated fructose sweeteners, but not the fructose found in fruits and vegetables) are vilified in the media is because they are a major component of processed foods that provide empty calories, but no nutrition.
In an April 2014 article published in the journal Diabetes Care (by the American Diabetes Association), the authors point out that sugar-sweetened “beverages do not suppress the intake of other food calories to an appropriate degree to prevent weight gain. Thus, beverage calories can be viewed as ‘add-on’ calories, enhancing the risk of obesity.” They also conclude that “adding fructose to the diet without subtracting other sources of energy produces weight gain”. Therefore, their recommendation is to choose “fruit rather than drink fruit juice or fruit drinks.” 16
If sugar, or fructose in particular, directly caused weight gain, then everybody who drank soft drinks every day would be overweight or obese (they’re not). Fruitarians would be among the most obese people on the planet (they tend to be underweight). What it all comes down to is simply energy (calories) in, energy (calories) out. Sugar is just one of several sources of calories in our diets, and what (and how) we eat and live, as well as genetic, medical, psychological and environmental factors all play a role in our ability to maintain a healthy body weight.
What About Fruit Sugar (Fructose) And Diabetes Risk?
I could write a completely different post on dietary sugar and its link to diabetes. Much like fructose and weight gain, looking at the big picture from browsing the scientific literature suggests that diabetes is not as simple as “eat sugar, get diabetes”. There are multiple risk factors for diabetes that include, but are not limited to, dietary sugar intake. Obesity, diet (not just sugar intake, but also fat intake and excess calorie consumption which leads to obesity), lifestyle, genetic, and environmental factors may all play a role in the etiology of diabetes.
While several published articles in scientific journals implicate high-sugar (added sugars, not fruit) intake as an increased risk factor for diabetes, studies consistently show that a higher fruit intake may lessen diabetes risk. This has been shown again and again.
An article published in the Mayo Clinic Proceedings on January 29, 2015, concludes that “added fructose in particular (eg, as a constituent of added sucrose or as the main component of high-fructose sweeteners) may pose the greatest problem for incident diabetes, diabetes-related metabolic abnormalities”, but that “whole foods that contain fructose (eg, fruits and vegetables) pose no problem for health and are likely protective against diabetes and adverse CV [cardiovascular] outcomes.” 17
An article published in the journal Preventative Medicine (January 2001) looked at how fruit and vegetable consumption was associated with diabetes incidence in a cohort of
U. S. adults aged 25–74 years who were followed for about 20 years. The researchers concluded that based on the evidence, participants who consumed five or more servings of fruits and vegetables per day had a lower incidence of developing diabetes in the 20-year period. 18
Another article in the Journal Nutrition found that while added fructose, and high-fructose diets can potentially lead to increased risk for chronic diseases such as diabetes, “the fructose that occurs naturally in fruits and vegetables provides only a modest amount of dietary fructose and should not be of concern”. 7
The results of a study that analyzed the dietary habits of 21,831 people during a 12 year period was published in the Archives of Internal Medicine in 2008. The researchers found that “higher plasma vitamin C level and, to a lesser degree, fruit and vegetable intake were associated with a substantially decreased risk of diabetes”. They concluded that a diet rich in fruits and vegetables may provide a protective benefit against developing diabetes. 19
And one more analysis of the diets of 71,346 healthy, female nurses aged 38-63 years old over an 18-year period published in the journal Diabetes Care in 2008 found that an increase of fruit consumption was associated with a lower risk of developing diabetes. They also found that instances of diabetes were lower in nurses who consumed at least one serving of leafy greens per day. However, they found that fruit juice consumption was associated with a higher risk of diabetes. 20
It’s no surprise that fruit juice, but not fresh fruits, would be implicated as a risk factor for diabetes. Fruit juice has been stripped of its fiber content, and commercially prepared juices are much lower in vitamin, mineral and antioxidant content than whole foods or freshly blended green smoothies, and many contain added sugars.
In short, the sugar (including fructose) content of fresh fruits do not pose any increased risk of developing diabetes. Rather, fruits may provide a protective benefit against diabetes along with other dietary and lifestyle factors.
How Does All This Relate To Green Smoothies and Weight Loss?
There are no published scientific studies on green smoothies, so we have to rely on the reams of anecdotal evidence, and make conclusions based on published studies of their component parts – fruits and vegetables.
Unlike sugar-sweetened beverages and fruit juice, green smoothies still have all of their fiber. A freshly-made green smoothie still contains all of its vitamins, minerals and antioxidants. And while green smoothies are broken down more than a whole fruit, the act of blending replicates thorough chewing. It would be silly to fret about the potential health implications of thoroughly chewing your apple over swallowing it whole.
We know, based on the evidence, that sugar itself, is likely not the sole cause of obesity, diabetes and other chronic diseases. Also, the research consistently points to increased fruit and vegetable consumption as a protective measure against obesity, diabetes and chronic disease.
We know that while the naturally-occurring sugar in fruit is molecularly similar to isolated sugars that are added to foods, these two methods of obtaining dietary sugars behave very differently in our bodies. It is not so much the amount of sugar, but the source of that sugar. There is not one scientific study that implicates naturally-occurring sugars in whole foods as a cause for concern for increased risk of obesity and diabetes.
Even if a meal-replacement green smoothie with 1 banana, 1 cup pineapple and 1/2 cup cherries has almost as much sugar (41.3 grams – less than half of which is fructose) as a 21-ounce cola (44 grams), the sugar content is still irrelevant because:
- The green smoothie is a meal, while the cola is not,
- It is still a whole food and contains fiber, which slows down the absorption of sugar,
- It also contains vitamins, minerals, and antioxidants that mitigate the potential negative effects of sugar,
- There is no scientific evidence that the fructose content of fruits pose any health risk in humans, especially for obesity or the development of diabetes,
- Long-standing recommendations for daily fruit and vegetable intake suggest a minimum of five servings per day, and more than five servings per day is associated with a lower risk of obesity and diabetes
- About half of the sugar content of a green smoothie is glucose, which is a required nutrient that your cells need for energy production,
- You are almost guaranteed to consume fewer calories (and total sugar) when you fill up on a nutrient-dense, fiber-rich, lower calorie green smoothie once per day.
I lost 40 pounds by consuming up to ten servings (usually more) of fruits and vegetables every day since 2008, and I have maintained that weight loss the whole time (find out exactly how I did it).
Numerous people have lost weight (and improved blood sugar numbers) with green smoothies and whole foods, and I have published some of their testimonials on my website. Furthermore, I have helped thousands of people lose weight with my weight loss meal plans, and on average, I have seen people lose 10 pounds within a 3-4 week period by drinking one to two meal-replacement green smoothies per day while following my meal plan.
Based on my personal experience, the research that I have done, and the work I do to help people lose weight through my website and information, it is my belief that green smoothies are an effective way to increase fruit and vegetable consumption in order to lose weight and reduce the risk factors for chronic diseases, including diabetes.
One of the biggest mistakes people make when looking at a whole foods diet is to hone in on single components of foods. Instead of seeing a piece of fruit for the healthy, hydrating source of vitamins, minerals, antioxidants, protein, omega-3s and fiber that it is, they get hung up on the sugar content and minimize the other health-giving properties.
Whole foods do not need to be treated the same way as boxed, packaged, processed and otherwise manufactured foods. Yes, by all means, pay attention to the (added) sugar content of your breakfast cereal. Let added sugar be the main reason that you stop drinking soft drinks, or indulge in a favorite baked good once in a great while. But when it comes to whole foods like fruit, the sugar content is irrelevant, unless you have a medical reason (such as diabetes) to manage carbohydrate intake per meal).
So stop worrying about isolated components (like sugar content) of whole foods, and enjoy your green smoothie.
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