Fructose
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Much of the fruit that humans consume today is not the same fruit that humans evolved to eat.
Take the peach, for example. The peach probably originated in China (1) and has been cultivated as far back as 4000 BC. The natural peach was 25 mm in diameter, consisted of 64% edible flesh and 36% seed, and tasted "earthy, sweet, sour, and slightly salty" (2). In 2016, the average peach is 100 mm in diameter, consists of 90% edible flesh and only 10% seed, and tastes "sweet, refreshing, and juicy" (2). After only a few thousand years, which is an inconsequential blip in evolutionary time, the peach has become 16 times larger not to mention juicier and sweeter than its wild cousin. Clearly, it's not the same peach it once was.
Consider the watermelon. The watermelon originated in southern Africa (3) and has been cultivated since 3000 BC, yet back then it would not have even been recognized as the watermelon of today. In 3000 BC the natural watermelon was only 50 mm in diameter (that's not a typo), consisted of only 1.9% sugar, and tasted "extremely bitter or bitter-sweet" (4). Today, the watermelon has been cultivated to a massive 660 mm in diameter, consists of 6.2% sugar, and tastes "deliciously sweet and so juicy that it sometimes explodes when ripe" (4). Wow! So again, after only a few thousand years, the marvels of human cultivation have produced a watermelon that is 1,680 times larger not to mention juicier and sweeter than its wild cousin. Just like the peach, it's not the same watermelon it once was (in fact, the watermelon may be the least natural fruit in existence).
Now I'm not here to pick on peaches and watermelons. In fact as far as fruits go, peaches and watermelons are nowhere near the worst offenders with regards to what I am going to talk about. They're not big-time, they're small-time. The big-time offenders, the Al Capones of the fruit world, well they would have to be dried fruit, the apple, the mango, the grape, the pear...umm, I'd better stop there.
The problem with fruit is fructose, a sugar that masquerades as a good guy when in fact it is a bad guy.
Dr Jekyll
Table sugar, or sucrose, can be found among plants such as the sugarcane whereupon it is harvested for human consumption. Too much sucrose in the diet leads to obesity, type II diabetes, and many other health problems. But why? To answer this, we must realize that sucrose is actually composed of two simple sugars - it is 50% glucose and 50% fructose.
The first component of sucrose, glucose, is found in certain plants in the form of sucrose, and it also circulates in the blood of animals as blood sugar. The effect of a food on blood sugar can be measured using a scale called the glycemic index, which gives glucose a value of 100 and measures all other foods against it (5). The higher a food's glycemic index, the more it spikes the blood sugar, and spikes in blood sugar crank up the body's release of a hormone called insulin which is public enemy number one when it comes to producing insulin resistance, the prelude to obesity and type II diabetes (5). Some of the foods with the highest glycemic index include white rice with an index of 72-89, white bread with an index of 70-75, and coca cola with an index of 63 (6). If spikes in blood sugar resulting in high blood insulin levels are bad for health, then white rice and white bread are truly evil.
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Insidious. |
Wrong - although fructose comes across as the Dr Jekyll of the sugar world, it's exactly the opposite.
Mr Hyde
Unfortunately, it is now known that although fructose does not raise blood sugar much compared to glucose, it is even more strongly linked to obesity and type II diabetes than glucose.
Let's look at a couple of great studies that have shown this. One 1980 study gave 15 healthy subjects an extra 1,000 calories per day as either glucose or fructose and measured their changes in insulin sensitivity over one week (7). While the glucose group showed no changes in insulin sensitivity, the fructose group showed a 25% worsening of insulin sensitivity - after just one week! Another 2009 study randomized their 32 participants to eat 25% of their calories as Kool-Aid sweetened with either glucose or fructose for ten weeks (8); the number 25% may seem high but actually, many people consume this much glucose or fructose daily (9). By the end of the study, the glucose group developed 3.2% more intra-abdominal fat as well as a mild reduction in insulin sensitivity, but the fructose group gained an unholy 14% more intra-abdominal fat and even worse developed pre-diabetes - after only ten weeks! So after just one week of excess fructose, a healthy person will develop insulin resistance, and by ten weeks, they will develop pre-diabetes - well, it's not much of a stretch to think about what years or decades of excess fructose consumption can do to the human body...obesity and type II diabetes, here we come.
How is it that fructose is even worse then glucose, when it hardly raises blood insulin levels compared to glucose? The reason has to do with how the two simple sugars are metabolized. Glucose can be used by almost every cell in the body - liver, muscle, brain, red blood cells - and it can also be easily stored as glycogen in tissues such as liver and muscle. The harmful effects of excess glucose are therefore dispersed throughout the body, the damage mitigated by many organs and tissues. Fructose, on the other hand, can only be metabolized by the liver, where excess fructose is converted to fat in the liver (5). The harmful effects of excess fructose are therefore directed solely at the poor liver; it cannot receive metabolic assistance from any other organ or tissue. Fructose forces the liver to become fatty liver, and fatty liver is critical for the development of insulin resistance (5). So while glucose spikes the blood sugar, cranking up blood insulin levels which are a prelude to insulin resistance later on, seemingly innocent fructose barely touches the blood sugar - instead, it targets the liver where it is converted into fatty liver, and just skips the prelude to directly stimulate insulin resistance then and there.
Hence, insulin resistance by a paltry one week. Hence, pre-diabetes by a mere ten weeks.
Fructose is Mr Hyde; its terrible effects are completely hidden by the glycemic index. The bottom line? It is best to avoid foods high in total metabolic fructose (combined sucrose and fructose). The worst offenders, starting with Al Capone, are processed foods containing lots of sucrose or high-fructose corn syrup (added to many fast foods as well as processed foods - check the label) followed by dried fruit (such as the raisin, dried fig, and date), and then the apple, mango, grape, and pear (10). The fruits with the lowest amounts of total metabolic fructose, starting with the best, would have to be the lime, the avocado, the lemon, the tomato, the guava, and the strawberry (10).
I'm not saying to abandon all fruit, however it is important to realize that human cultivation has increased the fructose content of the fruit of today and therefore it ought to be eaten in restricted amounts. Fructose does not spike the blood sugar as much as glucose, oh no, it does something even worse by smashing your poor liver.
Be nice to your liver. Go easy on the fruit, except for those fruits with the lowest amount of total metabolic fructose.
Solace (inspired by Barrie Phillips).
References
(1) Thacker C. 1985. The History of Gardens. Berkeley: University of California Press.
(2) Kennedy J. 2014. Artificial vs Natural Peach. https://jameskennedymonash.wordpress.com/2014/07/09/artificial-vs-natural-peach/.
(3) https://en.wikipedia.org/wiki/Watermelon.
(4) Kennedy J, 2014. Artificial vs Natural Watermelon and Sweetcorn. https://jameskennedymonash.wordpress.com/2014/07/14/artificial-vs-natural-watermelon-sweetcorn/.
(5) Fung. 2016. The Obesity Code: unlocking the secrets of weight loss. Scribe.
(6) http://www.glycemicindex.com/.
(7) Beck-Nielsen H, Pedersen O, Lindskov HO. 1980. Impaired cellular insulin binding and insulin sensitivity induced by high-fructose feeding in normal subjects. The America Journal of Clinical Nutrition 33, 273-278.
(8) Stanhope KL, Schwarz JM, Keim NL, Griffen SC, Bremer AA, Graham JL, Hatcher B, Cox CL, Dyachenko A, Zhang W, McGahan JP, Seibert A, Krauss RM, Chiu S, Schaefer EJ, Ai M, Otokozawa S, Nakajima K, Nakano T, Beysen C, Hellerstein MK, Berglund L, Havel PJ. 2009. Consuming fructose-sweetened, not glucose-sweetened, beverages increases visceral adiposity and lipids and decreases insulin sensitivity in overweight/obese humans. The Journal of Clinical Investigation 119(5), 1322-1334.
(9) Schulze MB, Manson JE, Ludwig DS, Colditz GA, Stampfer MJ, Willett WC, Hu FB. 2004. Sugar-sweetened beverages, weight gain and incidence of type 2 diabetes in young and middle aged women. JAMA 292(8), 927-934.
(10) http://www.reducetriglycerides.com/reader_triglycerides_low_fructose_fruit.htmm.
