Many scientific studies show that eating sugar-rich whole fruit is healthful, even for people who are diabetic (Annals of Int Med, June 16, 2020;172(12):826-827). However, I tried eating an extremely large amount of fruit (15-20 clementines or 10 regular oranges per day), and my previously normal fasting blood triglyceride level rose to levels higher than 460 mg/dL (normal is under 150). Eating that much fruit had raised my blood sugar level so high that it caused the high rise in triglycerides, which increased my risk for heart attacks. One month after returning to eating just one orange a day, my triglycerides had dropped back down to 146.
Some studies show that high consumption of fresh fruits is associated with significantly reduced risk of type II diabetes, and a reduced risk for heart attacks in diabetics (PLoS Med, April 11, 217;14(4):e1002279). However, most fruits are full of sugar, and some people, especially those who are overweight, prediabetic or diabetic, may be harmed by having a high rise in blood sugar when they eat large amounts of fruit (Indian J Endocrinol Metab, Jan-Feb, 2017;21(1):71–75). A high rise in blood sugar after meals can cause considerable cell damage, even in people who have normal fasting blood sugar levels (PLOS Biology, 2018;16(7):e2005143).
How Eating Large Amounts of Fruit Can Raise Triglycerides
I am susceptible to becoming diabetic because I genetically store most of my fat in my belly and liver. A fatty liver is the most common cause of insulin resistance and diabetes. Your body stores sugar only in your liver, muscles and fat. When your liver and muscles are full of sugar, all additional sugar is converted to a type of fat called triglycerides. A high rise in blood triglycerides is usually due to a high rise in blood sugar that has been converted to fat (Eur J of Int Med, Feb 7, 2014). As your blood levels of triglycerides rise, you use up your good HDL cholesterol to carry the triglycerides from your bloodstream to your liver, so your good HDL cholesterol goes down. Then the triglycerides collect in your liver to cause a fatty liver that can lead to diabetes. When your liver is full of fat, it loses its ability to respond to insulin, and blood sugar can rise to high levels. When you eat, blood sugar levels rise. Your pancreas releases insulin which lowers high blood sugar levels by driving sugar from the bloodstream into the liver, but if you have a fatty liver, it will not accept sugar from your bloodstream. The progression toward becoming diabetic is:
• A high rise in blood sugar after meals — blood sugar greater than 140 one hour after you eat (Atherosclerosis, Nov 17, 2016;256:15-20)
• A high rise in insulin to lower the high blood sugar levels
• A high rise in triglycerides — fasting blood triglycerides greater than 150
• A drop in the good HDL cholesterol — fasting good HDL cholesterol below 40
• A fatty liver, which can be diagnosed with a sonogram that will show if your liver is full of fat (Nutrition, Metabolism & Cardiovascular Diseases, 12/30/2015).
Fruits Contain Healthful Soluble Fiber
Fruits contain soluble fiber, which passes unabsorbed to the colon where bacteria convert it to short chain fatty acids (SCFAs) that help lower blood sugar (Diabetes Care, Dec 1991;14(12):1115-25), high blood pressure (Nutrition, Metabolism and Cardiovascular Disease, January 2018;28(1):3–13), high cholesterol (American Journal of Clinical Nutrition, Jan 1, 1999;69(1):30–42), and inflammation (Am J Clin Nutr, April, 2006; 83(4):760–766). Just getting diabetics to eat more foods containing soluble fiber for six weeks lowered fasting blood sugar, average blood sugar, high insulin levels, total and bad LDL cholesterol, fasting triglycerides, and sugar in the urine (NEJM, May 11, 2000;342(19):1392-8).
Sugar added to foods can increase risk for diabetes and heart attacks, while sugar in whole fruits usually does not (Mayo Clinic Proceedings, published online January 29, 2015). Sugar in fruit is safer than sugar in a cookie, but still can harm you if you store extra fat in you liver:
• Sugar in fruit is far less likely to cause a high rise in blood sugar than sugars added to drinks or foods.
• Fruits contain soluble and insoluble fiber that binds to the sugar while it is in your intestines, delaying absorption and blunting the rise in blood sugar levels.
• Fruits help people control weight by making them feel full earlier so they eat less. One apple contains 23 grams of sugar and is more filling than a 16-ounce bottle of cola that has 52 grams of sugar.
• The sugar in fruits is combined with antioxidants that help to block inflammation that damages cells.
Why Sugar in Fruit is Safer than Sugar in a Cookie
When you eat a sugar-added cookie, the sugar is absorbed quickly and your blood sugar rises to high levels unless your liver can remove the added sugar. Fruits contain soluble fiber, a gel that binds to some of the sugar and helps to prevent the sugar from being absorbed in your upper intestinal tract. The sugar that is bound to soluble fiber passes to your colon where bacteria there break down the soluble fiber to release the sugar, so much of the sugar is absorbed many hours after you have eaten the fruit. When you eat a cookie, your blood sugar level will rise more than twice as high as it would have had you eaten a fruit with the same amount of sugar in it (J Agric Food Chem, 1990;38 (3):753–757).
However, my experience with eating large numbers of oranges has shown me that not enough of the sugar in fruit may bind to the soluble fiber to prevent a high rise in blood sugar. This may be particularly true of the juicy fruits, those that release a lot of liquid during the chewing process, such as oranges, watermelon or grapes. Eating these fruits in large quantities may have almost the same effect as drinking fruit juice.
Eating whole fruit is healthful and can help to prevent diabetes. It is normal for blood sugar to rise moderately 20 to 40 minutes after you eat fruit and drop back down in less than an hour (Lancet, Oct 1,1977;2(8040):679-82). I think that everyone as they age should avoid liquid sugar in fruit juice because it will cause a high rise in blood sugar. Pre-diabetics, diabetics and obese people are at increased risk for developing harmful high blood sugar levels if they eat large amounts of fruit. To find out if you are pre-diabetic, get a blood sugar level exactly one hour after eating a full meal. If your sugar is >140, you are prediabetic or diabetic. If you are overweight, diabetic, or prediabetic or have triglycerides >150, good HDL cholesterol below 40, or a fatty liver, you may be harmed by eating more than five fruits a day.