Many scientific studies show that eating whole fruit is healthful, even for people who are diabetic. However, some people, especially those who are overweight, prediabetic or diabetic, may be harmed by eating very large amounts of fruit. Most fruits are full of sugar, and after eating 12 to 15 oranges or clementines a day for several months, my blood triglyceride level was 460 (normal is under 150). For me, eating that much fruit had raised my blood sugar level so high that it caused a high rise in triglycerides that can increase risk for heart attacks and premature death. Two weeks after cutting back to eating one or two oranges a day, my triglycerides dropped to 146.
Some people may need to limit the amount of fruit they eat, even though many scientific papers show that eating fruit can help to prevent and treat diabetes and obesity. The main benefit of eating whole fruit may come from its soluble fiber, which passes unabsorbed to the colon where bacteria convert it to short chain fatty acids (SCFAs). These SCFAs help to lower high blood pressure (Nutrition, Metabolism and Cardiovascular Disease, January 2018;28(1):3–13), cholesterol (The American Journal of Clinical Nutrition, Jan 1, 1999;69(1):30–42), blood sugar and diabetes (Diabetes Care, Dec 1991;14(12):1115-25), and inflammation (Am J Clin Nutr, April, 2006; 83(4):760–766).
How Eating Too Much Fruit Can Raise Triglycerides
Your body stores sugar only in your liver and muscles. 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 clear your blood of high blood sugar 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)
Research Showing that Fruit is Good for Everyone, Even Diabetics
Various studies have shown that the health benefits of fruits outweigh the potential harm from their high sugar content. In one classic study, people who took in more than 20 fruits a day for six months suffered no adverse effects on body weight, blood pressure, insulin levels, or cholesterol levels (S Afr Med J, 1971 Mar 6;45(10):253-61). Eating 20 fruits per day appeared to help prevent heart attacks and diabetes by not increasing weight, blood pressure or triglycerides and by lowering the bad LDL cholesterol by 38 points (Metabolism, April, 2001;50(4):494-503). The amount of sugar in 20 fruits is approximately 200 grams or the amount in eight 12-ounce cans of soda.
A high rise in blood sugar after eating can damage every cell in your body and fruits are usually full of sugar. However, overweight diabetics who markedly reduced their intakes of fruit for 12 weeks gained no advantage in body weight, belly circumference or HbA1c, compared to those who ate a lot of fruit (Nutr J, Mar 5, 2013;12:29). HbA1c measures cell damage in diabetics. 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 (N Engl J Med, May 11, 2000;342(19):1392-8).
Why Sugar in Fruit is Safer than Sugar in a Cookie
When you eat a cookie made with sugar, 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 and 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.
Sugar in Drinks is the Most Harmful
Drinking fruit juices and soft drinks increases diabetes risk by more than 130 percent (Arch Intern Med, 2008;168(14):1487-1492), while eating whole fruits helps to prevent diabetes (Diabetes Care, July 2008). You get the same high rise in blood sugar after drinking orange juice as you do after drinking a Coca Cola. No solid food is allowed to pass into your intestines. If solid food could enter your narrow intestines, it would cause an obstruction that could kill you. As soon as solid food reaches your stomach, your pyloric sphincter at the end of the stomach closes and only a liquid soup is squeezed past it. Fruit juice causes an almost immediate rise in blood sugar, while a whole fruit can take as long as five hours after eating to empty from your stomach into your intestines.
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). However, pre-diabetics, diabetics and obese people are at increased risk for developing high blood sugar levels if they eat large amounts of fruit.
A normal fasting blood sugar below 100 does not rule out diabetes. 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.
My recommendations for preventing and treating diabetes include:
• lose excess weight
• avoid drinking anything with sugar in it, including fruit juices
• avoid sugar-added foods
• limit fruit to no more than five servings per day, and eat your fruit with other foods
• restrict meat from mammals and processed meats
• restrict fried foods
• eat lots of vegetables, whole grains, beans, nuts and other seeds
• keep blood levels of hydroxy vitamin D above 20 ng/ml
• check with your doctor about starting or increasing an exercise program
See Treat Diabetes with Diet and Exercise
How Eating and Drinking Sugar Can Cause Diabetes
Why Meat May Increase Risk for Diabetes