Pamela A. Popper, Ph.D., N.D.

Wellness Forum Health

I’m receiving a lot of emails these days from people who are concerned that eating fruit is not good for health. They’ve been told that eating “too much” fruit increases the risk of type 2 diabetes, promotes weight gain, interferes with weight loss, and can both increase the risk of cancer and of dying from cancer.

Studies are of limited value in determining the link between fruit intake and the development of type 2 diabetes. This is because almost all of them involve self-reported data, which has been determined to be “…worse than nothing,” according to researchers.[1] [2] The reason is that people generally misreport their food intake. For example, analysis of NHANES data gathered by the USDA to determine eating habits of Americans and to provide information to guide development of public policy shows that almost all Americans should be normal-weight based on self-reported calorie intake. Obviously this is not true. So, it is not surprising that some studies show that eating fruit improves glycemic control for diabetics,[3] and that consuming apples decreased the risk of type 2 diabetes in the Women’s Health Study[4] and in a study conducted in Finland,[5] but not in the Iowa Women’s Health Study.[6]

Another problem in analyzing research concerning the effect of fruit on any marker of health is the reductionist approach that dominates almost all diet and health-related research today. Studies are designed to see if apples, or berries, or citrus fruit alone prevent or promote type 2 diabetes. It is difficult to find studies that have looked at the effect of a health-promoting dietary pattern that includes generous amounts of fruit on the risk of developing type 2 diabetes, or on glucose control for those who already have it.

According to some health professionals, the glycemic index of many fruits is responsible for the increased risk of developing type 2 diabetes or worsening glucose control for diabetics. The glycemic index assigns a number to foods according to how much they increase blood sugar levels. I have tried very hard to find something to like about this method of evaluating foods, but have concluded that it has no redeeming value. First, the valuation is calculated for one food consumed by itself, which is not the way food is eaten. No one knows the glycemic index for the endless combinations of foods people eat, which we call meals. But even using the glycemic index to evaluate individual foods presents a problem. Whole milk has a better GI, 41, than grapes, 59, and bananas, 62, and even sweet potatoes, 70.[7]  Does that mean that whole milk is better than these foods? Of course not. Use of the glycemic index to promote certain food choices involves cherry picking some comparisons and hoping that people don’t check out the online tables themselves, which seem to discredit the theory.

One thing we do know about type 2 diabetes is that higher-fiber diets reduce the risk of developing it[8], and higher fiber diets result in better glucose control.[9]  Fruit is a high-fiber food group. One apple contains 4.4 grams of fiber, and just one pear contains 6 grams.

For those who want to avoid diabetes or for diabetics who want to improve their health, the nutrient to be concerned about most is fat. A high-fat diet results in the accumulation of intramyocellular fat, fat inside the cells, which interferes with the action of insulin and results in insulin resistance.[10] [11] Eating a low-fat plant-based diet has been shown to lower intramyocellular fat levels.[12]  I can find no studies showing that fruit, when included in a low-fat plant-based diet worsens outcomes, or that limiting fruit intake improves health outcomes.

Studies regarding fruit and cancer suffer from the same methodological issues associated with studies of fruit and diabetes. Most of the studies involve evaluating both fruit and vegetable intake, or are reductionist, looking at single foods or nutrients. And most involve self-reported data, which is why the results are also inconsistent. Some case control studies show that higher fruit and vegetable intake lowers the risk of cancers of the oropharynx, esophagus, stomach, colon, rectum, and lung.[13] [14]  Other prospective cohort studies show no effect.[15] [16] [17]

What we do know about diet and cancer is that eating a plant-based diet with more fruit, vegetables, whole grains and legumes decreases the risk,[18] [19] and that such a diet can slow or stop the progression of cancer.[20]

Fears about fruit promoting cancer growth seem to result from confusion about the “sugar” in fruit. Fruits are high-carbohydrate foods, and almost all carbohydrate is converted to glucose, which is then used for energy by the body.  It is true that cancer cells take up glucose more than normal cells.[21] And it is also true that hyperglycemic cancer patients have worse outcomes.[22] [23]  But eating fruit by itself does not cause hyperglycemia. And since almost ALL carbohydrate is converted to glucose, in order to starve cancer cells by withholding glucose, foods like broccoli, kale, and sweet potatoes would need to be eliminated too.

Some research has shown that therapeutic water-only fasting, restricted calorie diets, and ketogenic diets may be useful for treating some forms of cancer, and turning cancer into a chronic disease rather than a life-threatening one.[24] [25] [26] The bottom line is that just restricting or eliminating fruit is not likely to change outcomes for cancer patients, however, much more comprehensive strategies are required.

As for fruit promoting weight gain or interfering with weight loss, most in the plant-based community agree that the calorie density of fruit and vegetables alone is insufficient to meet daily needs, which is why plant-based diets must include starchy foods like grains and potatoes.  A person living solely on fruit and requiring 2,000 calories per day would need to eat 20 bananas, or 20 apples, or 4.5 whole pineapples, or 6.5 pounds of grapes, JUST TO MAINTAIN CURRENT WEIGHT STATUS. Short of an eating disorder, it is difficult to imagine someone gaining weight while eating fruit.  And most people eat fruit in addition to vegetables, whole grains and legumes.

The focus on fruit in the diet is every bit as reductionist as focusing on individual nutrients in foods. Calcium by itself does not build strong bones, but a health-promoting diet, sun exposure and exercise do. Just eliminating animal foods does not improve health, however, adopting optimal diet and lifestyle habits and learning to say “no” to useless tests, drugs and procedures is the “secret” to great health. And just limiting fruit to 2 servings per day or eliminating it entirely will not prevent diabetes, weight gain, or reduce the risk of developing cancer or dying from it.

It is difficult enough to get people to adopt a plant-based diet and every additional restriction makes it harder to both get people on board and to keep them compliant. Most people like fruit, it’s easy to take to the office or on a plane, or in the car, and it is a much better way to satisfy a “sweet tooth” than many other choices. Let’s stay focused on the bigger picture, which is the dietary pattern a person eats, and that plant-based diets are better dietary patterns. Extra servings of fruit are not the problem; failure to get more people on board with dietary change is.

[1] Archer E, Pavela G, Lavie C. “The Inadmissibility of What We Eat in America and NHANES Dietary Data in Nutrition and Obesity Research and the Scientific Formulation of National Dietary Guidelines.” Mayo Clin Proc July 2015;90(7):911-926

[2] Archer E, Hand G, Blair S. “Validity of U.S. Nutritional Surveillance: National Health and Nutrition Examination Survey Caloric Energy Intake Data, 1971-2010.”  PLoS One October 9, 2013 DOI: 10.1371/journal.pone.0076632

[3] Jenkins D, Srichaikul K, Kendall C et al. “The relation of low glycaemic index fruit consumption to glycaemic control and risk factors for coronary heart disease in type 2 diabetes.” Diabetologia 2011;54:271-9

[4] Song Y, Manson J, Buring J, Sesso H, Liu S. “Associations of dietary flavonoids with risk of type 2 diabetes, and markers of insulin resistance and systemic inflammation in women: a prospective study and cross-sectional analysis.” J Am Coll Nutr 2005;24:376-84

[5] Knekt P, Kumpulainen J, Järvinen R et al. “Flavonoid intake and risk of chronic diseases.” Am J Clin Nutr 2002;76:560-8

[6] Nettleton J, Harnack L, Scrafford C, Mink P, Barraj L, Jacobs D Jr. “Dietary flavonoids and flavonoid-rich foods are not associated with risk of type 2 diabetes in postmenopausal women.” J Nutr 2006;136:3039-45

[7] http://www.health.harvard.edu/healthy-eating/glycemic_index_and_glycemic_load_for_100_foods

[8] The InterAct Consortium. Dietary fibre and incidence of type 2 diabetes in eight European countries: the EPIC-InterAct Study and a meta-analysis of prospective studies. Diabetologia. Published online May 29, 2015.

[9] Riccardi G, Rivellese A. “Effects of dietary fiber and carbohydrate on glucose and lipoprotein metabolism in diabetic patients.” Diabetes Care 1991 Dec;14(12):1115-1125

[10] Petersen K, Dufour S, Befroy D, Garcia R, Shulman G. “Impaired Mitochondrial Activity in the Insulin-Resistant Offspring of Patients with Type 2 Diabetes.”  NEJM Feb 12 2004;350(7):664-671

[11] Sparks L, Xie H, Koza R et al. “A high-fat diet coordinately downregulates genes required for mitochondrial oxidative phosphorylation in skeletal muscle.”  Diabetes July 2005;54(7):1926-1933

[12] Greco A, Mingrone G, Giancaterini A et al. “Insulin resistance in morbid obesity: reversal with intramyocellular fat depletion.”  Diabetes Jan 2002;51(1):144-151

[13] Block G, Patterson B, Subar A. “Fruit, vegetables, and cancer prevention: a review of the epidemiological evidence.” Nutr Cancer. 1992;18(1):1-29

[14] Liu C, Russell R. “Nutrition and gastric cancer risk: an update.” Nutr Rev 2008;66(5):237-249

[15] van Gils C, Peeters P, Bueno-de-Mesquita H, et al. “Consumption of vegetables and fruits and risk of breast cancer.” JAMA 2005;293(2):183-193

[16] See comment in PubMed Commons below Tsubono Y, Otani T, Kobayashi M, Yamamoto S, Sobue T, Tsugane S. “No association between fruit or vegetable consumption and the risk of colorectal cancer in Japan.” Br J Cancer 2005;92(9):1782-1784.

[17] George S, Park Y, Leitzmann M, et al. “Fruit and vegetable intake and risk of cancer: a prospective cohort study.” Am J Clin Nutr 2009;89(1):347-353

[18] Phillips RL. “Role of lifestyle and dietary habits in risk of cancer among Seventh-day Adventists.” Cancer Res. 1975;35(Suppl):3513-3522.

[19] Tanrtamango-Bartley Y, Jaceldo-Siegl K, Fan J, Fraser G. “Vegetarian diets and the Incidence of Cancer in a Low-risk Population.”  Cancer Epidemiol Biomarkers Prev February 2013 22; 286

[20] McDougall J. “Preliminary Study of Diet as an Adjunct Therapy for Breast Cancer.” Breast 10:18, 1984

[21] Hanahan D, Weinberg R. “Hallmarks of cancer: the next generation.” Cell 2011;144:646–674

[22] Derr R, Ye X, Islas M, Desideri S, Saudek C, Grossman S. “Association between hyperglycemia and survival in patients with newly diagnosed glioblastoma.” J Clin Oncol 2009, 27:1082–1086

[23] Krone C, Ely J. “Controlling hyperglycemia as an adjunct to cancer therapy.” Integr Cancer Ther 2005, 4:25–31

[24] Conyers R, Need A, Durbridge T, Harvey N, Potezny N, Rofe A. “Cancer, ketosis and parenteral nutrition.” Med J Aust 1979, 1:398–399

[25] Magee B, Potezny N, Rofe A, Conyers R. “The inhibition of malignant cell growth by ketone bodies.” Aust J Exp Biol Med Sci 1979, 57:529–539.

[26] Seyfried B, Kiebish M, Marsh J, Mukherjee P. “Targeting energy metabolism in brain cancer through calorie restriction and the ketogenic diet.” J Cancer Res Ther 2009;5 Suppl 1: S7–15