Claim #1: High insulin levels cause obesity.

Investigation: The subject of insulin and obesity is often addressed by those in the fitness industry. Many so-called nutrition experts blame obesity on elevated insulin. The scientific data does not support this claim. Insulin plays numerous roles in metabolism. Generally, popular diet gurus seem to have an incomplete understanding regarding insulin’s complexity and its effects on the body.

From Popular diets: A scientific review Freedman MR, King J, Kennedy E (2001) Popular diets: A scientific review. Obesity Research 9(S1):1–40 (3):

“Energy restriction, independent of diet composition (e.g. 15% to 73% CHO), improves glycemic control. The ability to lose weight on a calorie restricted diet over a short-term period does not vary in obese healthy women as a function of insulin resistance or hyperinsulinemia (high insulin levels). Golay et al. reported subjects consuming isocaloric diets (1,000 kcal) containing 15% CHO had significantly lower insulin levels compared with those consuming 45% CHO, yet there was no difference in weight loss between the two groups.

Grey and Kipnis studied 10 obese patients who were fed hypocaloric (1,500 kcal/d) liquid-formula diets containing either 72% or 0% CHO for four weeks before switching to the other diet. A significant reduction in basal plasma insulin levels was noted when subjects ingested the hypocaloric formula devoid of CHO. Refeeding the hypocaloric, high CHO formula resulted in a marked increase in the basal plasma insulin. However, patients lost 0.75 to 2.0 kg/week irrespective of caloric distribution.”

Popular fitness and nutrition literature often fails to mention the role of insulin in the central nervous system, where it acts to prevent weight gain. Recently, studies have shown that the selective genetic disruption of insulin signaling in the brain leads to increased food intake and obesity in animals, which demonstrates that intact insulin signaling in the central nervous system is required for normal body weight regulation (Freedman et al., 2001). When discussing insulin and its role in metabolism, why do authors generally not report this important information? My guess is that they are not aware of insulin’s role in the central nervous system and critical analysis is hard work.

Conclusion: Elevated insulin levels do not cause obesity. Obese people who eat excessive calories may also have elevated insulin levels. It is common logical fallacy to mistake correlation for causation. Fat gain is the result of excessive calories.

Claim #2: When dieting, avoid fruit.

Investigation: The fear of fruit comes from studies suggesting that 60 grams or more of fructose per day can up-regulate de novo lipogenesis (the process where excessive carbohydrates are converted to triglycerides in the liver), increase blood triglycerides, and induce insulin resistance. Keep in mind that fruit generally contains six to seven grams of fructose. That means it would take a bunch of fruit to get 60 grams of fructose. In most studies, the high consumption of fructose is generally due to the consumption of high levels of high fructose corn syrup (processing where varying portions of glucose are converted to fructose). Another consideration is that fructose causes minimal insulin secretion (Hale, 2007). Even if fructose consumption were high enough to elevate fat synthesis, a lack of insulin would probably result in increased fat burning. Assuming a calorie deficit, it all evens itself out at the end of the day.

What about the sugar content of fruit? Isn’t sugar a no-no when dieting? It’s important to realize that there are various types of sugar. According to the Advanced Carbohydrate Classification System, sugars include glucose, fructose, galactose, sucrose, maltose, lactose, and few others. In turn, fruits contain glucose, fructose, and sucrose in varying degrees. Do these sugars contribute to obesity? They can contribute to obesity if you consume excessive amounts. However, if eaten in moderation, there is no need to worry about fat gain.

Conclusion: I highly recommend eating fruit while dieting. Fruit is low in calories, is nutrient dense, contains fiber, and some types are very filling (particularly fruits that contain a fair amount of soluble fiber, e.g. an apple). In addition, fruit soothes many people's sweet tooth.