The Obesity Code: Unlocking the Secrets of Weight Loss

Chapter 15 focuses on artificial sweeteners. Several artificial sweeteners emerged after the discovery of saccharin in 1879, including aspartame (NutraSweet), acesulfame potassium, and sucralose (Splenda). In the second half of the 20th century, food companies increasingly relied artificial sweeteners to flavor their products, presenting low-calorie, sugar-free foods and beverages as healthy ways to lose weight. Consumption of artificial sweeteners increased more than 12-fold between 1965 and 2004, largely because of diet sodas (169). Both the AHA and the American Diabetes Association endorsed low-calorie sweeteners in 2010, claiming that these chemical additives supported weight loss. Fung contests this claim. Citing the American Cancer Society, the San Antonio Heart Study, the Northern Manhattan Study, and other surveys, he argues that foods high in artificial sweeteners, such as diet drinks, increase insulin levels and thus fuel obesity, diabetes, and heart disease. These findings are at odds with studies funded by food corporations, which do not show a relationship between sugar-sweetened drinks and obesity. Artificial sweeteners are low in calories, but they encourage overeating because they are less satiating than high-calorie, high-sugar sweets. In short, diet soda and other low-calorie, artificially sweetened foods do not support weight loss.

Chapter 16 explains the differences between “good” and “bad” carbohydrates. Refined foods have a high glycemic index, in contrast to traditional whole foods, even when they contain similar amounts of carbohydrates. The problem, then, lies not in carbohydrates per se, but in the way they are processed. A key issue is the removal of nutrients, which allows the body to digest and absorb carbohydrates very quickly. Technology plays a central role in how carbs are processed. Modern machine milling, for instance, pulverizes wheat into very fine flour. The body absorbs this powder more quickly than the coarse grains produced by traditional stone milling, causing glucose and insulin to spike. Modern milling also strips nutrients from foods, such as bran, germs, middlings, oils, vitamins, fiber, proteins, and fats, leaving only carbohydrates. Although whole grain and whole wheat flours retain some nutrients, notably, bran and germ, the body nevertheless absorbs them quickly.

Fiber, the part of a carbohydrate that is not digestible, is central to maintaining good health. However, studies reveal that fiber intake has decreased dramatically over the centuries. The Paleolithic diet consisted of 77 to 120 grams of dietary fiber per day. By contrast, Americans now consume an average of 15 grams of fiber per day (181).

The benefits of fiber are manifold. High fiber foods must be chewed thoroughly, thereby reducing food intake. The bulkiness of fiber also reduces consumption. This is particularly true of soluble fiber, which expands in volume by absorbing water, thereby increasing satiety. Fiber also helps with weight loss by bulking up stool and increasing caloric excretion. The most important benefit of fiber, however, lies in its ability to reduce absorption and digestion. Fung dubs fiber the “anti-nutrient” (182) because it subtracts sugars and insulin, as well as reduces carbohydrate absorption. Removing fiber makes food taste better and last longer, but it destroys the balance of whole foods and is harmful to human health. Indeed, studies show that fiber protects against elevated insulin, even in combination with a high glycemic diet. Fiber also protects against obesity and types 2 diabetes. By contrast, a high glycemic index diet, combined with low fiber intake, raises the risk of both illnesses. Vinegar serves a comparable protective function though researchers do not yet understand why. 

Chapter 17 examines the role of protein in maintaining a healthy weight. Most foods that are high in protein are also high in fat, but high-protein diets, such as the revised Atkins Diet of the 1990s, which promotes increasing protein consumption and cutting back on fat, do not lead to weight loss. The problem once again lies with insulin. Dietary proteins can cause surges in insulin levels, a process that occurs independently of blood sugar. As Fung observes, only 23% of the body’s insulin response relates to blood sugar. Since insulin (and not glucose) drives weight gain, any foods that raise insulin levels can cause obesity, including proteins. This explains why Fung takes issue with the glycemic index, which only measures carbohydrates. Glycemic index diets target the glucose response, falsely assuming it mirrors insulin. However, a low glucose response does not necessarily result in a low insulin response. Fung argues that incretins, hormones produced in the stomach, account for up to 70% of insulin secretion after oral glucose consumption.

The way the gastrointestinal tract operates is an important factor in weight loss. Given the gastrointestinal tract’s nerve cells, receptors, and hormones, Fung dubs it the “second brain” (191). The stomach and small intestine produce incretins, which increase insulin release by the pancreas. Though this response is most associated with glucose, glucose is not the only food that causes this response. Fats and amino acids also stimulate incretin (and thus insulin production), as do non-nutritive sweeteners, such as sucralose. The discovery of this new pathway of insulin release highlights the potential role of all foods in weight gain, not because of their caloric content, but because they stimulate insulin. Dietary fats, however, generally have the weakest insulin-stimulating effect.

Various dietary proteins differ in their ability to stimulate insulin and weight gain. Meat has traditionally been associated with obesity because it is high in protein and fat, a belief confirmed by several European and North American studies. Fung makes two key observations related to meat. First, most cows are fed grain rather than grass (the food they eat naturally) and are given antibiotics to keep them healthy, both of which impact the character of the meat. Second, North Americans tend to overconsume muscle meats, rejecting other parts of animals, such as cartilage, organs, and blood. (Fung compares this tendency to drinking fruit juice without pulp). Organ meats are generally the fattiest parts of animals. Hence, consuming muscle meat favors protein over fat. In contrast to meat, dairy is not linked to weight gain, despite causing spikes in insulin. Indeed, studies show that people with the highest dairy diets have the lowest rates of obesity (196). These studies also show that milk and cheese are weight neutral, yoghurt may have a slimming effect, and butter leads to minor weight gain. According to Fung, portion size explains the differences between dairy and meat. A person can easily consume a large steak in one sitting, but not gallons of milk or slabs of cheese. In addition to curbing over-eating because of its satiating effect, animal protein stimulates incretin, which slows gastric motility and makes people feel fuller longer.

Food is not the only factor that affects the body’s production of insulin: Dietary fats and proteins account for approximately 10% of the body’s insulin response while blood glucose accounts for 23%, leaving 67% unaccounted for. Genetics also impact insulin response, as does the consumption of dietary fiber and whole foods.

Fung debunks reductive explanations that blame single food groups or food types for obesity. All foods stimulate insulin. Hence, all foods can lead to weight gain. Instead of vilifying individual foods and food types, Fung urges readers to avoid processed foods. Followers of the Paleo Diet only eat foods that were available in Paleolithic times, favoring whole foods, such as fruits, vegetables, meats, nuts, and seeds, over processed foods, such as grains, dairy, sweeteners, and alcohol. The low-carb, high-fat diet similarly promotes eating real foods while also allowing dairy products. Fung recommends eating a variety of whole foods and only eating when hungry though he recognizes that avoiding processed foods is not always possible.