THE LINK BETWEEN DIET AND OBESITY
IN 1977 A U.S. Senate committee published a new set of dietary guidelines for Americans. Today, the U.S. Department of Health and Human Services and the U.S. Department of Agriculture (USDA) update and publish a new set every five years. To battle heart disease, which was the primary health concern in the 1970s, the guidelines recommended significant cuts to people’s consumption of dietary fat. Even as people became obese, these same guidelines were trotted out to do battle with this new enemy. The original 1980 food pyramid from the USDA suggested that Americans eat 6 to 11 servings of refined grains, such as bread, cereal, rice, and pasta every single day. I’m not sure that I know anybody who considers eating 11 slices of white bread daily to be a slimming diet. Yet this was the very diet recommended by the government of the United States and followed by other countries around the world. Virtually every health professional, doctor, and dietician in the world was soon giving this advice.
In addition to low-fat diets, the other big trend of the 1970s was the increase in leisure-time exercise. Before then, the idea of exercising for health or fun was as foreign as rap music to disco fans. Originally this advice was given to improve heart health, leading to a boom in “cardio” exercises such as aerobics and running. This advice was soon co-opted for weight loss as well, despite the utter lack of evidence supporting the efficacy of these exercise programs for weight loss.
Figure 4.2. The U.S. Department of Agriculture’s 1992 food pyramid3
Today, there are more gyms per capita than ever before. Local marathons and 10K races attract tens of thousands of runners. For most of her life, my grandmother never saw a gym. While exercise certainly has many health benefits (improved muscle tone, improved flexibility, increased bone mass, etc.), weight loss is not one of them. Scientific studies repeatedly confirm the minimal weight-loss effect of exercise programs. Two main reasons explain why. First, doing more exercise generally leads to eating more food, which will negate much of the weight-loss effects. They don’t say that you are “working up an appetite” for no reason. Second, doing more exercise reduces a person’s overall activity at other times of the day. For example, if you work a physically demanding job like construction for eight hours every day, then it is unlikely you will get home and decide to go on a 10K run just for fun. If you have been sitting in front of a computer all day, then that 10K run may sound quite appealing, but increasing leisure time exercise does not change total daily activity.
From the 1970s on, we have continued to believe that a low-fat diet combined with exercise will reduce weight and that people who are obese are just lacking in willpower. What we now know is that while what we eat does affect our weight, dietary fat is not the culprit. To understand why, we need to look at what happens to food when it enters the body.
DIGESTION: HOW THE BODY BREAKS DOWN FOOD
ALL FOODS ARE a combination of three major components called macronutrients:
1.Proteins
2.Dietary fats
3.Carbohydrates
In turn, each macronutrient is composed of smaller units or building blocks.
Proteins are chains of building blocks called amino acids. In the human body there are at least 20 amino acids, which can be combined to form thousands of different proteins. Nine amino acids are considered essential because the human body cannot synthesize them, which means they must be obtained through diet. If you don’t eat enough of these proteins, you will become malnourished. Food sources of protein include meats, poultry, and seafood; dairy milk, cheese, and yogurt; eggs; beans and legumes.
Dietary fats are molecules called triglycerides, which are composed of a glycerol backbone and three fatty acids. Certain types of fat are also considered essential and must be obtained through diet. These include the omega-3 and omega-6 fatty acids. Food sources of fats include oily fish; dairy milk, cheese, and yogurt; eggs; nuts and seeds; coconuts; avocados.
Carbohydrates are chains of sugars such as glucose, fructose, or lactose. Table sugar, called sucrose, is composed of one molecule of glucose linked to one molecule of fructose. Starches, like flour, are composed of long chains of glucose in the form of amylopectin or amylose. There are no essential carbohydrates. Food sources of carbohydrates include grains; fruits and vegetables; beans and legumes; energy drinks and alcohol.
Food also contains microscopic amounts of vitamins (A, B, C, D, E, K, etc.) and minerals (iron, copper, selenium, etc.), which are known as micronutrients.
Digestion is the process of breaking down macronutrients—proteins, dietary fats, and carbohydrates—into their smaller components for absorption by the body. The amino acids and fatty acids that make up proteins and fats, respectively, can be either used as building blocks for cell components or burned for energy. The sugars that make up carbohydrates are burned for energy, but they are not used to build other cell parts.
The chemical reactions involved in creating this energy and building cell parts are collectively called metabolism. And each macronutrient is metabolized differently. Why is this important? Because these differences affect how energy is stored and used.
Protein metabolism
Protein, like lean meat, is broken down into its component amino acids during digestion and transported to the liver. Amino acids are mainly used to rebuild proteins in blood cells, bone, muscle, connective tissue, skin, etc. Think of this process as being similar to taking the letters from a Scrabble board and reshuffling them to create new words. We eat animal and plant proteins, break them into amino acids, and then recombine them to form our own proteins.
The primary function of ingested protein is to rebuild cell components, and burning it for energy is only a secondary function. If you eat more protein than is needed for rebuilding, there is no way to store these extra amino acids. Instead, the liver changes them into glucose by a process called gluconeogenesis, or “the formation of new glucose.” (This word is derived from “gluco” meaning “glucose,” “neo” meaning “new,” and “genesis” meaning “the formation of.”) For an average American adult, an estimated 50 to 70 percent of ingested protein is turned into glucose for energy.4 However, this percentage varies greatly depending upon your body weight and how much protein you are eating.
Dietary protein takes significant processing by gluconeogenesis before it is converted to glucose. By this time, the body has activated multiple hormonal systems to deal with the expected increase in glucose availability. Thus, blood glucose remains stable even if you eat lots of protein.
Insulin is released when eating protein, especially in patients with type 2 diabetes, and signals the cell to start synthesizing new proteins. Certain animal proteins, such as the whey in dairy, generate almost as much of an insulin response as carbohydrates.
Fat metabolism
Digestion of dietary fat requires bile to mix and emulsify it. Bile is secreted by the liver, stored in the gallbladder, and released by the small intestine. Once the fat is absorbed by the small intestine, it is in droplets known as chylomicrons that are absorbed into the lymphatic system, which empties directly into the bloodstream. These chylomicrons are carried to fat cells called adipocytes, where they deliver a form of fat called triglycerides that are taken up for storage.
Dietary fat is absorbed more or less directly into our stores of body fat. While it may appear that dietary fat is far more conducive
