I had a discussion recently with a guy regarding the ever-prevalent dietary advice about calories. According to most dieticians, losing weight is as simple as eating fewer calories than you burn off. And while it is that simple, it's not that easy. The source of the calories makes a huge difference in how they affect the body. This is a part of my article "The Myths of Healthy Eating Part II" from awhile back.
We have all heard the news that we should reduce calories in order to improve bodyweight. Unfortunately, this is an overly simplistic way of looking at the body. The low-fat diet is based largely on the fact that a gram of carbohydrate contains fewer calories than a gram of fat: four vs. nine. The signs in the grocery store bread aisle read "Gram for gram, carbohydrates contain fewer calories than fat." While true, this statement is terribly misleading. "A calorie is a calorie" thinking assumes that the human body is a perfect thermodynamic engine; that is, it assumes that all calories are burned the same within the body regardless of their source. It also negates the fact that the hormonal pathways of processing the three macronutrients – carbohydrate, protein, and fat – are different.
The assumption that "a calorie is a calorie" means that a person following a 2,000 calorie per day diet could eat 2,000 calories of Twinkies or 2,000 calories of lettuce or 2,000 calories of chicken all with the same effects. I'm willing to bet that the person eating only Twinkies would not fare as well as the person eating lettuce or chicken.
We need to consider the "Thermic Effect of Food." The Thermic Effect of Food is the way in which a gram of protein, carbohydrate, or fat changes the metabolic rate. Basically, it takes the body twice the energy to break down a gram of protein as to break down a gram of carbohydrate or fat. This means that the gram of protein may report to the stomach with four calories, but following digestion and, taking into account the energy used to break it down, only about 70% of those calories are available. Fat has a thermic effect of about 3%, meaning a net gain of 97% of the calories in the fat you eat, while carbohydrates have a thermic effect of 5-10%.37 As you can see, different macronutrients affect the metabolic rate in different ways.
So based on the Thermic Effect of Food, it appears that restricting fat is a good idea since protein and carbohydrates cause a greater increase in the basal metabolic rate. However, this ignores the very real, very important hormonal effects of food. There is a simple reason that low carbohydrate diets work and work well: restricting carbohydrates decreases circulating glucose, which decreases insulin, which allows the body to access stored fat. When people think about energy, they think about that which they eat. However, most of us have a huge amount of energy stored around our waists (some more than others) in the form of fat.
But how does food affect hormones? In order for nutrients to be shuttled into the muscles, liver, and fat storage of the body, they need help. This help comes in the form of insulin. Insulin is secreted in response to carbohydrates and protein; carbohydrates cause a significant insulin dump, while protein has only a small effect on insulin. A large fat meal can cause an insulin response, but fat is effectively hormonally neutral. The rub here is that when insulin is circulating, the body will not release stored fat. You can see that the carbohydrate-heavy, low-protein, low-fat diet that is commonly touted keeps glucose and insulin levels elevated, which makes losing weight hard without following the starvation diet. So hormonally, a calorie is not a calorie – some calories (those from carbohydrates) tend to put the body into fat storage mode.
A calorie cannot be a calorie. If all calories were equal, then the macronutrient composition of the diet would have no effect on bodyweight. Studies, however, show this to be absolutely untrue. One study showed that a diet "high in protein and/or low in carbohydrate produced an ~2.5-kg [ 5.5 lbs] greater weight loss after 12 wk of treatment."38 Another study points out that "The ideal weight loss diet, if it even exists, remains to be determined, but a high-carbohydrate/low-protein diet may be unsatisfactory for many obese individuals" and that there are metabolic advantages to a low-carb diet.39 Finally, a study from 1957 by Kekwick and Pawan showed that even on a 1000 calorie diet (hypocaloric by any measure), patients on a 90% fat diet (with 10g of carbohydrate) and 90% protein diet (5g of carbohydrate) lost weight, while those on a 90% carbohydrate diet (225g of carbohydrate) lost little or none. Those on the fat diet lost the most, while some of those on the carbohydrate diet actually gained weight that they had lost on the fat or protein diets.40
"A calorie is a calorie" is dying a slow, painful death. Many in the nutrition field refuse to let go of this trite and worn out way of thinking despite mounting evidence that it just isn't true. In fact, just altering the level of omega-3 fats in the diet changes body composition. 41 To steal a phrase from John Berardi, "A fat isn't even a fat, let alone a calorie a calorie!" This isn't free license to eat all the protein and fat you want, ala Atkins, but it does illustrate that simply replacing "high-calorie" fat with "low-calorie" carbohydrates or protein doesn't work by the simple mechanism hoped for.
40 Kekwick, A, Pawan, GLS (1957) Metabolic study in human obesity with isocaloric diets high in fat,
protein or carbohydrate Metabolism 6,447-460
My big beef with "calorie is a calorie" thinking is food quality. Telling someone to eat more or less calories (depending on whether the goal is adding or reducing mass) doesn't tell them what foods they should be eating. People that focus on calories don't generally change what they eat so much as they change the package that it comes in. Instead of potato chips, they eat Baked Lay's. Instead of regular snack foods, it's fat-free this and low-fat that. That doesn't solve the underlying problem of inadequate vitamin and mineral intake and improper macronutrient portioning.
It also doesn't address differences between different types of the same macronutrient. For instance, is 50g of carbs from grapes identical to 50g of carbs from a Hershey's bar? The smart money is on "No!" It all breaks down to glucose, but along with those 200 calories (50g * 4 cal/g) from the grapes comes fiber, vitamins, and minerals. But if you only think calorically, the source is irrelevant. Are monounsaturated fats no different than trans fats? Both contain 9 calories per gram. But it doesn't take a genius to know that these two fats affect the body in drastically different ways.
There is a very simple solution that is also easier to implement. Eat like a Modern Forager: meat, vegetables, nuts, oils (olive, coconut, and palm), fruits, tubers, and squashes. Along with the reasons detailed in the article such as biochemical responses to food in the body and the thermic effect of eating, protein and fat stimulate the release of the appetite-suppressing hormones: peptide YY, cholecystokinin, and leptin. This effect of these two macronutrients will help you control your caloric intake. On the other hand, eating a standard low-fat, high-carbohydrate diet is very difficult because the overabundance of carbs plays with your blood sugar causing you to have constant insulin spikes and hunger.
Stop thinking about calories and start thinking about food.