5 Kinds Of Reason That Junk Food Makes You Fat

1 Jul

doughnutWe all know that a diet high in processed foods such as pizza, chocolate, and doughnuts is more fattening than a diet that primarily consists of nutritious whole foods like meat and vegetables. But why is that? If you ask a dozen people on the street this question, you’ll likely get a wide variety of answers. Some will probably tell you that it’s all about the sugar, others may point out the difference in fiber content between the two diets, and some will likely say the primary problem is that candy, burgers, and other highly processed foods are very high in calories.

However, if you ask ask specifically why large amounts of high fructose corn syrup or sucrose are so fattening, how a low intake of dietary fiber can promote obesity and disease, or why calorie density matters, most people will probably come up blank. Because that’s the thing – we all know that “junk food” is unhealthy and fattening, but many of us find that it’s difficult to explain the actual mechanisms linking processed food with ill-health. I think if more people knew how highly processed food affects the body, healthy eating patterns would increase in popularity.

The problems with junk food

Before we begin, let’s start with a quick definition of what junk food really is:

Junk food is a derisive slang term for food that is of little nutritional value and often high in fat, sugar, salt, and calories. It is widely believed that the term was coined by Michael Jacobson, director of the Center for Science in the Public Interest, in 1972. Junk foods typically contain high levels of calories from sugar or fat with little protein, vitamins or minerals. Foods commonly considered junk foods include salted snack foods, gum, candy, sweet desserts, fried fast food, and sugary carbonated beverages. Many foods such as hamburgers, pizza, and tacos can be considered either healthy or junk food depending on their ingredients and preparation methods (1)

As I think most ancestral health enthusiasts would agree, the Paleo template is the best starting place for designing a healthy human diet. By looking at nutrition and health through the lens of evolution we can answer questions such as: What is normal for our species? What types of foods are we best adapted to eat? What types of foods were available in an ancestral, natural environment? How have modern processing techniques changed our food?

With this evolutionary framework in mind, let’s get to the problems with modern junk food…

1. Junk foods are extremely palatable and have a powerful effect on the reward centers in the human brain

Imagine driving home after a long day at work. You know the fridge and pantry at home are packed with meat, vegetables, fruits, and healthy oils, but for some reason it seems so much more tempting to stop at one of the many fast food joints conveniently placed along the road than to wait and prepare a meal at home. Not necessarily because you’re that hungry – more because the taste of something juicy, salty, and fatty seems so appealing right now. If you’ve become accustomed to eating a healthy diet, this need for something quick and tasty on the way home from work is probably not very pronounced; however, if your dietary regime is relatively loose, chances are today’s dinner ends up being burgers and fries instead of grass-fed beef and zucchini.

Why are we so drawn to these modern food products? The convenience aspect, as well as a lack of self-control, certainly play a role, but that’s not all. Hamburgers, doughnuts, and cookies have a supramaximal reward value compared to anything you’d find naturally in the wild, and for some, the hyper-rewarding combination of starch, salt, sugar, glutamate, and refined fat causes their “Stone Age bodies” to essentially becoming addicted to fast food (2, 3, 4).

Fast food is designed to be very palatable. Of course, we all want to eat good food, but the problem is that highly processed foods are too good. Modern food processing equipment allows us to combine several different ingredients and create food products very different from anything our primal ancestors had access to.

The ancient hard-wired mechanism that make us seek out foods that are safe and high in calories are working against us in today’s toxic food environment. The energy homeostasis system goes into a tailspin when we have constant access to highly rewarding and palatable food.

2. Most junk foods have a very high calorie density

With a few exceptions (e.g., honey), most junk foods are extremely calorie dense compared to “natural”, whole foods. This doesn’t really come as a surprise, as the foods you can pick up at fast food joints have all the characteristics of calorie-dense food items. Most importantly, they are low in fiber and water and high in sugar, starch, and refined fat.

Is calorie density a good indicator of the healthfulness and fattening effect of food? Not necessarily. Some of the “healthiest” foods on the planet, such as certain types of organ meats, are relatively calorie-dense. However, these types of animal meats are clearly in a very different category than the foods you’ll find at MacDonald’s or Burger King.

One of the problems with eating foods that are very high in calories is that they tend to have a low satiety index score, meaning that they aren’t especially filling per calorie. Getting 550 kcal at McDonald’s (one Big Mac) is an easy and quick task compared to getting the same amount of energy from more filling foods such as fruits, vegetables, and meats.

3. Junk foods tend to be low in protein

Of all the macronutrients, protein has the most potent effect on satiety and thermogenesis, and there is convincing evidence that protein intake has a significant impact on total calorie intake. If you eat a diet that contains inadequate amounts of protein, you would likely consume more total calories than you would on a diet containing adequate protein. The average protein intake in western societies (~15% of total calories) is very low compared to the intake of 19-35% among hunter-gatherers (5, 6).

The causes of this discrepancy become clear when we look at the difference between the ancestral and modern food environments. Our Paleolithic ancestors ate exclusively wild, unprocessed meats and plants, whereas in the modern world, a grain-based diet is the norm, and calorie-dense foods rich in refined fat, refined grains, and sugar are cheap and easily accessible.

Is the average doughnut-eating American compensating for a low percentage of protein in the diet by eating more total calories? Probably, yes.

4. Junk foods have an unnatural nutrient composition

You won’t find any foods in nature with a nutrient composition even closely resembling that of chocolate bars, cupcakes, or Big Mac’s. Junk foods tend to be low in protein (as mentioned), fiber, micronutrients, and water, and high in starch, salt, sugar, and refined fat. This combination is absolutely devastating, both to our waistline and general health.

Consumption of foods with this abnormal nurient composition results in alterations of the gut microbiota (which will have systemic effects on metabolism, immunity, and overall health), decreased insulin sensitivity, decreased leptin sensitivity, and a suboptimal gene expression pattern (7, 8, 9). This orchestra will promote fat accumulation and poor health.

5. Junk foods have a low satiety index score

As mentioned, junk foods have a low protein content, poor micronutrient profile, and high sugar content. This can help explain why these food products score so poorly on the satiety index score (10). Basically, they don’t fill us up as well as do nutritious, whole foods. This difference in satiety value can help explain why people who adopt an ancestral diet tend to reduce their caloric intake (7, 11).

Just imagine the difference between eating fruits, seafood, meat, and vegetables for most meals and surviving on calorie-dense prepackaged meals and convenience food. The whole foods diet consists of foods that are much more satiating per calorie, and in combination with all of the previously mentioned benefits of eating a whole foods diet, this will likely lead you to eat less when you’re on the healthy diet.


Some people claim that it doesn’t really matter much what you eat, as long as you stay on top of your caloric intake. However, as most people with some understanding of nutrition will tell you, this is a way too simplistic perspective on diet and obesity. To really understand why we get fat, we have to look at how different foods affect our metabolism, microbiota, hormone levels, and satiety, among other things.

Dairy products, cereal grains, refined sugars, refined vegetable oils, and alcohol were never or rarely consumed by Paleolithic people, but today they make up 70% of the total daily energy consumed in the U.S. (7). Often, these types of foods are combined to produce modern, highly processed foods such as chocolate cake, doughnuts, and pizza, which have several unfavorable characteristics that make them especially unfit for human consumption. These foods wreak havoc on our hormone levels, immunity, appetite regulation, and microbiota, and promote sugar cravings and fat gain, particularly when they are eaten in large quantities.