Four Reasons Junk Food Is Fattening

By on December 29, 2014

For years, those of us working in the field of nutrition treated all calories as the same when it came to estimating energy needs. A calorie is a calorie we used to say and to our defense in the early days of nutritional science it made sense.

In biochemical terms, a calorie is defined as the amount of energy (heat) needed to raise 1 litre of water, 1 degree Celsius. A concept that is at best abstract. Essentially, the energy we get from the digestion of food is used to run the body like gasoline runs the engine of a car.

In the early days of nutritional research, the energy potential of different foods was assessed by putting food in a contraption called a bomb calorimeter; a type of mini furnace where the food was completely burned down to ash. Because all foods could be completely incinerated, 100% of the energy potential could be extracted and therefore 100 calories of celery was equivalent to 100 calories of sugar or 100 calories of a cookie.

The human body however, is not a bomb calorimeter and we do not extract calories equally from different foods and so, a calorie is not a calorie and the concept of calorie quality was born.

What’s this got to do with junk food?

It’s long been understood that certain foods tend to be more ‘fattening’ than others. That said, with proper portion control even so-called fattening foods can be part of a healthy diet but we all know that some foods are easier to overeat, or don’t fill us up for as long as other wholesome foods like vegetables, meat, fish, fruit and nuts do. Most of us will say that junk food is simply higher in calories, or has lots of sugar, which is why it’s problematic but it’s more than that. How junk food is processed and its impact on calorie quality and intensity of flavours also influences how our bodies deal with those calories. 

While the definition of junk food isn’t standardized, it’s largely agreed that it’s a food or food stuff that is highly refined and processed, with added refined sugars and fats with little protein, fiber, vitamins and minerals.

4 ways in which junk food is more fattening

1. Junk food is highly processed and refined

One of the ways wholesome foods help to keep us satisfied is through their bulk. Even a well-chewed meal of roast beef, steamed kale, and mashed sweet potatoes is bulkier than a fast food burger made from a highly minced meat patty and refined white flour bun; it takes less time to eat the burger. Refined and processed foods are easy to over eat for this reason and because they are processed, are often digested more quickly. As a result they have a low satiety factor, or index – this is why people who eat more whole foods can often eat more total food (increased food volume) yet end up eating less calories. Consider how easier it is to eat the 550 calories in a Big Mac versus having to eat 7 medium apples to get the same number of calories.

2. Junk foods are highly rewarding

Let’s face it. If you had to choose between an apple, or any other food you basically like and your favourite indulgence (for me, it’s fresh cut fries with lots of salt), the natural, more simple food, would be less rewarding. It’s OK, you’re not a bad person. You and I (and everyone else) are hard-wired that way. It’s all about how foods stimulate the reward centers in our brain (think increased levels of serotonin and dopamine) which is good, it makes eating pleasurable. The problem is that with the advent of food science, we are able to make some food more palatable than most foods found in nature ever have been throughout human history. Food companies invest enormous amounts of money to increase the reward factor of junk foods in a sense to hijack our brains.

Trying to overcome our biology and resist the temptations that surround us when it comes to inexpensive, highly rewarding foods that are everywhere isn’t easy.

3. Junk foods are usually very high in calories

This shouldn’t come as a surprise to anyone. A large doughnut has 300 calories, a can of pop 140 calories, a subway sandwich about 456 calories, and a slice of most quick breads like lemon loaf found that your local coffee shop have about 450 calories. Because they are also low in water, fiber and protein, these calorie-dense foods are easy to eat in no time and don’t have a high satiety index. All of these affect the quality of their calories and since the body is not a bomb calorimeter, the 300 calorie doughnut will count more than if those 300 calories came from salmon.

4. Junk food is cheap and the portions ain’t small

Like point number 3, this isn’t news either but bears repeating. The portions are out of control and the food is cheap. It doesn’t cost a lot of money to make junk food from ingredients that are highly subsidized like wheat, refined vegetable oils and sugar. Junk food can be sold at rock-bottom prices and still have a large profit margin. Those on tight budgets are naturally going to be attracted to these so-called bargains. Likewise, it doesn’t cost a lot to increase the serving size either; selling extra large fries and pop is only pennies compared to the regular sizes but the higher cost is passed on to the customer. It’s not hard to get a hamburger combo meal for under $5 that packs a whopping 1400 calories.

What’s the bottom line?

The focus on weight loss has traditionally been on counting calories with an emphasis on the quantity, or total calories versus the quality of those calories. It not only becomes burdensome to try and count them, but the total number doesn't tell us anything about the nutritient content of the overall diet. Knowing that junk food is not only about calories, but also how those calories are processed and handled by they body is an important piece of knowledge; an critical educational piece I use with clients all the time. By focusing on minimally processed, wholesome foods you'll enjoy calories that ultimately work best with your biology and metabolism that work to postively influence your body-fat regulation while supplying an abundant of vitamins, minerals, protein, fiber and phytonutrients. 


Feed-forward mechanisms: addiction-like behavioural and molecular adaptations in overeating

The Western diet and lifestyle and diseases of civilization

A satiety index of common foods.

Doug Cook RD, MHSc is a Registered Dietitian and Nutritionist with a focus on functional medical nutrition therapy. He uses an integrative and holistic nutritional approach providing science-based guidance on food and diet along with the judicious use of nutritional supplements where appropriate. He is the coauthor of Nutrition for Canadians for Dummies (Wiley, 2008) and The Complete Leaky Gut Health & Diet Book (Robert Rose, Spring 2015). You can learn more about Doug by visiting his Facebook page, following him on Twitter, or by checking out his website

Photo credit: The Group Gallery

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