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Is A Calorie A Calorie?

Well, yes. But actually, no.

See, the thing is, the huge emphasis put on the caloric values on food labels have caused us to think like robots. As my friends would always say to me at convenience stores, "yo, this item has lower calories so we should buy this." To determine which snacks they should buy, they merely compare the caloric values of different foods and choose those with lower said values. At least that's what my friends would do. While they are right in that they consciously check the food label of different items, they are wrong in that they have omitted other important numbers and factors that everyone should consider when choosing food items that are the best for their health.

I am not saying that I totally disagree with their emphasis on calories. They are right. The most important factor in determining whether your body gains or loses weight is in the overall caloric intake versus the overall caloric output. Therefore, the fewer calories you eat, the more likely you are of hitting a caloric deficit.

A calorie is a calorie, strictly in the sense of a scientific method to determine the amount of energy a food contains. However, a calorie is not a calorie when you consider the biochemical pathways different foods go through after you've consumed them, as well as their varying impacts on your hormones and brain center (1). Below, I will be listing 5 reasons why a calorie is not a calorie, so that the next time you shop for food items, you will hopefully have a better understanding of what to look for.

1. The Thermic Effect of Different Foods

In short, the thermic effect of food (TEF) is defined as the increase in metabolic rate after the ingestion of a meal (2). It measures how much different food items increase the body's energy expenditure due to the energy required to ingest, digest, and metabolize the nutrients (1). That said, because different types of foods go through different metabolic pathways, the varying efficiencies of said pathways causes a difference in the amount of the food's energy used for digestive work (1).

Generally, the thermic effect is around 0-3% for fats, 5-10% for carbs, and 20-30% for proteins (3). This means that when you intake 100 calories of fat, you would end up "actually" absorbing 97-100 calories, as the other 0-3 were used up due to the TEF. However, if you intake 100 grams of protein, you would end up absorbing only around 70-80 calories. So, a calorie might not be a calorie after all, and your metabolism might benefit from a high-protein diet.

2. Food Processing

According to eatright.org, processed food includes food that has been cooked, canned, frozen, packaged, or changed in nutritional composition with fortifying, preserving, or preparing in different ways. Here we are not focusing only on the chemical, biological, and/or mechanical manipulation (although all of those do count as processed food), but just food processing in general, including cooking. In general, food processing can increase the bioavailability of the nutrients in your food and increase the percent of metabolizable calories (3). For example, when drinking a cup of orange juice, you would actually absorb more calories than if you ate a whole orange, due to the food processing involved.

One way food processing increases the number of metabolizable calories is by facilitating digestion and absorption through uncovering the nutrient containing cells in food and increasing their particle sizes. Another way is through heat, and thereby altering the digestibility of each macronutrient (e.g. causing proteins to unfold and starches to gelatinize) (3). Lastly, processed and whole foods generally have different TEF (mentioned above), which also contributes to the varying net calories absorbed.

3. Reduction of Appetite from Protein

The most challenging part of staying on a calorie-restrictive diet is fighting off hunger. The hungrier you are, the more likely you are to eat more and stray off the diet. Simple as that. However, a new study on mice has shown that eating a high-protein diet can boost the release of a hunger-suppressing hormone called peptide YY, which generally reduces appetite in humans. 

Moreover, protein has shown to be effective in reducing ghrelin levels, a hunger hormone that the stomach releases when empty, signalling to your brain that you should eat.

Therefore, while eating a protein-dense 200 calories meal and a carb-dense 200 calories meal might provide you with the same amount of calories (ignoring the TEF), the protein-dense meal is way more beneficial in controlling your hunger and hence increasing the chance of you adhering to your diet plan.

4. Difference in Satiety

Different foods of the same calories can give you a drastically different feeling of fullness after consuming them. The satiety index is a measure of the satiety values of different foods by taking into account the ability of foods to reduce hunger, increase feelings of fullness and reduce calorie intake for the few hours after you've consumed them (1). Foods like vegetables, eggs, oatmeal, boiled potatoes, and greek yogurt generally possess a high satiety index, while foods like cake and donuts are generally lower on the satiety scale. Therefore, you might feel really full after eating 300 calories of vegetables, but would still be left feeling hungry after eating a 300 calories cake. You make the choice.

Another important factor to consider is the effect that dietary fiber has on satiety. Fiber is a form of carbohydrate that the human body cannot digest, and it improves your overall satiety without adding contributing to your calorie intake. Therefore, eating a 100-calorie high fiber food gives you a way higher feeling of fullness than a 100-calorie low fiber food.

5. The Glycemic Index

We all know that refined carbs like sucrose are bad, but why? They all have a relatively high glycemic index (GI), which, according to the Mayo Clinic, is a system assigning a number to carbohydrate-containing foods according to how much each food increases blood sugar (4). By eating foods that rapidly increasing your blood sugar level, you are bound to experience a blood sugar crash in the upcoming few hours, and when that occurs, your hunger increases and you crave another snack. This phenomenon is called the "blood sugar roller coaster" (1). Constantly consuming foods with a high GI will lead to more cravings and cause you to eat more. So, 100 calories from a high GI food might not be the same as 100 calories from a low GI food, as the latter has a significantly less impact on your blood sugar and overall hunger level, and the former just makes you want to eat more.

References:

(1) https://www.healthline.com/nutrition/6-reasons-why-a-calorie-is-not-a-calorie

(2) https://pubmed.ncbi.nlm.nih.gov/8561055/

(3) https://chanapdavis.medium.com/when-a-calorie-is-not-a-calorie-82062c6bd740

(4) https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/in-depth/glycemic-index-diet/art-20048478