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Fat, carb, protein — does it really matter what form a calorie comes in? 

According to the tidy little illusion of calorie math, no. We treat calories like currency — all totals being equal, whether counted in coins, bills, or silver bars.

Yet, is a calorie really a calorie, no matter the form or source? Under all conditions to all individuals at all times?

It’s tempting to think this way. Yet science begs to differ. 

Technically speaking, a calorie is a measure of how much energy something releases when it’s combusted in a closed system (called a bomb calorimeter in laboratory settings). One calorie is the amount of energy required to raise the temperature of one gram of water by one degree Celsius.

The calories reported on food labels are actually kilocalories, often written with a capitalized “C.” (That’s why you’ll see me capitalize references to Calories for the rest of our discussion (or list them as kcal).

But the real question is this: is a Calorie really a Calorie, no matter what? Let’s break it down.


Questionable Calorie Counts

I hate to break it to you, but the Calorie counts you see on food packages are flawed. This may explain why some struggle to manage their weight despite meticulous “Calorie counting.”

The USDA maintains one of the most extensive, free food databases in the world. The Calorie numbers measured in lab settings — which are tightly controlled, closed systems where energy cannot be “lost” — don’t always behave predictably when applied in real-world settings.

Our bodies are open biological systems, so the exact Calorie dynamics observed in lab settings don’t carry over perfectly.

The scientists who perform the lab analyses to determine the Calorie content of foods recommend the public treat these numbers as estimates at best. Go figure. 

To report Calories on a food label, a food manufacturer can either perform its own bomb calorimeter testing (costly and time-consuming), or use existing ingredient data from the USDA database to extrapolate the data to their product (the quick and dirty method). To control costs and maximize profits, which method would you follow?

When reporting final Calorie counts for labels, these values are required to be accurate within 20 percent of actual measures, but in all reality, no one really checks this before a product reaches our shelves. 

Calorie accuracy is all over the map when researchers compare actual Calories against labeled amounts on commercially prepared foods. A more truthful name for 100-Calorie packs could be “somewhere around 80- to 120-Calorie packs — we think.”

To muddy the waters a bit more, many free Calorie-counter apps source their data from the huge USDA database and allow their millions of users to also create custom foods for all other users to select from. This explains why there may be upward of 30 choices for chicken breast when you go to log your food. Your Calorie counts are only as accurate as Joe-user’s interpretation or best guess.

Sadly, even if you have the best intentions by trying to track your Calories accurately, your 1,500-Calorie plan could really be supplying anywhere from 1,200 to 1,800 Calories! Could this be one of the reasons your meal plan didn’t work as predicted?

Of course, being aware of what you’re eating and how much you’re eating can be helpful for changing nutrition behaviors. Despite the many potential layers of inaccuracy with Calorie counting, I still believe monitoring your diet somehow is important for raising awareness. It just doesn’t appear to matter whether you simply write down what you eat, log your food intake with a photo journal, or meticulously enter measurements and Calorie info into a digital app. All these methods can be effective.

Manipulating your food choices based solely on Calories isn’t really the best idea for a few other reasons. Let me explain.

Thermic Effect and Satiety

Fun fact: When you eat different foods, your body actually expends different amounts of energy digesting, absorbing, and metabolizing the nutrients. It’s called the thermic effect of feeding. A Calorie is not just a Calorie once we eat it. 

Protein-rich foods have the greatest thermic effect, and they seem to suppress hunger better than any other macronutrient .

A whopping 25 to 30 percent of the Calories in the protein you consume are spent just digesting it. 

This means when you eat 100-Calorie portion of protein (3 to 4 ounces), you’ll burn about 25 Calories digesting it, so you’ll only net about 75 Calories.    

Contrast the high thermic effect of protein with that of carbohydrates (6 to 8 percent) and fats (0 to 3 percent), and it seems that anyone who wants to “boost” their metabolism would benefit from eating protein with every meal. 

Perhaps even more surprising is that certain types of carbohydrates — simple sugars such as glucose or fructose and refined grains — are particularly stealthy at eluding our sense of fullness. They have low thermic effect and are low in fiber, which decreases their thermic effect even further compared to high-fiber carbs like veggies, fruits, or whole grains. 

Fructose, in particular, is likely the worst source of Calories to consume. This abundant, super sweet, super cheap form of added sugar follows different metabolic pathways than other carbohydrates, alters appetite-system signaling, and bypasses hunger-hormone responses. The amount of fructose we consume may even be a main driver of the development of insulin resistance and its progression to diabetes.

Minimizing intake of processed carbohydrates, and especially added sugars like fructose, is critical if you want to optimize your health and manage your appetite.

Researchers have observed that whole, unprocessed meals containing complex, fiber-rich carbohydrates demand nearly 50 percent more energy to digest and absorb compared to processed meals containing the same amount of Calories. 

The bottom line is this: To keep your metabolism humming, your hunger managed, and fat storage averted, eat plenty of fibrous vegetables, ample protein, and minimally processed complex carbs based on your activity level. And, of course, minimize your intake of processed grains and added sugars. 

Even beyond the thermic effect, however, there’s more to the Calorie story.

We Are What We Eat, Digest, and Absorb

Most people I know eat food, not Calories. They digest proteins, carbohydrates, and fats, not Calories. They absorb nutrients and metabolize energy, not Calories. The amount of energy absorbed and metabolized can be expressed as Calories, but that’s not what we eat, digest, and absorb.

We may think we know plenty about the Calories contained in our food choices and even understand a great deal of how our bodies metabolize various foods. But we’re just beginning to learn how our gut microflora (the bacteria living in your digestive tract) influences our entire biochemical relationship with what we eat. 

What you absorb and what your gut microflora absorbs can determine a lot about your weight and health. 

The different types of bacteria living in our colons seem to have a profound effect on how many Calories we extract from our foodrendering Calorie counting unpredictable depending on which bacterial pattern you have.

Researchers have shown marked difference in bacterial profiles of normal weight and obese humans and mice. Multiple studies have shown that transplanting gut microbes from obese mice into normal-weight mice was enough to make the normal-weight mice gain weight, despite no change in diet or exercise.

Our genes appear to play a role in determining which strains of bacteria thrive and which struggle in our individual systems, but our food environment also has tremendous influence. Even those with ideal bacterial balance may be subject to harmful bacterial balance shifts if exposed to high enough doses of artificial sweeteners, which are commonly used as a way for people to help control Calorie intake.

Conversely, diets rich in wholesome, fiber-rich produce and naturally fermented foods can positively shift the gut microbiome and influence a number of healthier metabolic outcomes.

Beyond energy absorption, the types of bacteria living in the gut have also been shown to influence whole-body metabolism, and even play a role in preventing metabolic syndrome and diabetes.

Should You Ignore Calories?

Calories can’t be ignored. Energy balance still matters for weight and body-composition regulation.

The evidence simply suggests that Calories are tougher to monitor, count, or control than we’d like to believe. Not all Calories are equal for all people, at all times, under all conditions. And they’re clearly not the only factor that matters for metabolism and weight regulation.

If monitoring Calories helps you stay on track with your nutrition and health goals, then keep monitoring them.

If you hate counting Calories because it isn’t helpful, stresses you out, or appears misleading to you, don’t focus on them as much.

Focus on other factors like eating nutrient-dense, minimally processed foods rich in protein and fiber. Chances are, if half your plate is filled with colorful produce and much of the other half is rich in protein, you’ll get full before you have a chance to over-consume Calories.


Keep the conversation going.

Leave a comment, ask a question, or see what others are talking about in the Life Time Health Facebook group.

Paul Kriegler, RD, CPT

Paul Kriegler, RD, LD, CPT, CISSN, is the director of nutritional product development at Life Time. He’s also a USA track and field coach.

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