Are all calories the same?

Are all calories the same? – the evidence

This guide is written by Dr. Bret Scher, MD and was last updated on September 28, 2022. It was medically reviewed by Dr. Ted Naiman, MD on August 1, 2022.

The guide contains scientific references. You can find these in the notes throughout the text, and click the links to read the peer-reviewed scientific papers. When appropriate we include a grading of the strength of the evidence, with a link to our policy on this. Our evidence-based guides are updated at least once per year to reflect and reference the latest science on the topic.

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You have likely heard the question multiple times: Are all calories the same? 

Or you may have heard it phrased differently, such as “a calorie is a calorie.”

Does lowering your calorie intake lead to weight loss no matter where those calories come from? Is weight loss as simple as a math equation of eating fewer calories than you expend?

The answers to these questions are both yes and no. As with many topics in nutrition science and healthy weight loss, the details matter.


The short answers are:

  • All calories are the same by scientific definition, but they are not all the same as far as how your body responds to them.
  • For weight loss, calories count but that doesn’t mean you have to count them.
  • Your body reacts differently to various calorie-containing foods.

It makes sense that 300 calories of donuts aren’t the same as 300 calories from chicken. But does scientific evidence support this belief?

All calories are the same outside the body

A calorie is a unit of energy. A calorie is the amount of energy needed to raise the temperature of one gram of water one degree celsius.

In this case, it is just math. Calories are a standard unit for a precise amount of energy. In other words, all calories measure the same amount of energy (although in nutrition we commonly use the word calorie to refer to a kilocalorie).

But how does this apply to food?

Fat provides 9 calories per gram, while protein and carbs each provide 4 calories per gram. And alcohol provides 7 calories per gram.

And based on the first law of thermodynamics — which states that the amount of energy in any given system is constant — energy can be transferred or stored but can’t be created or destroyed. 

This applies to your body, too. All energy consumed must be accounted for. It has to be either burned, excreted, or stored. 

So, if you eat and absorb 100 calories, you have to either burn it or store it. Simple.

Is weight loss simply a math problem?

If you expend more calories than you take in, you will lose weight. This is a true statement. 

After all, Kansas State professor Mark Haub, PhD, lost 27 pounds (12 kilos) in 10 weeks eating mostly twinkies and other processed “junk food” such as cookies, chips, and sugary cereal. He made sure he maintained a negative calorie balance and watched the pounds disappear. However, a subsequent article uncovered that he was funded by Coca Cola, calling into question the validity of his results. 

But was it easy for him to stop eating each day when he reached his calorie target? How did he physically feel after eating such food? Would he have maintained his weight loss long term without significant hunger and cravings? And what about his long-term health? 

These questions go far beyond calories in and calories out. But, at least in the short term, it appears that reducing calories — no matter what the type — can lead to weight loss.

Humans are complicated

When it comes to weight loss and food metabolism, the human body is complicated. We don’t function like machines with simple inputs and outputs.

For starters, not all weight loss is the same. At Diet Doctor, we focus on healthy weight loss — meaning losing mostly excess fat mass, improving metabolic health, and doing it in an enjoyable and sustainable way.

Second, understanding how many calories we expend is complicated. The clearest example is the thermic effect of food. This has nothing to do with exercise or even metabolic rate. Instead, it is a measurement of how many calories you burn simply by digesting food.

Studies consistently show that protein has the highest thermic effect — meaning you burn more calories digesting protein than carbs or fat.

In addition, your resting energy expenditure can change based on what you eat or how you lose weight.

Weight loss in general, but especially loss of lean mass, can reduce your resting metabolic rate. When your metabolic rate is reduced too much, it can make it very difficult to maintain weight loss long term. But by losing mostly fat mass and maintaining or building lean mass, you can minimize the reduction in resting energy expenditure.

And what you eat can change your energy expenditure. Studies have shown that people eating ketogenic diets increase their total energy expenditure compared to those eating control diets. While controversy exists about how large of an effect this has, the data we have point to a probable metabolic advantage. 

Plus — and this is important — what you eat can change how much you eat. 

Kevin Hall, PhD, and colleagues performed a trial that allowed subjects to eat as much as they wanted, and carefully tracked their caloric intake. The study showed that participants ate 500 calories more each day when they ate ultra-processed foods, compared to less processed foods. The study authors attempted to match the percentage of calories from different macronutrients in both the ultra-processed and less-processed offerings, thus suggesting that the level of processing alone was the main cause of excess calorie intake. 

Therefore, all calories are not the same when it comes to human nutrition and metabolism.

Hormones matter

Calories come from macronutrients, which can also affect your hormones in different ways.

Insulin is perhaps the most common example. Sugar and high-glycemic-index carbs tend to trigger a greater insulin response than protein- or fiber-containing carbs. And fat appears to have the least immediate insulin action, although it may have a mild longer-term effect, especially when eaten to excess.

Although still controversial, many studies point to hyperinsulinemia as being a cause of obesity. Whether or not it is proven as causal, elevated insulin levels are associated with obesity and metabolic dysfunction, and lifestyle therapies that reduce insulin levels can help with weight loss. Therefore, calories from foods that minimize insulin secretion may be better suited for weight loss in some individuals.

Further, studies report that elevated insulin levels can lead to a rapid decline in post-meal blood sugar, which then leads to increased hunger and calorie intake.

Foods can also affect our satiety (“fullness”) hormones differently. Certain foods may trigger the release of leptin, CCK, PYY, GLP-1 and inhibit ghrelin (a “hunger” hormone) more than others. 

These hormones don’t appear to respond simply to the number of calories. Instead, protein-containing foods trigger satiety hormones more than other foods, and fibrous carbs trigger them more than refined carbs. 

So, once again, we see that not all calories have the same effects. 

Hunger matters

It’s one thing to evaluate how your body processes and responds to calories. It’s another thing to evaluate how your brain responds. 

As we discussed in our podcast with Stephan Guyenet, PhD, your brain may be the most important organ for regulating weight loss and regain. Your brain can unconsciously alter feelings of hunger, hormone responses, and metabolic set points in reaction to different foods and degrees of weight loss.

And the foods you eat dramatically influence your hunger. As we detail in our guide on managing hunger while losing weight, your hunger will change depending on the number of calories and the macronutrient mix you eat, and your body’s innate metabolic response.

Increased hunger due to forced calorie restriction may be the most common reason people fail to sustain weight loss. 

Fortunately, you can follow certain principles that help reduce your hunger — and improve your satiety — while still getting all the nutrition you need and enjoying your meals.  We call this our higher-satiety eating approach, and you can learn more in our introductory guide

Higher-satiety eating prioritizes foods with higher protein percentages, lower energy density, higher fiber levels, and low hedonic factors. Eating this way allows you to decrease your calories with less effort. It’s as if your body counts the calories for you.


Don’t eat less; eat better

Instead of focusing on eating less, we suggest you focus on eating better. In the end, you may end up eating fewer calories, but you won’t have to think about it. That means you don’t have to track or weigh your food, which can reduce the feelings of restriction and hunger that come with it!

Some may argue that eating better in order to eat less is the same thing as counting calories to eat less. But we believe the mindset is different and your approach matters. 

This is true for maintenance of weight loss, too. Beating back hunger by eating better is a sustainable way to avoid weight regain; a life of calorie counting to avoid regain is not only tedious, but it does not address hunger. It’s a more difficult path that fails most people.

Once you understand how calories from various foods affect your metabolism differently, it should be clear why eating better can ultimately lead to healthy, sustained weight loss and improved metabolic health. 

So, are all calories the same? Definitely not when you consider their effect on your health, your weight, and how they make you feel.

  1. This is the definition of a kilocalorie, which is what most people mean when they use the term “calorie.”

  2. Although the base of Haub’s diet was junk food, he also drank a protein shake and ate at least one serving of vegetables plus a multivitamin every day. His story was covered by CNN. However, it was later reported that he was funded directly by Coca Cola for doing his experiment, thus calling into question its legitimacy.

  3. Nutrition and Metabolism 2004: Diet induced thermogenesis [overview article; ungraded]

    Nutrition and Metabolism 2014: A high-protein diet for reducing body fat: mechanisms and possible caveats [overview ;article; ungraded]

  4. PLoS One 2009: Metabolic and behavioral compensations in response to caloric restriction: implications for the maintenance of weight loss [randomized trial; moderate evidence]

    Nutrition and Metabolism 2021: Metabolic adaptation is associated with less weight and fat mass loss in response to low-energy diets [randomized trial; moderate evidence]

  5. Asia Pacific Journal of Clinical Nutrition 2001: New approach for weight reduction by a combination of diet, light resistance exercise and the timing of ingesting a protein supplement [randomized trial; moderate evidence]

    Obesity 2012: Resistance training conserves fat-free mass and resting energy expenditure following weight loss [randomized trial; moderate evidence]

  6. BMJ 2018: Effects of a low carbohydrate diet on energy expenditure during weight loss maintenance: randomized trial [randomized trial; moderate evidence]

    American Journal of Clinical Nutrition 2016: Energy expenditure and body composition changes after an isocaloric ketogenic diet in overweight and obese men [randomized trial; moderate evidence]

  7. Cell Metabolism 2019: Ultra-processed diets cause excess calorie intake and weight gain: An inpatient randomized controlled trial of ad libitum food intake [randomized trial; moderate evidence]

  8. American Journal of Clinical Nutrition 1997: An insulin index of foods: the insulin demand generated by 1000-kJ portions of common foods [non-controlled study; weak evidence]

  9. Nutrition and Metabolism 2016: Fatty acids stimulate insulin secretion from human pancreatic islets at fasting glucose concentrations via mitochondria-dependent and -independent mechanisms [non-controlled study; weak evidence]

  10. Journal of Endocrinology 2017: A causal role for hyperinsulinemia in obesity [overview article; ungraded]

  11. Pediatric Clinics of North America 2019: Childhood obesity and the metabolic syndrome [overview article; ungraded]

    Minerva Endocrinology 2012: Prevalence of metabolic syndrome and insulin resistance in overweight and obese women according to the different diagnostic criteria [observational study; weak evidence]

    Nutrition & Diabetes 2017: Enhanced insulin sensitivity in successful, long-term weight loss maintainers compared with matched controls with no weight loss history [case control study; weak evidence]

    Journal of the American College of Nutrition 2013: Improvements in glucose metabolism and insulin sensitivity with a low-carbohydrate diet in obese patients with type 2 diabetes [uncontrolled study; weak evidence]

    JCI Insight 2019: Dietary carbohydrate restriction improves metabolic syndrome independent of weight loss [randomized trial; moderate evidence]

    Obesity 2015: Weight loss on low-fat vs. low-carb diets by insulin resistance status among overweight adults & adults with obesity: A randomized pilot trial [moderate evidence]

  12. Nature Metabolism 2021: Postprandial glycaemic dips predict appetite and energy intake in healthy individuals [non-controlled study; weak evidence]

  13. Gastroenterology 2017: Blaming the Brain for Obesity: Integration of Hedonic and Homeostatic Mechanisms [overview article; ungraded]

  14. In one study, when people deliberately ate 250 calories less at a meal, they were hungrier within a few hours, which didn’t happen after they burned the same number of calories during exercise:

    Appetite 2014: Appetite and gut peptide responses to exercise and calorie restriction. The effect of modest energy deficits [randomized crossover trial; moderate evidence]

    Eating a high-protein, low-carb breakfast of eggs has been shown to help overweight people feel full longer and eat less at their next meal compared to eating a high-carb, low-protein bagel breakfast:

    Journal of the American College of Nutrition 2005: Short-term effect of eggs on satiety in overweight and obese subjects [randomized crossover trial; moderate evidence]

    And studies have shown that responses to hunger and fullness can vary a lot from person to person:

    Current Obesity Reports 2019: Issues in measuring and interpreting human appetite (satiety/satiation) and its contribution to obesity [review article; ungraded]

  15. Researchers have found that chronic calorie restriction and dieting often fail to achieve sustainable weight loss:

    Perspectives on Psychological Science 2017: Reducing calorie intake may not help you lose body weight [overview article; ungraded]

    Repeated dieting attempts may actually lead to gaining more weight over time:

    Obesity Reviews 2015: Pathways from dieting to weight regain, to obesity and to the metabolic syndrome: an overview [overview article; ungraded]