Written by: Yuchen He, MS
Reviewed by: Emily J., MSc RD
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"Calories in, calories out" is a highly debated view of weight and weight loss. But what's the science behind the idea? Is there a better alternative to calorie tracking?
In the dieting world, “Calories In, Calories Out” (CICO) tells us that if we restrict the calories we eat — and increase calories burned — we can be in an energy deficit and lose weight. Sounds pretty simple, right?
However, research indicates that only 20% of weight loss attempts result in meaningful weight loss, and it’s common to regain lost weight after the initial 6-12 months of weight loss [1, 2]. So can CICO really help with weight management? And if not, is there a better approach?
Calories In, Calories Out (CICO) essentially refers to the net energy balance of the body.
It can be thought of as an equation:
CALORIES IN - CALORIES OUT = CALORIES STORED/USED
The word “calories” is simply a unit of energy. “Calories in” refers to the food we eat during the day, in the form of macronutrients, such as carbohydrates, fat, and protein. “Calories out” refers to energy expenditure through exercise and natural metabolic processes.
CICO tells us that if you burn more energy than you’re taking in, you’ll be in an energy deficit, which leads to weight loss.
While the equation is valid (it’s a rule of physics), it’s an oversimplification of a complicated process. It’s also flawed as a sustainable weight loss methodology. Here’s why.
While energy intake is trackable to a certain extent, energy expenditure is often impossible to accurately assess in your daily life. This is because your body expends energy in four ways:
In other words, CICO has pretty significant limitations, even though it’s theoretically valid as an energy balance equation. While it’s helpful for understanding the basics of weight management, it’s not a sufficient or particularly useful model to actually help you lose weight sustainably and consistently.
Let’s say you decide to use CICO to track your calorie intake and energy expenditure. Even though your measurements may not be the most accurate, you can tell, compared to your non-dieting days, that you are eating less and moving more — and therefore in an energy deficit.
But does an energy deficit always equate to weight loss?
As it turns out, it’s not just about the number of calories you consume, but the source. Depending on what exactly you’re eating, you’re affecting different aspects of your health — including glucose levels and hormones — in different ways, all of which may impact your ability to lose weight and keep it off.
According to CICO, eating 200 calories worth of refined carbs (such as white bread) and 200 calories worth of protein (such as chicken) are, at the end of the day, simply 200 calories — but is that actually true? In this case, the calories aren’t created equal, especially from the perspective of your blood glucose levels. When you eat 200 calories of carbs, your digestive system breaks them down into glucose, leading to a rise in blood sugar levels (a post-meal spike). Your pancreas releases the hormone insulin to unlock your cells so that this glucose can be picked up and used by cells for energy — thus bringing your blood glucose levels back down.
But if you consistently eat a diet high in refined carbs (even if it’s fewer calories than you’re expending), your body will experience surges in glucose levels that push your pancreas to produce even more insulin to catch up, leading to a major crash. To bring your levels back up, you may find yourself craving something sweet — perpetuating the cycle and potentially leading to reduced insulin sensitivity and weight gain.
In addition to insulin, the foods you eat can affect other hormones, such as leptin and ghrelin, which regulate your appetite. Research has indicated that increasing your protein intake can lower levels of ghrelin (which makes you hungry), while people who eat diets high in carbs have higher levels of ghrelin .
You've probably heard of a weight loss plateau, when your weight loss efforts, despite being consistent, are no longer yielding the same results. In other words, you're still eating less and exercising, but have stopped losing weight.
That’s because your body is constantly trying to maintain homeostasis, or equilibrium among all physiological processes. Both gaining and losing weight — even if intentional — tell your body that its equilibrium is off, and it tries to correct this. In fact, research indicates that your body will adjust its energy expenditure (i.e., “calories out”) proportionally in response to weight loss and reduced intake . It does this because it needs less energy to keep up basic metabolic functions when you have less body mass.
In other words, while you can override your body’s attempts to maintain homeostasis, it will always try to correct for any short-term overeating or undereating by increasing or decreasing energy expenditure respectively.
Approaching weight loss with a model like CICO isn’t sustainable and tends to fail in the long run. While you may lose weight temporarily by restricting your calorie intake and exercising more, putting your body in a state of “famine” (i.e., telling your body that there’s a lack of nutrients and food) can increase your cortisol (stress) levels, make you hungrier, and promote binge eating — triggering a guilt-ridden cycle of starting a diet, breaking it, and restarting it again . Research in postmenopausal women has found that this kind of calorie-restricting and disinhibited eating (overeating sweets and highly-palatable foods, typically when in a negative state of mind) can lead to weight rebounds over time .
The bottom line? CICO is a useful model for understanding the basics of energy balance, and a deficit will result in some amount of weight loss — but its usefulness stops there.
Long-term weight loss studies point out that successful weight management strategies focus on sustained behavioral changes rather than counting calories eaten and burned .
So instead of struggling to restrict calories every day on CICO — and setting yourself up for failure — it’s key to look beyond CICO and focus on methods that focus on long-term behavior change, such as glucose monitoring and stabilizing your blood sugar levels.
Ultimately it’s not just knowing the number of calories you consume and expend, but where they’re coming from — in addition to other lifestyle factors like exercise, stress management, and sleep — that can help you lose weight.
Instead of relying on CICO alone, dialing in on your blood glucose levels — and using a tool like a CGM to track your levels in real-time — can be a key way to make the behavioral changes needed to successfully manage your weight. With a CGM, you can understand your own health on a personal level and unlock insights about your own body’s response to specific foods, exercises, sleep habits, and other activities.