CICO diet: Does calories in, calories out work? A doctor’s viewpoint

You probably already know that losing weight requires a calorie deficit: taking in less energy than your body uses. That’s the idea behind CICO, which stands for “calories in, calories out.” The concept is simple: Your body needs energy (calories) to function, and when you consistently consume fewer calories than you burn, it draws on stored energy to make up the difference.

CICO is often presented as a straightforward formula for weight loss: Eat fewer calories, burn more, and the weight will come off. But that simplicity has also made it a subject of debate. Some question whether CICO really works, while others argue that it overlooks the many factors that influence how our bodies regulate weight.

The reality is that energy balance matters, but factors like hormones, metabolism, appetite, and food quality can influence both sides of the equation. And maintaining a calorie deficit isn’t always as simple as eating less and moving more.

We asked Dr. Karen Mann, MD, Noom’s Medical Director, to explain the science behind CICO, why it may be hard to maintain a calorie deficit, and what other kinds of support may help.

Q: What does CICO mean?

Dr. Mann: CICO stands for “calories in, calories out.” It’s a shorthand for energy balance: the relationship between energy taken in from food and drinks and energy the body uses.

Your body uses energy all day, not only when you exercise. It needs energy for essential functions like breathing and keeping your organs working. It also uses energy to digest food and to move, whether you’re walking, doing household tasks, or working out.

CICO is a basic accounting idea, not a specific meal plan or a complete explanation of why people gain or lose weight. It doesn’t say what you should eat or how to treat obesity. Think of it as a concept, not a strategy. And it’s not a shortcut to weight loss. 

Q: How is CICO related to a calorie deficit?

Dr. Mann: A calorie deficit is when your body uses more energy than it gets from food and drinks. It’s one state of energy balance: when it’s sustained, the body draws on stored energy, and weight tends to decrease over time. Here’s a helpful calculator to look at how the numbers are calculated and what yours may look like. 

Q: What is the CICO diet?

Dr. Mann: People often call CICO a “diet,” but it doesn’t have required foods or fixed rules about carbohydrates, fat, or protein. It usually refers to calorie tracking with the goal of eating less energy than the body uses.

“CICO” can sound like a simple set of numbers that anyone can control precisely. In real life, both sides of the equation are influenced by biology and environment, and the numbers are estimates. 

If you’re looking for a specific way to eat, my advice is to focus on the quality of what you’re putting into your body. CICO can make it seem like it doesn’t matter what goes in as long as it’s less than the output, but it’s often not that simple. 

Focusing on prioritizing protein, increasing fiber, limiting processed foods, and eating fruits and vegetables can matter just as much as hitting a specific number. The same number of calories from protein- and fiber-rich foods tends to be more filling than foods from processed sources. In the long term, these habits can help you maintain weight loss, not just cause a quick change that you can’t sustain. 

Q: Is CICO too simplistic to explain weight loss?

Dr. Mann: It can be, especially when it’s presented as “just eat less and move more.” Energy balance describes the relationship between intake and expenditure, but it doesn’t explain on its own why appetite changes, why energy use shifts, or why creating a deficit can be easier for some people than others.

Research shows why the simple “calories in, calories out” idea isn’t fully accurate:

• Your body can respond to weight loss by making it harder to lose weight. Sometimes when you lose weight, your body fights back against this change. A review of weight-loss research describes appetite and energy changes that may make weight regain more likely. That means the same plan that helped you lose weight in the beginning can feel harder to maintain, or why someone may reach a weight loss plateau.
• Eating less can make your body push back. When people reduce how much they eat, the body may respond by increasing hunger and changing how much energy it uses. That’s why saying “just eat less and move more” leaves out how the body responds and how you might really feel.
• A calorie number doesn’t show how much energy your body uses to process a meal. In addition to calories, your body uses energy to digest and process what you eat. That amount is called the thermic effect of food, and it can vary based on what you eat. So the calorie number alone doesn’t tell you everything about how a food affects your body, it’s also about the specific foods themselves. 

CICO describes the energy balance, but it doesn’t explain the whole story behind it. 

Q: Does CICO work for everyone?

Dr. Mann: It’s true that if you restrict calories you lose weight, so in that sense, CICO does work. Where it gets complicated is that the focus on calories can take emphasis away from food quality and nutrient density. Restriction is unsustainable from a psychological standpoint, and obsessing over calories when they aren’t from high-quality foods doesn’t necessarily make us healthier or more successful in weight loss.

Calorie needs are estimates, too. Online calculators and activity trackers can’t measure exactly how much energy your body uses. Weight can fluctuate for reasons that don’t reflect changes in body fat, so I encourage people to look at longer-term patterns instead of reacting to every weigh-in.

Health conditions, medications, sleep, stress, and your daily environment can all affect appetite, activity, and weight. If calorie tracking isn’t working for you—or it’s affecting your well-being—you can use a different method and still work toward better health. The best approach is one that supports your health and fits your life. It shouldn’t require you to be perfect or cut out every food you like.

Q: What are common mistakes people make with CICO?

Dr. Mann: CICO can be useful as a broad concept, but it’s easy to oversimplify it. Some common pitfalls include:

• Treating calorie estimates as exact. Nutrition labels, restaurant information, food logs, and activity trackers can all be off. Think of the numbers as guides, not perfect measurements.
• Assuming weight loss will be linear. The same calorie target may lead to different results over time because your body’s energy needs and appetite can change.
• Focusing only on the calorie number. Calories measure energy, but they don’t tell you whether a food provides protein, fiber, vitamins, or minerals—or how filling it may be. Two foods with similar calories can differ in nutrition and how they affect fullness.
• Trying to “earn” food with exercise. Movement supports health and can contribute to energy use, but exercise calories are hard to estimate. You don’t need to punish yourself with a workout for eating.
• Assuming a larger deficit is always better. Cutting too much can make it difficult to get enough nutrients and keep up with everyday life.

Again, I’d focus on building meals that are satisfying and provide the nutrients your body needs. The overall pattern is what matters most, not what happens on one day. 

Q: Can you eat too little while trying to lose weight?

Dr. Mann: A bigger calorie deficit isn’t automatically safer or more effective. Eating too little can make it harder to get the nutrients your body needs and may leave you feeling weak, tired, or unwell. There are major safety concerns with diets that restrict calories below a certain point, and very low-calorie diets of ≤800 kcal/day require medical supervision.

Another concern is if you’re on a GLP-1. With these medicines, it’s common for your appetite to be lower, resulting in a calorie deficit through less intake. S,o being mindful to take in enough energy while despite the deficit is important. The type of energy matters, too. I encourage everyone to meet their daily protein requirements to maintain muscle.

If a plan leaves you persistently fatigued, dizzy, intensely hungry, or distressed about food, talk with a healthcare professional or registered dietitian. If you’re under 18, pregnant or breastfeeding, or have a history of an eating disorder, get medical guidance before trying to lose weight.

The goal isn’t to see how little you can eat. It’s to find an approach that supports your health and gives you enough nourishment.

Q: What if maintaining a calorie deficit feels impossible? 

Dr. Mann: Hunger, fullness, and energy use are influenced by biology. After weight loss, the body may respond with changes in appetite and energy expenditure that make maintaining that loss harder. That’s not a personal failure.

For some people, medication can help address the biological signals involved in appetite and fullness. GLP-1 medications, like Wegovy, Foundayo, and Zepbound, can reduce appetite and increase feelings of fullness; some also slow how quickly food moves through the stomach. Medication isn’t right or necessary for everyone, but it can be one treatment option to discuss with a prescriber.

Q: How do GLP-1 medications fit with CICO?

Dr. Mann: GLP-1 medications can change the appetite and fullness signals that influence how much someone eats. That can make it easier for some people to take in less energy without feeling constant hunger or food noise.

It’s important to work with your prescriber on a plan that supports nutrition, hydration, and muscle health, especially if your appetite is much lower.

Learn more about GLP-1 medications for weight loss and Noom’s medication options.

The bottom line: Calorie counting doesn’t work for everyone

CICO is an energy-balance concept, not a diet or a full explanation of obesity. A calorie deficit is associated with weight loss, but calorie counting is only one way to approach it—and the body’s response can make a deficit harder to create or maintain over time. 

Energy balance applies to everyone. What doesn’t suit everyone is calorie counting as a method. Nutrition, appetite, hormones, metabolism, health conditions, medications, sleep, stress, and your environment all matter. Some people do well with habit changes or structured eating patterns, while others may benefit from additional medical support.

The goal is to find an approach that supports your health, provides enough nourishment, and feels realistic to continue. If calorie tracking feels stressful, a deficit is hard to maintain, or you’re unsure what’s safe for you, talk with a healthcare professional about other options. If you want to explore whether prescription weight loss treatment is appropriate for you, Noom can connect eligible members with a clinician to provide personalized care.