CICO (calories-in-calories-out) is one of the simplest weight-loss diets. But this popular diet’s simplicity ignores some crucial aspects of long-term weight loss and health. Read more to learn about the CICO diet, how it works, its potential benefits, and downsides to watch out for.
During a CICO (calories-in-calories-out) diet, there is no focus on the type of food that you eat. Only the number of calories consumed is taken into consideration. CICO’s approach is very simple: its proponents believe that if you eat fewer calories than you burn (fewer calories in than calories out), you will lose weight.
The goal of this diet is for the body to end up burning more calories than the amount being consumed. As a result, the body needs to burn storages of fat for the extra energy, resulting in weight loss [1, 2].
Body weight only changes when energy balance (energy intake vs. expenditure) changes. For CICO to work, energy use must be greater than energy intake. Energy expenditure occurs through exercise and rest (resting metabolic rate) .
One important point most people forget is that your weight isn’t the only measure of your body’s overall health. The CICO diet is only concerned with reducing the number of calories you consume. On its face, it doesn’t take the body’s nutritional needs into account .
While the health complications associated with excess weight are improved by weight loss, it is very important to consider the overall effects of different types of food on your body .
If you plan on using CICO to lose weight, we recommend working with a doctor or nutritionist to ensure that you aren’t deficient in any important nutrients.
All the cells in our body require energy to function. This energy (which can be measured in calories) is constantly powering important reactions and processes in the body. We provide this energy to the body through food and burning stored fats. Any unused energy remaining in the body becomes stored as fat [3, 4].
The calories-in and the calories-out principle is a basic equation. Calories-in refers to the consumption of food and drinks, where energy is being added. Calories-out refers to the energy that is being used in the body during activity and rest. If more energy is used than added, fat stores are burned away as extra fuel [3, 4].
CICO relies on the principle of “energy imbalance.” In CICO, daily energy intake should be less than TDEE. Burning off more calories than you consume will result in weight loss .
According to proponents of CICO:
- Weight Gain: CALORIES IN > CALORIES OUT
- Weight Loss: CALORIES IN < CALORIES OUT
- Weight Maintenance: CALORIES IN = CALORIES OUT
Decreasing the number of calories consumed is the most common way to lose weight. The body must burn 3500 calories to shed 1 pound of fat. For example today you eat 2000 calories and burn 2500 calories. This leads to a total loss of 500 calories. If this is done for a week (500 calories burned x 7 days) you will burn 3500 calories, and 1 pound of fat [8, 9, 10].
Determining your calories-in number largely depends on your weight, age, height and calories-out number .
- Resting Metabolic Rate (RMR): the amount of energy needed to fuel the body at rest (breathing, heartbeat, blinking, etc.). This can vary from individual to individual and is mostly influenced by fat-free body mass. Age, sex, and fat mass also affect RMR.
- Thermic Effect of Food (TEF): energy used to digest food (absorption and metabolism). The more food you consume, the greater the TEF. It’s usually around 10% of the total amount of food you eat.
- Energy Expended through Physical Activity (EEPA): is the element that you can control the most. It also has the strongest influence on determining your calories-in number. This value is determined by the duration and energy cost of physical activities. This will differ depending on the intensity of the exercise.
The same amount of calories will have different energy costs to digest (TEF) depending on the type of food. This is greatest for proteins (20-30% energy content of the protein), followed by carbohydrates (5-10% energy content of carbs), and fats (0-3% energy content of fats) [12, 13].
However, the TEF is not nearly as important as your physical activity (EEPA) for calculating your total energy expenditure (TDEE). There are several online calculators that may help you determine your TDEE and EEPA (based on your activity levels) .
Once you have determined what your total energy expenditure (TDEE) and physical activity (EEPA) goals are, you can set the appropriate target for daily calories consumed.
CICO’s straightforward approach to weight loss can be highly beneficial for obese individuals who are suffering from weight-related health complications. The CICO diet is considered safe when calorie deficits are balanced with nutrient intake.
To avoid nutrient deficiencies and other potential adverse effects, work with your doctor or nutritionist to design and implement a healthy and balanced CICO diet.
Calorie restriction, when done correctly, refers to reducing calorie intake by 30-40% while maintaining proper nutrition (vitamin, protein, mineral, and water intake). If done this way, calorie deficits are believed to have many health benefits including [14, 15, 16, 17, 18, 19, 20]:
- Inverse association with tumor and breast cancer risk
- Reduced oxidative stress
- Reduced incidence of heart disease (particularly atherosclerosis) and stroke (due largely to decreased blood pressure)
- Reduced tissue injuries caused by lack of oxygen (ischemia)
- Reduced cholesterol (hyperlipidemia) and obesity (through intermittent fasting)
- Hormonal balance: increased insulin sensitivity (useful for diabetics) and ghrelin reduction (causes hunger)
- Less inflammation
- Eye protection (protects the retina and may help reduce the symptoms of cataracts and glaucoma)
- Slowed aging and increased longevity
- Improved brain and memory function
- Slowed progression of brain diseases (e.g. Alzheimer’s Disease)
- Reduced risk of heart disease and stroke (reduced blood pressure)
- Reduced risk of diabetes
- Reduced risk of different cancers
- Reduced risk of asthma
- Reduced risk of gallbladder disease
- Improvements in osteoarthritis
- Improvements in chronic back pain
- Improved cholesterol levels
- Improved movements
- Reduced risk of stroke
Successful weight loss in the long-term can be maintained by:
- Continued monitoring of your caloric intake 
- A support system that helps with the regulation of your health goals [24, 25]
- Maintaining physical activity 
- Good mental health 
Note that these benefits are limited to people who start out overweight or obese. If you are already at a healthy weight, further weight loss is not recommended. Your doctor can help you determine and reach your ideal weight.
The CICO diet is focused on a calorie deficit, and there is no guidance on how to achieve it. The path to weight loss can be either healthy or unhealthy. CICO diets may lead to health problems if improperly approached (e.g. only eating chips and candy but remaining below the calculated calorie intake).
Problems may arise due to a lack of nutrients, energy, and exercise. Nutrient deficiencies may occur when just reducing calories without eating enough essential nutrients. Lack of energy can cause fatigue, lowering the drive to exercise and leading to further health risks [28, 29].
To avoid these potential dangers, work with your doctor to design a healthy CICO diet.
There is a prevalence of nutrient deficiency in individuals following some popular diet plans. This may also occur in individuals following the CICO diet. Nutrient deficiencies can lead to a variety of problems such as :
- Cancer: according to some researchers, diet contributes to about a third of preventable cancers (same amount as smoking) [31, 32].
- DNA Damage: a deficiency in any micronutrient (folate, vitamin B12, vitamin B6, niacin, vitamin C, vitamin E, iron and zinc) damages the DNA (mimics damage via radiation) .
- Chronic Energy Deficiency (CED): Deficiencies in vitamin A, vitamin B, and proteins caused chronic energy deficiencies in 658 (out of 1540) drought-affected adults (observational study) .
- Osteoporosis: diet is important for the prevention of osteoporosis. Nutrient deficiencies (vitamin D, calcium, magnesium, silicon, vitamin K and boron) may reduce bone growth and maintenance [34, 35, 36, 37].
- Heart Disease: 509 patients with heart disease and iron deficiencies were given iron supplements over 5 to 36 weeks (meta-analysis), resulting in a 60% decrease in heart failure risk .
- Birth Defects: Vitamin supplements are strongly recommended during pregnancy. Micronutrient deficiencies may lead to detrimental long-term outcomes for newborns such as survival, cognitive function, as well as increased risk of diabetes, heart disease, and stroke [39, 40, 41].
- Diabetes: the body requires vitamins and nutrients (cofactors) to maintain glucose and insulin levels. Deficiencies in these nutrients may impair the glucose and insulin pathways, which may contribute to diabetes .
- Obesity: Nutrient deficiencies are common in individuals with obesity, who usually eat high-calorie foods with low nutrient content. Nutrient deficiencies impact glucose and leptin breakdown, which may lead to obesity [43, 44].
- Hair Loss: Hair loss can be caused by deficiencies in iron, zinc, niacin (vitamin B3), fatty acids, selenium, vitamin D, vitamin A, vitamin E, folic acid, biotin, and proteins. Since hair loss can be a result of many factors, it is unclear whether nutrient supplementation will help restore hair growth .
- Mental Health Problems (especially Depression): Vitamin B12 and folate supplements improved mood in depressed patients (RCTs), suggesting that deficiencies in these vitamins can worsen depression .
- Brain Problems: Zinc deficiency has been linked to low physical activity and reduced brain activity among children. Additionally, vitamin B12 deficiency has been associated with lower cognitive functioning in adults [47, 48].
- Weakened Immune Response: several experiments over a period of 6 years showed that nutritional stress (protein and calorie deprivation) leads to a variety of changes in the immune response of mice, rats and guinea pigs. This may also apply to individuals deprived of nutrients .
- Inflammation: supplementation with vitamin C, vitamin E, and carotenoids reduce inflammation in the body .
Some individuals who adopt the CICO diet may be doing so due to its lack of food restrictions. These people may end up eating unhealthy food while still maintaining a caloric deficit. On the other hand, other individuals may not eat enough calories. This may cause weight loss, but that doesn’t make it healthy [2, 51].
Unhealthy calories (sweets, processed foods, fried foods, etc.) or an inadequate number of calories may lead to health problems like :
- Inflammation: high-carb, high-sugar, and high-fat meals can be a direct cause of inflammation. Increased inflammation can make it difficult to stick to a diet by causing cravings, fatigue, psychological stress, and lack of motivation [50, 52, 53].
- Stunted Growth Via Chronic Energy Deficiency: energy deficiency is associated with weight loss, reduced metabolism and reduced exercise when not enough nutrients are consumed. CED may cause stunted growth, especially in children. These individuals don’t “lose weight”, they simply grow less [54, 33].
- Fat Gain: Individuals with chronic energy deficiencies naturally adapt by reducing their body’s energy consumption. This means that they will not lose weight but instead will have less overall energy. Once these individuals start eating normally again (supplemented), they will experience very rapid fat gain .
- Reduced Immune Response: a deficiency in total calories and proteins severely reduces the immune response. This makes individuals extremely susceptible to a variety of illnesses’ such as bacterial infections .
- A Risk to Offspring: poor dietary habits will likely negatively impact the immune system response of any offspring. Unhealthy foods will lead to protein and gut bacteria changes in the body, which influence the immune system. These immune modifications may be passed down to your offspring .
- Heart Disease: a poor diet is one of the most common causes of heart disease. Inappropriate eating habits (high fat, high salt, lower number of meals but larger portions) significantly increases the risk of heart disease [56, 57].
- Cancer: high fat, salt, and sugar diets with low fruits and vegetables strongly increase the risk of cancer (1 in 3 cancers may be diet-related) .
- Stress: low-calorie diets have been associated with an increase in cortisol levels. Increased stress levels lead to more unhealthy eating habits, causing weight-gain in many cases [58, 59].
While a diet may be effective in weight-loss, diet combined with exercise is the most effective and healthiest method of weight-loss. Added to weight-loss, exercise has numerous positive benefits such as reducing the risk of heart disease, diabetes, stroke, treating osteoporosis, and a variety of other benefits [60, 61, 62].
Maintaining consistent exercise during a diet can be difficult for some. But the effects of caloric deficits (fatigue, stress, lack of motivation) make it even more difficult to exercise often .
Additionally, the body of individuals with caloric deficiencies adapts by using less energy (total daily energy expenditure). Since the energy burned during rest (RMR) doesn’t vary much, individuals will have less remaining energy to use for exercise (EEPA) .
A study in 12 healthy wrestlers found that short-term caloric deficits decrease exercise power (compared to a normal diet). The wrestlers had restricted calorie intake for a week .
To avoid adverse effects during weight loss, work with your doctor to determine the best diet for you.