What Happens if I Start Eating Fewer Calories?

Cutting calories, which are measures of energy, is a major principle of most diets — if you want to lose weight, you should consume fewer calories than you burn. And while eating fewer calories can lead to weight loss in the short run, it can also also affect your body and mind in other ways.

The daily recommended caloric intake for any individual depends on various factors like age, height, weight, gender and physical activity level. The Office of Disease Prevention and Health Promotion's ​Dietary Guidelines for Americans​ advises that healthy adult women should consume between ​1,200 and 2,400​ calories per day and men should consume between ​2,000 and 3,000​ calories.

There are different methods to reduce your caloric intake: Calorie restriction, which involves eating fewer calories below what is typical while still eating essential nutrients, and fasting, which involves eating at only certain times of the day or week. One example of the latter is intermittent fasting, in which periods of eating are alternated with periods of no eating and overall calories are reduced because there is less time to consume them.

​Read more:​ Healthy Ways to Lose Weight Fast

A November 2018 study in the ​American Journal of Clinical Nutrition​ showed that both intermittent fasting (eating unrestricted for five days and then eating a restricted caloric intake for two days, reducing overall calories by 20 percent) and continuous calorie restriction (reducing daily calories by 20 percent) led to weight loss and prevention of metabolic diseases.

An August 2015 study in the ​International Journal of Obesity​ also showed that while calorie restriction resulted in weight loss, there were other ​physiological effects.​ There are many complex processes involved in maintaining body weight, and the body can change and adapt when it is taking in fewer calories.

For example, total energy expenditure (the number of calories burned by basic body functions, digestion and physical activity) is reduced after weight loss because less energy is required to maintain a smaller body and metabolism becomes more efficient, which can both lead to weight gain. This means in order to keep losing weight, an individual has to continuously cut more and more calories from their diet, which is not a sustainable long-term option.

It is possible to lose weight with diet via cutting calories alone. An October 2014 review in ​PLOS One​ found that long-term weight loss was greater with diet vs. aerobic exercise, but adding aerobic exercise to a diet had slightly ​better results​ than diet alone.

And an August 2015 article published in the ​British Journal of Sports Medicine​ argues against the claim that having obesity is due solely to a lack of physical activity because diet plays an important role, and "you can't outrun a bad diet."

However, eating too few calories (i.e., under the recommended daily range) can have counterproductive effects on your body, such as depriving it of essential nutrients. In addition, your body is designed to maintain certain energy stores as a survival tactic, and by reducing energy (calorie) intake, your metabolism lowers to compensate.

An August 2016 study in ​Obesity​ found the reduced resting metabolic rate caused from weight loss can persist over time, even after the diet period.

​Read more:​ Can Eating Too Few Calories Prevent Weight Loss?

The ​International Journal of Obesity​ study notes that calorie restriction can decrease levels of leptin, the hormone that manages satiety, leading to increased appetite, hunger, the desire to eat and prospective consumption. Calorie restriction can also lead to an increase in ​cravings​, an increased emotional and sensory response to food, and decreased cognitive control of food intake. All of these factors can contribute to weight regain.

There is some limited evidence that eating fewer calories could have long-term benefits. A July 2015 study in the ​Journals of Gerontology​ looked at individuals who didn't have obesity over two years and found that reducing calories (maintaining a 12 percent reduction in calories over the entire time period) helped reduce risk factors for age-related diseases such as blood pressure, insulin resistance and cholesterol.

While eating fewer calories can result in short-term weight loss, evidence shows that it is ​not an effective long-term strategy​ for body weight management. Instead, individuals should aim to maintain an overall healthy diet and physical activity level. Before taking any drastic reductions in caloric intake, you should consult with a medical professional to make sure nutritional requirements are met, and it is a healthy option for your lifestyle.