How many calories should you eat in a day? The USDA's guidelines for men, women and children – USA TODAY

Corrections & clarifications: An earlier version of this story misspelled the name of the spokesperson for the Academy of Nutrition and Dietetics. She is Caroline Susie.
Few things are as essential as nutrition, yet few things can feel as nebulous. Even the question of how to lose weight, once regarded as basic, is contested by experts, who say there’s more to it than simply eating less and exercising more. 
How many calories should you eat a day? 
There is no single answer to this question either, experts say. It depends on a number of variables, including your gender, age and weight among others.
But the U.S. Department of Agriculture puts out rough calorie ranges as part of its dietary guidelines report, published every five years.
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Adult females need anywhere from 1,600 to 2,400 calories a day and adult males need anywhere from 2,000 to 3,000 calories a day, according to the USDA’s latest “Dietary Guidelines for Americans” report released in 2020. 
Daily calorie needs for toddlers younger than 2 falls between 700 and 1,000 calories. Children aged 2 to 8 need between 1,000 and 2,000 calories, the report says. Daily calorie intake estimates vary widely for older children and teenagers, according to the USDA, with a range of 1,400 to 3,200.
The ranges vary based on gender, age, weight and activity level.
The USDA also refers Americans to an online Dietary Reference Intake calculator that makes individualized daily calorie intake suggestions based on the information you input along with daily macronutrient, vitamin and mineral intake recommendations. 
While online calculators provide calorie rough intake estimates, Caroline Susie, a registered dietician and spokesperson for the Academy of Nutrition and Dietetics, recommends consulting a dietician to best figure out your calorie needs and how to meet them. Health goals and medical history may factor into formulating a recommended diet in a way that can’t be picked up by an online calculator.
A calorie is a unit of energy in food, according to the National Institute of Health.
 “And our body uses that energy derived from foods to help us live our best life and go throughout our day,” Susie told USA TODAY.
1 gram of carbohydrate is 4 calories, 1 gram of protein is 4 calories and 1 gram of fat contains 9 calories, according to the South Dakota Department of Health. 
One way to lose weight is by reducing calorie consumption, but experts warn against cutting back too much. 
Susie said she comes across people on extreme calorie deficit diets who consume less than 1,000 calories a day. 
“It’s just not enough to keep you fueled long term,” Susie said. “And what’s scary is that when it’s so extreme and too low, you put yourself at risk for certain vitamin and nutritional deficiencies.” 
And research has shown that taking in too few calories can stunt your metabolism. 
“I see a lot of people undereat because they do a ridiculous calculation online,” Christina Barth, a health and nutrition professor at Arizona State University, told USA TODAY. “And they wonder why they’re not losing weight. Well, when you cut your calories too low, your body slows down. Your metabolism slows down.” 

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