Although there are a variety of theories behind the growing obesity problem plaguing North American adults and children, the most consistent findings point to caloric intake as the culprit. Here’s a simple equation to get to the root of the problem:

Calories eaten > calories spent = weight gain.

According to National Health Examination Surveys, adult obesity trends in the United States between 1976 and 2014 indicate the percentage of the adult population classified as obese has roughly doubled to more than 38 percent in the last three decades. Children may be learning eating habits from their parents, potentially contributing to rising obesity rates in children as well. Recent findings from the Centers for Disease Control and Prevention indicate one in five school-aged children and young people in the United States is obese. In Canada, the Public Health Agency says roughly one in seven children is obese.

Teaching children healthy eating habits starts at home and can extend to what students are given to eat while at school. The Center for Science in the Public Interest says schools across the country are working hard to improve school nutrition. Here’s how parents and school districts can help make school lunches more nutritious and delicious and lower in calories.

Control snack intake. The U.S. Department of Agriculture says that more than one-fourth of kids’ daily caloric intake comes from snacking. Choosing smarter snacks may help reduce overeating. Good snacks can include grain products that contain 50 percent or more whole grains by weight; snacks in which the primary ingredient is a fruit, a vegetable, dairy product, or lean protein; snacks that are a combination food that contain at least a quarter cup of fruits or vegetables; and foods that contain no more than 200 calories.

Read nutritional information. When selecting foods for school lunches, parents should read the nutritional information to make sure they know exactly what they are feeding their children. Select foods that are low in saturated fats and cholesterol and high in fiber and nutrient-rich fruits, vegetables, grains, and legumes.

Go with water. Rethinking beverage choices can help control kids’ caloric intake. Many people don’t realize just how many calories beverages add to their daily intake. Even a six-ounce, 100-percent apple juice can include as many as 96 calories. Sodas and other soft drinks pack a hefty caloric punch. Water, seltzer and unsweetened iced tea are healthy beverage options. If milk is the go-to beverage, choose a reduced-fat version.

Introduce new foods. Children can be notoriously picky eaters, but with patience and perseverance, parents can introduce new, healthy foods at lunchtime. Yogurt, hummus and salsa are healthy and can add flavor to vegetables and fruit. When making sandwiches, exchange refined breads for whole-grain varieties. Choose lean protein sources, and go heavy on vegetables and fruits for natural fiber, which will create feelings of satiety.

Read the school menu. Let children indulge in ordering from the school menu when healthy options are featured. Urge them to try something unexpected, rather than sticking to chicken nuggets or pizza days.