hen students return to school in a few weeks, the biggest changes they see may be in the food they eat. It will be healthier and served in smaller portions, with more fruits, vegetables and whole grains, and less trans fat and sodium.

The food transformation comes from two major overhauls in the regulations governing school food taking effect this fall.

The US Department of Agriculture has issued its first major rewriting of nutritional guidelines for government-subsidized school lunches and breakfasts in more than 15 years. And the state has created its first set of nutritional standards for all other school food products — sold a la carte in the cafeteria, in vending machines, and immediately before and after school — to make them healthier and less calorie-laden.

“All align with the Dietary Guidelines for Americans,” said Karen McGrail, assistant director of the John C. Stalker Institute of Food and Nutrition at Framingham State University. “They’re all helping to promote healthy food in the same directions.”

The dietary guidelines, released last year, were the federal government’s response to increasing numbers of overweight and obese Americans. They urge all Americans to eat less, fill half their plates with fruits and vegetables, switch to fat-free or lower-fat milk, eat less salt, and avoid sugary drinks.

Students who get school lunch will be required to have at least one vegetable or piece of fruit on their tray. Foods like pizza won’t disappear from school lunch menus but they will become healthier. And chocolate and other flavored milk, which will have limits on sugar this year, could disappear entirely next year: State regulations will require flavored milks to have no more sugar than low-fat plain milk by the 2013-2014 school year.

The new state regulations, which took effect Aug. 1, ban deep-fat fryers in school kitchens and trans fat in “competitive food” — anything sold in schools outside the subsidized lunches and breakfasts. Juice cannot be served in portions larger than 4 ounces, or ½ cup. Serving sizes of all food, except for entrees, must be 200 calories or less.

The new rules also require schools to offer water to students all day without charge. Fruits and unfried vegetables must be offered anywhere that food is sold in schools, other than vending machines that are either unrefrigerated or that only sell beverages.

And by the 2013-2014 academic year, schools must offer students nutritional information on all competitive food and beverages that are not prepackaged, except for fresh fruit and vegetables. This recommendation reflects studies suggesting that many people eat fewer calories when they are given nutritional information about food they are buying.

Some schools have already started making their lunch lines healthier.

In Needham, the school system’s nutrition service director, Ruth Griffin, phased out white bread. A few years ago, cafeterias in her district began serving sandwiches with one slice of white bread and one slice of wheat. Then they switched to wheat bread for both pieces.

“The thing I think is wonderful about the new meal pattern is the increased fruits and vegetables,” Griffin said, with at least one serving of legumes required each week. “The challenge is to find ways to make beans taste wonderful and enticing for kids.”

This week, Framingham State’s Stalker Institute held a three-day conference in Marlborough to help schools understand the new regulations. The institute has an online nutrition evaluator, where school officials can find out whether a specific food meets the new standards.

The institute has also created the “A-List,” which started as a way to make recommendations to schools about which processed snacks are healthiest. When new standards were released, the institute revised the list — and it went from 2,000 items to 600, McGrail said.

For instance, the only Doritos tortilla chips now on the list is a reduced-fat version with 130 calories per serving. Small packages of goldfish crackers are there, but only those made with whole grains. Some ice cream meets the new standards, including So Delicious Organic Sandwiches in chocolate.

The conference this week included sessions on how to encourage students to eat healthier food. At other conferences, McGrail said, she’s heard a mantra: “It’s not nutrition unless they eat.”

“Obviously, having kids being open to healthy foods is part of why this is all being done,” she said.