Sandwich chain Subway has developed a new healthier bread recipe it says contains the nutritional equivalent of one glass of milk.

The restaurant chain launched its new calcium- and vitamin D-fortified bread across the US Tuesday, replacing its previous recipe. Each six-inch (15 cm) serving of each type of bread is said to provide 30 percent of the daily recommended value of calcium and 20 percent of the daily recommended intake of vitamin D - or akin to drinking one glass of milk.

The 9-Grain Wheat and Honey Oat breads also contain 20 percent of the recommended daily value of fiber.

The new bread recipe was developed with the help of dieticians and is available across the 24,000 restaurants in the US.

Citing statistics from the Centers for Disease Control, Subway points out that on average, men and women in the US consume less than the daily recommended amount of calcium they need.

The Institute of Medicine states that less than one-third of older people meet the recommended intake for vitamin D.

The Subway website also has a column penned by nutritionists explaining the importance of the two nutrients.

"Not getting enough calcium means weak, porous bones and increases your risk of osteoporosis...Getting plenty of calcium when you're older can help minimize calcium loss from your bones. Vitamin D helps the body to absorb calcium and build and maintain strong bones."

Furthermore, research has also linked low levels of calcium to high blood pressure and vitamin D deficiency to increased risk of cardiovascular disease.

Fast-food retailers have been increasingly adding healthier menu options in the last few months.

Recently, American chicken retailer Chick-fil-A followed McDonald's lead and launched a breakfast oatmeal option last week, while Burger King is reportedly testing out its own oatmeal offering as well.

A healthy fast-food chain helmed by the former chief operating officer and global president of McDonald's and a former personal chef to Oprah Winfrey, is set to open its first restaurant in Palo Alto, California. Each dish at Lyfe Kitchen, which stands for Love Your Food Everyday, will contain less than 600 calories.