Recently, we went over the Alternate Healthy Eating Index (AHEI) and the recommendations on what to include in a food pattern as stated by Harvard School of Public Health (HSPH).  There were some differences from the national dietary guidelines.  

Today I would like to address one of them in as briefly a manner as I am capable.

Calcium.  This mineral is vital for bone health and the most usable source appears to come from dairy products.  The Dietary Guidelines for Americans includes 2-3 servings of dairy in its sample food plans.  The nutritionists at HSPH remind us that milk/dairy is not the only source of calcium and may not even be the best.  Two reasons against it are the high saturated fat content and the digestive difficulties some people have with lactose.  With regard to the fat component, to reduce consumption of saturated fats, which all the guidelines suggest, one must choose the 1% or less version of dairy across all the products.  This means your milk, yogurt, ice cream and cheese!

There is good news.  Green leafy vegetables are a very low calorie, no fat, exceptional source of calcium AND vitamin K.  These are two of the three bone health factors.  The other is vitamin D which is very hard to find in food sources (salmon is one, mushrooms another).  You will find kale, collards, brocolli raab, and many other similar vegetables (spinach, mustard and turnip greens) loaded with calcium and K. Less leafy vegetables and legumes are options too.  For instance, broccoli, Brussels sprouts, white beans,  okra, soy beans and some forms of tofu.  (here you could have one cup of dairy a day (milk or yogurt or cheese) and then two of these others as one option in meeting recommendations.

Every one of these veggies is a low calorie food with the exception of the white beans - which have more calories but are a superb source of protein. 
Here are some numbers for you:
(1 cup of leafy greans, 1/2  cup otherwise)
Calories are below 50 per serving of each item listed.
Collards - 266 mg of calcium and 836 mcg of K (the winner)
Kale - 94 mg of calcium and 1062 mcg of K
Broccoli raab (rabi) - 100 mg calcium and 217 mcg K
Brussel Sprouts - 28mg calcium and 109 mcg K

What to learn more?  You can look up a particular food and assess its macronutrient, vitamin and mineral content.  When you are scrolling through, don't stop at potassium which has a k by it.  Actual vitamin K is lower in the list. Access the nutrient data base by clicking here.
My knowledge comes from reading the information available from the Nutritionsource website of HSPH - You can read it yourself by clicking here.