Your body uses calcium from food and drink to build and maintain the skeleton, but it cannot manage this alone. Without vitamin D, the process comes to a halt and the result can be a porous skeleton. There are two ways to acquire this important vitamin:
Through dietary intake, especially fatty species of fish such as salmon, trout, herring and mackerel.
Through the skin, when exposed to sunshine. However, for all who live north of 46°N this happens too seldom. In Europe, everyone who lives north of the Alps produces too little vitamin D and must therefore obtain it through dietary intake.
Source: Norwegian National Council for Nutrition report: ”Dietary guidelines to promote public health and prevent chronic illnesses in Norway – Methodology and scientific knowledge basis”, chapter 24: ”Diet and prevention of osteoporosis”.