Is plant based milk as good a calcium source as whole milk? Is reduced fat milk as good as whole milk? What do we mean by ‘good’? Why not a couple of calcium pills and forget about drinking any kind of milk? Simple question, complicated answers. Why complicated? Because we are talking about one the main issues in staying alive and healthy – getting proper nutrition.

WHAT DO WE MEAN BY ‘GOOD’?

Is Food Calcium Absorbed Efficiently

As far as we are concerned, the calcium in a ‘good’ food source is ‘available’ to the body. And ‘available’ means it can be readily absorbed by the intestines into the blood. The word ‘bioavailable’ is – for us – best used this way: Is the food calcium absorbed efficiently? How efficiently? What fraction of the calcium in the food can be absorbed? This article is about liquid foods, milks of all sorts, from cows, plants, whatever. We view cow’s milk as the gold standard but we know not everyone uses it. In fact some plant based milks can achieve the same level of bioavailability as cow’s milk.

The milk is one thing, the intestines are another – they are alive and have their own idea about what fraction of calcium they will absorb. That fraction is regulated. So bioavailability as measured by absorption will vary from person to person. Studies of any one food will therefore have an average value and a spread depending on age, sex, and everything else that controls calcium absorption by the intestines.

What makes the food part of the equation interesting is that the food itself affects how well the intestines can absorb the calcium. The intestines have their goal – so much calcium absorbed per minute. The food offers more or less resistance to absorption depending on how it interacts with calcium – makes it more or less ‘available’ to the intestine. Fats, proteins, phosphates, phytate even oxalates in food interact with calcium, bind to it or not. Stomach acid changes foods, a lot, and therefore calcium binding.  But people can measure the overall fraction of calcium absorbed and that is a measure of ‘bioavailability’.

Is Food Calcium Retained By Bone?

Some authors use ‘bioavailability’ to mean not only absorption of food calcium into blood but the use of that calcium by the body, especially to build and maintain bone. That is another matter altogether. Some calcium is secreted from blood into the intestine and lost in the stool ‘endogenous fecal calcium’. Saliva, pancreatic and biliary secretions, duodenal secretions. That subtracts from overall ‘absorption’: some calcium is coming in, some is leaving via the intestine. There there aare the kidneys. They filter and calcium out of the blood and let a certain amount escape into the urine – to cause stones in some people. So when we are talking about bioavailability in terms of bone we mean the net of absorption from food into blood, minus endogenous fecal calcium loss from blood into the stool, minus loss of calcium in the urine. Stone formers are unique in having a high frequency of ‘hypercalciuria’, more calcium loss in urine that is usual among people in general. That puts their bones at special risk.

To measure ‘bioavailability’ of a food as defined by bone calcium balance requires a lot of effort. One can use isotopes to mark uptake from food, endogenous fecal loss and if you add in simultaneous urine measurements you get an estimate. We do not much like this use of the word because regulation of urine calcium is independent of diet calcium, though very dependent on diet sodium, sugar, and protein. It makes little sense to isolate one food with calcium in it and speak about overall balance because that one food is just one part of what matters. So we use the word to mean the fraction of the calcium in a food that people will absorb into the blood.