What were thought of as certainties can fall apart in the light of new circumstances, new thinking.
For years we have been told that the Omega 3 fatty acids from oily fish such as salmon, tuna and mackerel are good for the heart and brain.
Now, some of the people who proved that the world’s fish catches were not rising but falling - a critical milestone in this century’s thinking - have produced a report that shows that the evidence for the beneficial effects of Omega 3s is somewhat less impressive than one might think.
The study actively challenges dietary advice that we actually should be eating more oily fish.
It says that people in the Northern hemisphere who eat a balanced diet may be getting enough Omega 3s anyway but people in the Southern hemisphere are having their essential proteins and fish oils stolen from them by diet faddism in the North.
Oily fish can actually be bad for overweight or unhealthy people with heart problems, it says.
Was the fad for Omega 3s perhaps just a marketing strategy worked up by the fishing industry? That’s going too far, but it certainly looks as this dietary obsession is something we should be looking at a whole lot more critically now we know that wild fish stocks are in trouble almost everywhere.
Policies can be skewed by vested interests - including our own perceived health interests.
In fact what the human race needs are balanced policies that promote healthy people and healthy wild fish populations, able to feed a human population that will rise by a third over the next half century.
That is what we should be planning for, together with ways of producing Omega 3 fatty acids from vegetables and algae, to take the pressure off fish.
In the meantime, we may actually need to eat less of certain fish - salmon and tuna for instance - rather than more as the dieticians have been calling for.