There have been a lot of questions about the reliability of nutritional science. We should respond with an assertive statement: Nutrition is a hard science. By just about any comparison, much of what is known about nutrition and the methods that have built that knowledge is as robust as classical physics, biochemistry and other basic sciences generally recognised as rigorous. We know how people handle hundreds of dietary compounds, understand many of the mechanisms and pathways, and are usually able to predict metabolic responses to lack or excess. If somebody wants to know which foods are good sources of vitamin B₁₂ and how it gets from ingested food into the bloodstream, we can be confident that the answer today will be the same as 50 years from now. The knowledge about this and many other specific nutrition concepts can be justifiably considered mature with low likelihood to change materially.

We can also predict with confidence that vitamin B₁₂ stores will decline without any source of vitamin B₁₂, say in a person with a vegan food intake pattern not using dietary supplements, although it may take decades to reach the point of deficiency. It is easy enough to prevent vitamin B₁₂ deficiency with food supplementation for people at risk. Biomarkers like methylmalonic acid (MMA) concentration in plasma help to recognise deficiency when …