The threshold effect: consequences of change in the frequency of food intake in the presence of a functional threshold. 
Studies on diseases thought to be diet-induced have so far assumed that the effect of a food depends solely on the amount ingested.  When the metabolism of a food derivative is dependent on a threshold-dependent mechanism this single variable assumption is invalid, as the net effect of the derivative will also depend on intake frequency.  In this paper the consequences of varying intake frequency in the presence of a notional threshold are analysed mathematically.  It is shown that at the low frequency end of the scale relevant to human diet, if a metabolic threshold exists then the net effect of a food will be critically dependent on intake frequency, and that the effect of changing frequency may exceed that produced by even substantial changes in quantum.  It is concluded that epidemiological studies which have not included data on frequency of intake involve a potential error and could be misleading.