从头合成（de novo synthesis）是指从最简单的组成分子，如二氧化碳和氨基酸等开始，消耗较多能量，逐步合成生物大分子。核苷酸的从头合成与补救合成是相对的概念。

De novo synthesis refers to the synthesis of complex molecules from simple molecules such as sugars or amino acids, as opposed to recycling after partial degradation. For example, nucleotides are not needed in the diet as they can be constructed from small precursor molecules such as formate and aspartate. Methionine, on the other hand, is needed in the diet because while it can be degraded to and then regenerated from homocysteine, it cannot be synthesized de novo.