Transgenic lettuce (Lactuca sativa L. cv. 'Cortina') lines expressing the asparagine synthetase A gene from Escherichia coli were produced to alter the plant nitrogen status and eventually enhance growth. The relative molecular abundance of water-soluble metabolites was measured by 1H NMR in transgenic and conventional plants at early developmental stages and grown under the same conditions. NMR metabolic profiles assessed that a transgenic line and the wild-type counterpart shared the same compounds, but it also revealed side effects on the carbon metabolism following genetic modification. Concerning the nitrogen status, the amino acid content did not vary significantly, except for glutamic acid and gamma-aminobutyric acid, which diminished in the transgenics. As for the carbon metabolism, in transgenic leaves the contents of sucrose, glucose, and fructose decreased, whereas that of inulin increased up to 30 times, accompanied by the alteration of most Krebs's cycle organic acids and the rise of tartaric acid compared to nontransformed controls. Lettuce leaf inulins consisted of short oligomeric chains made of one glucose unit bound to two/four fructose units. Inulins are beneficial for human health, and they are extracted from plants and commercialized as long-chain types, whereas the short forms are synthesized chemically. Hence, lettuce genotypes with high content of foliar short-chain inulin represent useful materials for breeding strategies and a potential source for low molecular weight inulin.