Although [1-13 C]pyruvate is polarized along with [1-13 C]glycine, it is not visible in the ZULF spectrum in Fig. . We believe there are several reasons, considering that the [2-13 C]isotopomer was observed in previous work . First, the peaks are expected at lower frequencies than those of the [2-13 C] isotopomer, since the 13 C in the first labeling is weakly coupled to the three protons in the methyl group. As previously discussed, this XA 3 group would yield peaks at 3 J CH = 1.4 Hz and 2 × 3 J CH = 2.8 Hz . This poses a challenge since at very low frequencies, the magnetometer sensitivity is decreased due to 1/f -noise. Second, pyruvate is known to have two forms in aqueous solutions, i.e., the hydrate and non-hydrate form, which show peaks at additional frequencies, which decreases the overall signal amplitude . Nonetheless, these low-frequency peaks were recently observed at zero field, using PHIP as the hyperpolarization method. Is it worth noting that in Fig. , the formate peak shows a non-Lorentzian profile. We believe this is due to uncompensated magnetic-field gradients that hampered the shimming procedure at the sample location. This effect is also present in Fig. , albeit concealed on account of a broader linewidth.