Recent studies suggest that the growth and fecundity of northern ungulates may be coupled to their summer nutrition. Here, we compare summer dietary nitrogen availability of the five major browse plants (comprising approximately 79% of the diet) of moose (Alces alces) in Denali National Park and Nelchina Basin, Alaska, USA. In recent years the productivity of Denali moose has been significantly higher than that of Nelchina moose, prompting this comparison. We examined the phenological progression of leaf nitrogen concentration, tannin-protein precipitation capacity, and digestible protein over three summers in both regions. We then modeled the potential nutritional consequences for a cow moose consuming representative diets on each range, predicting both net protein intake (NPI) and lean body mass accumulation each year. We found that leaf nitrogen and digestible protein decreased, while tannin-protein precipitation capacity increased throughout the summer for all forages. There was 23% more digestible protein in Denali leaves than Nelchina leaves on average, and this difference was significant in all three years. Tannins accounted for a large (mean = 46%) reduction in protein availability, suggesting a key role of these secondary compounds in the nitrogen balance of moose in these regions. Finally, our NPI model predicted that Denali cows were in positive protein balance 17 days longer than Nelchina cows and accumulated 18 kg more lean body mass over the summer, on average. We conclude that summer dietary nitrogen availability may act as a nutritional constraint on moose and suggest that more emphasis be placed on elucidating its role in population dynamics and conservation of northern ungulates.