Dietary climate impact correlates ambiguously with health biomarkers- a randomised controlled trial in healthy Finnish adults.

Purpose: A transition to more plant-rich diets is an effective way to reduce the climate impact of a diet. Using a whole-diet approach, we studied how partial replacement of animal-sourced with plant-sourced proteins affected the dietary climate impact while simultaneously considering diet-related health biomarkers.

Methods: In a 12-week randomised controlled trial, 107 women and 29 men were assigned into three diet groups (ANIMAL, 50/50, PLANT) with animal-to-plant-protein ratios of 70/30, 50/50, and 30/70, respectively. Life-cycle-assessment-based coefficients for foods were used to assess the climate impact of the diet groups, based on four-day food records. Correlations between climate impact and biomarkers were assessed.

Results: The climate impact (CO₂ eq.) for PLANT was 3.32 kg per day, 3.05 kg per 2,000 kcal, and 0.04 kg per gram of protein, for 50/50 4.34, 4.20, and 0.05 kg, and for ANIMAL 4.93, 4.94, and 0.06 kg, respectively (p < 0.05 for all except ANIMAL vs. 50/50 /g protein and /2,000 kcal). Climate impact correlated weakly positively with colorectal cancer risk markers and a positive status of bone turnover, but not with cardiometabolic risk markers. Animal-based iron intake and climate impact (per 2,000 kcal) had a strong positive correlation 0.70 C.I. [0.60, 0.77], while saturated fat (0.29 [0.13, 0.44]) and calcium (0.37, [0.22, 0.51]) intake had a weak positive correlation, and fibre intake (- 0.37, [- 0.51, - 0.21]) a weak negative correlation with climate impact.

Conclusion: Replacing animal-sourced proteins with plant-sourced proteins reduced the climate impact of the diet. The relationship between climate impact and biomarkers was more ambiguous indicated by both beneficial and harmful indicators within lower climate impacts.

Clinical trial registry: NCT03206827; registration date: 2017-06-30.