Discrepancy between predicted and measured exercise intensity for eliciting the maximal rate of lipid oxidation.

Background And Aims: Ectopic lipid storage is implicated in type 2 diabetes pathogenesis; hence, exercise to deplete stores (i.e., at the intensity that allows for maximal rate of lipid oxidation; MLO) might be optimal for restoring metabolic health. This intensity ("Fat") is estimated during incremental exercise ("Fat test"). However, in "the field" general recommendations exist regarding a range of percentages of maximal heart rate (HR) to elicit MLO. The degree to which this range is aligned with measured Fat has not been investigated. We compared measured HR at Fat, with maximal HR percentages within the typically recommended range in a sample of 26 individuals (Female: n = 11, European ancestry: n = 17).

Methods And Results: Subjects completed a modified Fat test with a 5-min warmup, followed by incremental stages starting at 15 W with work rate increased by 15 W every 5 min until termination criteria were reached. Pulmonary gas exchange was recorded and average values for V˙ o and V˙ co for the final minute of each stage were used to estimate substrate-oxidation rates. We modeled lipid-oxidation kinetics using a sinusoidal model and expressed MLO relative to peak V˙ o and HR. Bland-Altman analysis demonstrated lack of concordance between HR at Fat and at 50%, 70%, and 80% of age-predicted maximum with a mean difference of 23 b·min.

Conclusion: Our results indicate that estimated "fat-burning" heart rate zones are inappropriate for prescribing exercise to elicit MLO and we recommend direct individual exercise lipid oxidation measurements to elicit these values.