Cardiorespiratory Fitness May Influence Metabolic Inflexibility During Exercise in Obese Persons

Francisco J. Amaro-Gahete; Guillermo Sanchez-Delgado; Ignacio Ara; Jonatan R. Ruiz


J Clin Endocrinol Metab. 2019;104(12):5780-5790. 

In This Article

Abstract and Introduction


Context: We examined whether obese individuals have a reduced maximal fat oxidation (MFO) and the intensity that elicit MFO (Fatmax) compared with normal weight and overweight persons, taking into account their level of cardiorespiratory fitness.

Methods: The study subjects were 138 sedentary adults (87 women) aged 30.1 ± 13.6 years. Based on their body mass index, subjects were categorized as being of normal weight (n = 66), overweight (n = 48), or obese (n = 24). MFO and Fatmax were determined for all subjects by indirect calorimetry, using a walking graded exercise test. MFO was expressed in absolute terms (g/min) and relative to whole-body lean mass (mg/kgleanmass/min). Cardiorespiratory fitness was assessed via a maximal treadmill test.

Results: No differences in absolute MFO and Fatmax values were seen between the obese, normal weight, and overweight subjects (all P > 0.2), although after adjusting for cardiorespiratory fitness, the obese subjects returned significantly higher values than did their normal weight and overweight counterparts (all P < 0.03). However, when expressed with respect to lean mass, the MFO of the normal weight subjects was significantly greater than that of the overweight and obese subjects, independent of age, sex, or cardiorespiratory fitness.

Conclusions: Obese individuals have higher absolute MFO values when cardiorespiratory fitness is taken into account, but when expressed with respect to lean mass, normal weight individuals show a greater capacity to oxidize fat during exercise per unit of metabolically active tissue independent of age, sex, or cardiorespiratory fitness. These findings suggest that obese individuals may suffer from metabolic inflexibility during exercise.


Obesity is a major public health problem[1] and one of the most important risk factors for mortality.[2] Metabolic flexibility is defined as the capacity to adapt one's nutrient oxidation balance to alterations in metabolic or energy requirements.[3,4] Traditionally, obese persons have been thought to suffer from metabolic inflexibility, which is characterized by reduced fat oxidation by the skeletal muscles under fasting conditions[5] and in the postprandial state.[6]

Maximal fat oxidation during exercise (MFO), and the intensity of exercise that elicits MFO(Fatmax), are key determinants of metabolic flexibility.[4,7] Whether MFO and Fatmax during exercise (hereafter referred to simply as MFO and Fatmax) differ depending on weight category is uncertain. A number of studies report MFO and Fatmax to be lower in obese people,[8–10] but others indicate them to be similar in normal weight and obese individuals[11–13] and higher in obese men than in fitness-matched overweight counterparts.[14]

Sex, age, muscle mass, and cardiorespiratory fitness are also determinants of MFO and Fatmax.[7] Greater cardiorespiratory fitness is associated with a lower risk of cardiovascular disease and all-cause mortality,[15] and different studies indicate a strong, positive association to exist between cardiorespiratory fitness and MFO and Fatmax in endurance athletes[16–18] and both active and sedentary individuals,[19–21] although others report no such association in Ironman athletes.[22] However, the studies that have examined MFO in obese subjects did not take cardiorespiratory fitness into account, making it difficult to know whether their apparent lower MFO and Fatmax are a consequence of their greater fatness or lesser fitness.[14] Taking their cardiorespiratory fitness into account, the present work examines whether obese individuals have a lower MFO and Fatmax compared with normal weight and overweight persons.