Fructose Feeding during the Postabsorptive State Alters Body Composition and Spares Nitrogen in...

Fructose feeding in the context of high energy intake is recognized as being responsible for metabolic dysregulation. However, its consumption in the postabsorptive state might contribute to reducing the use of amino acids (AAs) as energy substrates and thus spare nitrogen resources, which could be beneficial during catabolic states.

We hypothesized that fructose feeding during a catabolic situation corresponding to protein-energy restriction (PER) in older rats would reduce AA utilization for energy purposes, thus slowing down the loss of body weight (BW) and improving body composition.

For 45 d, 22-mo-old male Wistar rats (average weight: 716 g) were fed a control ration (13% protein) either at normal (20 g/d), restricted (PER: 10 g/d), or at PER levels supplemented with glucose (3 g/d) or fructose (3 g/d) and then studied in the postabsorptive state. We measured BW, body composition, and enzyme activities and metabolite concentrations related to glucose, fructose, and AA metabolism.

Both glucose and fructose feeding reduced PER-induced loss of BW and lean mass (-27% compared with PER), but only fructose reduced the loss of fat mass (-28% compared with PER). Fructose feeding prevented the PER-induced loss of muscle and intestinal mass. Fructose feeding also reduced circulating branched-chain AA concentrations by 50% (compared with PER) and increased those of alanine (+65% compared with PER). A reduction in hepatic enzymes related to AA catabolism was also observed during fructose feeding (compared with PER), whereas glycogen concentrations were enhanced in both intestine (+300%) and muscle (+21%).

We showed that in PER older rats, fructose feeding improved body composition and the weight of several organs by reducing AA catabolism and utilization for energy production and liver autophagy potential. This could be advantageous in sparing body proteins, particularly during catabolic states, such as those related to malnutrition during aging.