The Effects of a Low Vitamin A Diet on the Mechanism of Intramuscular Fat Development

Commercial steers consisting of a minimum of 75% Angus genetics and purebred Simmental steers were used to evaluate the effect of a low vitamin A (LVA) diet on marbling in cattle in a 2x2 factorial design. After 3 months of backgrounding on a LVA diet (1017 IU/kg DM), 64 steers (32 Angus cross, 32 Simmental) were allocated one of two dietary treatments for finishing. The LVA diet contained 723 IU vitamin A/kg DM; the control diet (CON) consisted of the LVA diet supplemented with 2200 IU/kg DM vitamin A. At the completion of finishing steers were slaughtered at a commercial abattoir. Muscle biopsies were collected from the longissimus dorsi at the beginning, middle, and end of finishing. Histology was used to visualize the protein fatty acid binding protein 4 (FABP4). RNA was extracted from the biopsy tissue and reverse transcribed to cDNA. Gene expression was measured by qPCR. Target genes include FABP4, PREF-1, PPARγ, ADH1C, RALDH1a1, C/EBPα, and RXR-α, β, and γ. Blood samples were collected at the beginning and end of finishing. These were used for retinol quantification (via HPLC) and hormone analysis (via RIA) for insulin-like growth factor 1 (IGF-1) and growth hormone (GH). There was an interaction (P<0.01) between treatment and breed for marbling score. The LVA treatment resulted in a 16% increase in marbling in the LVA treatment compared to CON within the Angus cross group, but had no effect in Simmental cattle. No difference was found in FABP4 protein expression. Serum retinol displayed a treatment by time interaction (P=0.01), with LVA levels lower than CON at both time points. There was a difference (P=0.01) in GH between breeds, Simmentals having greater levels. The LVA treatment tended (P=0.09) to result in lower IGF-1 serum concentrations when compared to the CON group. Angus cross cattle had greater (P=0.001) expression of FABP4, as well as a tendency (P<0.10) for greater expression of RXR-α, β, and γ. The CON group showed a significantly (P=0.04) higher expression of FABP4 and a tendency (P<0.10) for higher expression of RXR-α, C/EBPα, PREF-1 and ADH1C. There was no differences in PPARγ or RALDH1a1 expression. Therefore we conclude that in steers of moderate to high marbling potential (such as Angus cross), vitamin A restriction is associated with increased marbling and differential expression of adipogenic genes within the vitamin A pathway.