Study 1: A study was conducted to determine the effects of geriatric foods utilizing 40 healthy geriatric cats (>= 12 years, initially 4.6 kg). Cats were fed a control food during a 30-day pre-feeding period, blocked by age, gender, and body fat percentage, and randomly assigned to 1 of 4 foods (A = experimental geriatric food, B = lams Active Maturity, C = Royal Canin Mature 28, and D = Purina ONE Senior Protection Formula) for the 90-day testing period. Chemistry screens and microalbuminuria were evaluated at Day 30 and Day 90 along with dual-energy x-ray absorptiometry (DEXA) scan at Day 90. Cats fed all foods maintained weight and body fat content throughout the study. At Day 90, cats fed food D had reduced (P < 0.01) bone mineral content. Cats fed food A and C had reduced (P < 0.03) lean. Cats fed food A tended to have a lower percentage of cats with renal damage (P < 0.08) compared with cats fed food B. For vital organ measures, cats fed food A had lower (P0.15). However, cats fed food H weighed less and had less fat (P < 0.03) compared with cats fed food E. Cats fed food F also had less fat (P0.35). By combining the data from both studies and plotting increasing lysine:calorie ratio versus lean loss for each corresponding ratio, a strong linear relationship could be detected with the line representing 75% (R-2 = 0.75) of the variation in lean loss. A moderate linear relationship was detected as bone mineral content decreased with increasing crude protein concentration (R-2 = 0.24). These studies suggest that feeding a food with reduced protein/phosphorus and higher levels of polyunsaturated fatty acids as well as antioxidants improved markers of kidney function compared with other commercial formulas. Additionally, increasing the lysine:calorie ratio prevented the loss of lean body mass in the geriatric cat and provided a similar response to high protein foods.