Fat Cat, Thin Muscles: 5-Point BCS, MCS and 1-2% a Week

Two numbers decide a weight plan: how much fat the cat carries, and how much muscle it has left. A 6.8 kg cat at body condition score 4 of 5 is roughly 30% over ideal, and its target is 5.2 kg, not 6.0 kg. Lose faster than 2% of body weight a week and the cat gives up muscle first, which is the part that keeps it jumping onto the bed at 12 years old.

Two scores, two different problems

Body condition score (BCS) measures fat. Muscle condition score (MCS) measures lean mass. They move independently: a cat can be BCS 4 of 5 and still lose muscle over the spine and hips, a pattern called sarcopenic obesity. That combination matters more than either number alone, because fat pads hide the loss until the cat is weak. Weigh, score, and palpate at every visit, not once a year.

BCS on the 5-point scale, in 60 seconds

Stand the cat on a flat table, stand behind it, and run both hands along the rib cage with the thumbs on the spine. On the 5-point scale, 3 is ideal, 1 is emaciated, and 5 is obese. Ribs felt under a thin fat layer with a visible waist behind the ribs means 3. Ribs felt only with firm pressure and no waist means 4, which is 15% to 20% over ideal. No ribs at all, a rounded back, and a belly that swings when the cat walks means 5, or 30% to 40% over. Each point above 3 is worth about 15% to 20% of body weight, so score honestly before you pick a target.

MCS: four places your hands must check

Work down four landmarks: the flat of the skull behind each eye, the top edge of each shoulder blade, the bumps of the lower spine, and the two hip points. With normal muscle all four feel covered, and the spine reads as a rounded ridge. Mild loss sharpens the spine and scapulae while leaving padding. At moderate loss the hip points and vertebrae read through the coat and the skin over each temple sinks into a hollow. Severe loss leaves bare bone at the pelvis, with nothing to pinch. Grade it at every weigh-in, because a cat losing 100 g a week of muscle and 100 g of fat is not on track even when the scale looks right.

The 1 to 2 percent rule, in grams

Safe loss is 1% to 2% of current body weight per week. For a 6.8 kg cat that is 68 to 136 g a week, or 0.3 to 0.6 kg a month, and a 30% excess takes about 6 to 8 months to clear. Seniors over 10 years lose at 0.5% to 1% a week, or 30 to 60 g, because they have less muscle to spare. Log one number a week: the reading taken before breakfast on the same machine. A 200 g swing between two weigh-ins is usually the scale, not the cat.

Calories and protein for the target weight

Resting energy comes from 70 multiplied by target weight in kg raised to 0.75, which is about 240 kcal at 5.2 kg; a loss plan feeds 0.8 of that, near 190 kcal a day. Protein should land at 6 to 7 g per kg of target weight, so 31 to 36 g a day, with fiber at 8% to 10% of dry matter to keep the cat full. Never feed below the resting requirement: under 180 kcal a day for a 4 kg cat invites hepatic lipidosis, especially after 2 to 3 days of not eating.

Why a 12-year-old cannot lose like a 2-year-old

Older cats digest protein less efficiently, use less of what they absorb for muscle, and often carry CKD, hyperthyroidism, arthritis, or diabetes at the same time. A T4 above 4.0 µg/dL can mask weight loss for months, and a creatinine of 1.8 mg/dL changes the protein target entirely. Over-restriction in a senior costs muscle, then strength, then the ability to reach the litter box. Arthritis pain also cuts activity, so treat pain before cutting calories further.

When the scale is reporting disease, not diet

Eating normally while shedding 10% of body weight across 6 months is a bloodwork case, not a stricter diet. That pattern points to hyperthyroidism, diabetes, CKD, or gastrointestinal disease. Losing weight with a normal appetite is covered in detail at weight loss with a normal appetite, and the age-based risk bands sit in the cat age calculator. Appetite loss after a companion dies is a separate problem, not a calorie math error.

When the loss stalls for 3 weeks

If weekly loss falls under 0.5% for 3 weeks, cut calories by 10%, not 30%. Recheck the actual grams fed with a kitchen scale, because a 40 g scoop of kibble can vary by 15% between bags. Split the day into 4 meals, use a puzzle feeder for 2 of them, and add 5 minutes of play twice a day. Do not switch to a light diet and free-feed it; measured meals are the only way to know what the cat eats.

The weekly sheet that keeps it honest

1. Weigh before breakfast, same scale, log to 10 g.
2. Score BCS every 2 weeks and MCS every 4 weeks.
3. Weigh the food in grams; recalculate whenever the bag changes.
4. Ask for T4, creatinine, and urine at the start and at 6 months.
5. Stop the diet and call the vet if the cat skips 24 hours of food.

BCS (1 to 5)	Weight vs ideal	What your hands feel	Target for a 6.8 kg cat	kcal a day	Loss per week
2	15% under	Ribs and spine sharp, no fat cover	Gain to 5.6 kg	260 to 300	Gain 50 to 70 g
3	Ideal	Ribs under a thin layer, waist visible	Hold 5.2 kg	240	0 g
4	15% to 20% over	Ribs need firm pressure, waist soft	5.2 kg, about 4 months	190 to 210	70 to 100 g
4 with MCS loss	20% over, muscle down	Ribs padded, spine and hips sharp	5.2 kg, 6 to 8 months	200 with 36 g protein	35 to 70 g
5	30% to 40% over	No ribs, broad back, hanging belly	5.2 kg, 4 to 6 months	180 to 200	70 to 136 g

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