How 6 kg body weight changes mercury risk
Smaller cats accumulate toxins faster relative to their size—what's a negligible dose for a 10 kg cat becomes significant for yours. The oil's purification process removes most mercury, but "most" isn't zero, and daily dosing means weekly micrograms add up over months. Watch for subtle neurological changes: misjudging jumps, excessive vocalization at night, or sudden dislike of being petted.
Older cats often have reduced kidney function even without diagnosed disease, slowing mercury elimination. If your cat drinks more water lately or has dilute urine, the supplement's cumulative effect could be stronger than the label suggests.
Why capsule form isn't automatically safer
Encapsulation concentrates the oil's components—while it prevents oxidation, it also delivers all the tuna's fat-soluble compounds at once. Some manufacturers add vitamin E to their oils as a preservative, which can interact with certain joint supplements. Check if the capsule itself contains gelatin (often fish-derived), doubling the marine-source exposure.
The timing of doses matters—giving the capsule with a meal slows absorption and may reduce mercury uptake, while empty-stomach administration maximizes nutrient (and contaminant) absorption. If your cat vomits within an hour of taking it, she may have gotten a concentrated bolus of the oil before digestion.
Reading labels for hidden tuna sources
"Marine oil" or "omega-3 complex" often means tuna unless specified otherwise—contact the manufacturer if the label is vague. Look for batch testing documentation; reputable brands provide mercury assay results upon request. Avoid supplements combining tuna oil with other high-mercury fish like swordfish or shark.
Some joint formulas use tuna oil as a carrier for other ingredients—check if glucosamine or chondroitin are suspended in it. Your cat might tolerate switching to a non-fish-based version of those same compounds, like green-lipped mussel or algae-derived supplements.
Monitoring your cat between vet visits
Weigh your cat monthly—unexplained weight loss can signal mercury toxicity before obvious symptoms appear. Use the same scale at the same time of day (post-litter box but pre-meal). Keep a dated log of any tremors, even brief ones you might dismiss as dreaming.
Ask your vet about baseline bloodwork—a one-time mercury test establishes your cat's current level before continuing the supplement long-term. Request kidney values (SDMA test) and urinalysis annually, as mercury excretion relies heavily on renal function. If her creatinine rises, discontinue the tuna oil immediately.
The full verdict, portion guidance and serving notes are collected on the Tuna (canned) food facts page.