Supplements.

The supplement shelf is the most optimistic place in the pet shop. Joint support. Kidney support. Gut support. Calm support. Shine support. Every tub promises to support something, which is a word chosen with great care by people who know that "treats" would be illegal and "does nothing" would not sell.

Some of it works. Some of it is well-proven to do nothing at all, and has been for years, and is still the best-selling product in its category. Some of it is a tub of dead bacteria with a misspelt label. And almost none of it has been tested on cats before it reached you, because in the UK it does not have to be.

This guide goes through the three categories owners spend the most on, joints, omega-3 and probiotics, and says plainly which ones have evidence behind them, which ones do not, and why you will probably be convinced the useless one worked anyway. That last part is the important bit, so it comes first.

Here is a study that should be printed on the side of every tub.

Fifty-nine cats with confirmed arthritis were enrolled in a trial of a popular glucosamine and chondroitin joint supplement. For the first two weeks, before anyone got the real product, every cat was given a placebo: an identical capsule with nothing in it. The owners did not know this. By the end of those two weeks, 78 percent of owners reported that their cat had improved. On nothing.

The trial then ran for six weeks with half the cats on the supplement and half still on placebo. Both groups improved, according to their owners, and there was no real difference between them. When the researchers put activity monitors on the cats' collars to measure how much they actually moved, there was no difference between the two groups at all.

This is the caregiver placebo effect and it is not stupidity. Cats hide pain, so you are already guessing at how she feels. When you start giving her a daily capsule, you watch her more closely, you handle her more, you want it to work, and you see what you are looking for. Your eyes are not a measuring instrument. For a cat, there almost is no measuring instrument, which is why every supplement that has ever been sold has a drawer full of glowing reviews and why the glowing reviews mean nothing.

Keep this in mind for the rest of the guide. Where something is described as working, it is because it worked on a collar sensor or a blood test, not because an owner said so.

Arthritis shows on X-rays in about 60 percent of cats over six and 90 percent of cats past twelve. It is the thing most supplements are bought for, so it is where the evidence matters most.

Glucosamine and chondroitin. The two most common joint ingredients on the planet, in every tub, chew and sprinkle. The theory is sound: they are the raw materials cartilage is built from, and they are absorbed when a cat eats them. The problem is that absorption is not the same as doing anything. A systematic review of 72 trials in dogs and cats found what the authors called a very marked non-effect, and concluded that these products should no longer be recommended for arthritis pain in either species. Supplying bricks does not stop the demolition. The enzymes breaking the joint down and the inflammation causing the pain carry on regardless. The 78 percent placebo study above was a glucosamine and chondroitin product. Safe, yes, and limited data suggest little effect on blood sugar, though a diabetic cat should be monitored. Useful, no.

Green-lipped mussel. A freeze-dried extract of a New Zealand shellfish, and a different proposition, because it is not one ingredient but a whole package: some of the same cartilage building blocks, plus a lipid fraction containing a group of unusual fatty acids that, in laboratory studies, block both of the main inflammation pathways at once. In a blinded, controlled trial using collar accelerometers rather than owner opinion, cats fed a diet with green-lipped mussel, fish oil and glucosamine became measurably more active while the control cats became less so. The honest caveat is that the mussel was tested alongside fish oil and glucosamine, so nobody has fully separated them. But it is a real effect on a real instrument, which is more than glucosamine has ever managed.

Marine omega-3. The one with the strongest evidence, and the subject of the next section.

What the money should go on instead. Weight, if she carries any extra; every spare hundred grams is load on a sore hip. Ramps and steps so she does not have to jump. A warm bed within reach. And the licensed painkillers your vet can prescribe, which are the only thing in this list proven to reduce pain rather than merely support it.

First, the flaxseed problem. A great many omega-3 products for pets are made from flaxseed, hemp or chia oil. These contain a short-chain omega-3 called ALA, which humans and dogs can convert, slowly and partially, into the long-chain forms the body actually uses. Cats can convert very little. The liver enzyme that does the conversion works at only a fraction of the level in other animals. Plant omega-3 passes through a cat as, essentially, expensive fat. If the label says flaxseed, linseed, hemp or chia, it is a dog product with a cat on the front.

What a cat needs is the long-chain forms ready made: EPA and DHA, from cold-water fish, algae or squid. That is the only kind that counts.

What it does. EPA and DHA work their way into the membranes of every cell, where they push out the fatty acid the body uses to manufacture inflammation. The inflammatory signals made from EPA instead are much weaker, and EPA and DHA are also the raw material for a set of molecules whose entire job is to switch inflammation off once it has done its work. The result is less pain and a system that calms down faster.

For joints, cats on fish oil move more on a collar sensor, groom their own backs again, and keep moving through winter when arthritic cats usually seize up. For kidneys, which is the bigger story, studies, mostly in dogs, show fish oil lowering the pressure inside the remaining filtering units and cutting the protein leaking into the urine, and in one study of cats with chronic kidney disease, the renal diet richest in fish oil was linked to the longest survival. It is why veterinary renal diets add it. For a cat with a kidney diagnosis, this is part of treatment, and your vet should set it.

How to buy it.

• The label must give the EPA and DHA in milligrams per ml or per capsule. A total for fish oil is not a dose; most of fish oil is not EPA or DHA.
• It must contain vitamin E, listed as mixed tocopherols, at a decent level. Fish oil without an antioxidant goes rancid, and rancid oil is a dose of toxic by-products that irritate the gut and strip her antioxidant reserves. If it smells of a harbour, bin it.
• Dark bottle, fridge after opening, pump or capsule rather than a wide-necked jar.

How much. Your vet will set the dose, because it depends on what she is being treated for and what else she takes. The figure usually quoted for arthritis is 100 to 120 mg of combined EPA and DHA per kilo of cat per day, and similar amounts are used in kidney disease. That is EPA plus DHA, not the total oil: a teaspoon of a typical fish oil holds around 1.4 g of EPA and DHA, roughly three times the arthritis dose for a 4 kg cat. For an average cat that is a few hundred milligrams a day, which is a specific quantity and not a glug. Start at a quarter to a half of the target and work up over two weeks, or the gut will object.

A probiotic is a live bacterium that is supposed to reach the gut alive and do something useful there. That sentence contains three claims, and most products fail at least two of them.

Two veterinary microbiologists bought 25 commercial pet probiotics and took them to the lab. Four did not name the bacteria they contained. Of those that did, a third spelt the name wrong. Ten gave no indication of how many live bacteria were in the tub. Of the fifteen that did, only four actually contained what they claimed. When the researchers asked how many products both correctly identified their contents and delivered the promised number of live organisms, the answer was two. Eight percent.

The reasons are not mysterious. Bacteria are alive, and alive things die: in the warehouse, in the heat of the chew-making process, which can kill many of them unless they are specially coated, and in the stomach, where the acid is strong enough to dissolve bone. A probiotic baked into a treat is, in most cases, a treat.

Strain matters, not species. Saying a product contains Lactobacillus is like saying a kennel contains dog. The useful work has been done on individual strains with codes after their names, and in cats there is really one with proper evidence: Enterococcus faecium SF68, sold by Purina as FortiFlora. In a blinded trial of 217 stressed shelter cats, it significantly shortened bouts of diarrhoea. It also reduces the gut upset that comes with a course of antibiotics, and may ease herpes flare-ups in some cats (in a small pilot study). It does not colonise; it passes through, does its job, and leaves, so it works while she is taking it and stops when she stops. That is a real, bounded, useful effect, and it is the exception.

The kidney probiotic. There is a product sold for cats with kidney disease on the theory that its bacteria eat the waste products that build up in the blood, a sort of dialysis in the bowel. It is a beautiful idea. In a small blinded trial on cats with stable kidney disease, the product sprinkled on food did no better than a fibre placebo: the blood waste markers did not move. Any money spent on it is money not spent on the phosphate binders, renal diet and fluids that do extend a kidney cat's life.

A licensed veterinary medicine in the UK has to prove it is safe and that it works before it is sold, and it goes through the Veterinary Medicines Directorate to do so. Almost no supplement is a licensed medicine, and that is a choice the manufacturers make, because licensing is expensive and the alternative is free.

The alternative is to register the product as a complementary feed. Legally it is then food: a feedstuff with a high content of certain substances, to be given alongside other food. Nobody checks whether it does what the picture on the tub implies. The only rule is that the words on the packaging cannot claim to treat, cure or prevent a disease, which is why every label says supports, maintains, promotes and helps, and never says anything you could hold it to. The VMD has a team that steps in when a label crosses the line, usually after a complaint. That is the extent of the oversight.

In the United States the situation is the same with different acronyms. There is no legal category for an animal supplement at all; it is either feed or an unapproved drug, and the vague wording keeps it on the feed side.

What exists instead is voluntary. The yellow NASC Quality Seal, from the US National Animal Supplement Council, means the manufacturer has agreed to independent audits every two years, has its products randomly tested by an outside lab against what the label says, and reports adverse reactions to a shared database. It does not mean the product works. It means what is in the tub is what it says on the tub, which, given the probiotic audit above, is not a small thing. Look for the seal, or for a stated third-party test. A product without either is asking you to take its word.

Even a harmless supplement is not free of consequences, and a few of them matter.

Calories. Fish oil is fat, and fat is nine calories a gram, more than twice what protein or carbohydrate gives. A teaspoon poured over dinner every night is around 40 calories, which for a 4 kg cat is roughly a fifth of what she should eat in a day. Six months of that is a fat cat with sore joints, which is where you came in. Measure it.

Pancreatitis. A sudden jump in dietary fat can tip a cat with a history of pancreatitis, high blood fats or liver trouble into a flare. Build up slowly, and tell the vet before you start.

Phosphorus. This one catches people out. The pill pockets, flavoured pastes and soft chews that supplements come wrapped in can be made with bone meal, yeast or organ meat, which add phosphorus. Controlling phosphorus is one of the main things that determines how long a cat with kidney disease lives, and it is the whole point of the expensive renal diet. Delivering an unproven supplement inside a high-phosphorus treat can undo the diet it is sitting next to.

Bleeding. High doses of omega-3 slightly thin the blood. On its own this is rarely a problem, but alongside the anti-inflammatory painkillers often prescribed for arthritis, it is worth the vet knowing, and it is worth stopping a week before any planned surgery.

None of these are reasons not to supplement. They are reasons the vet should know what she is getting, which is the part most people skip.

If you are going to try something, try it in a way that can tell you whether it worked. Otherwise you are the 78 percent.

1. Pick the thing you will measure before you start. Not "seems brighter". Something you can count or see. For joints: can she get onto the windowsill, does she go up the stairs, has she started grooming her own lower back again. For kidneys: the blood and urine numbers your vet runs, specifically creatinine, SDMA, phosphorus and the urine protein ratio. For the gut: a stool score, one to seven, every day. A cheap activity collar removes your opinion from the joint question entirely.
2. Record a baseline. Two weeks of the measure before anything changes.
3. Start low. A quarter to a half of the target dose, building up over a fortnight.
4. Give it 60 to 90 days. Less than that and you are measuring your own enthusiasm.
5. Set the stop rule in advance. If the measure has not moved at day 90, stop. Do not carry on because it might be helping in some way you cannot see. That is the sentence the industry runs on.

One thing at a time. If you start the fish oil, the joint chew and the new food in the same week and she improves, you have learned nothing except that something happened.

• Does the label give the actual milligrams of the active ingredient, or the strain code of the bacterium? If it says proprietary blend, put it back.
• Is there a third-party quality seal or stated independent testing?
• Does the vet know she is on it, and what else she is on?
• Have you weighed the calories and the phosphorus of whatever it comes wrapped in?
• Do you know what you are measuring, and when you will stop?

The whole problem with supplements is that nobody measures. The tub goes in the cupboard, the capsule goes in the cat, and six months later you are telling the vet she seems a bit better on it, which is exactly what 78 percent of people said about an empty capsule.

The Maleb's Tracker is the instrument. Log the supplement and the dose on the day you start. Log the thing you chose to measure every day: the windowsill, the stairs, the stool score, the mobility notes. Put the 90-day review in the calendar when you begin, not when you remember. At the end you will have a line on a chart, and the line either moved or it did not, and that is the first time in the history of the supplement industry that anyone has asked a cat rather than her owner.

She cannot tell you whether it worked. The data can. Collect it.

This guide is general information. Supplement doses, especially omega-3 for kidney disease, depend on your cat's weight, diagnosis and other medication, and should be set by your vet. Tell your vet about every supplement your cat takes, including the ones from the supermarket.