The truth about cats and dogs

According to the RSPCA, 35% of Aussie cats and 40% of dogs are overweight or obese, with figures for pets in the UK and US similarly high.

Apart from overall lower quality and shorter life, there are specific conditions associated with obesity, including arthritis, heart and respiratory conditions, diabetes, irritability and certain cancers. Pets become less willing to play and exercise, reducing the wellbeing that comes from these interactions with humans.

While there are many reasons why obesity is becoming more prevalent in pets, it basically comes down to energy intake exceeding energy expenditure. Factors on the food side of this equation are more high-energy treats and overfeeding of healthy food. And pets, like humans, now have more sedentary lifestyles. Neutered pets are more prone to weight gain due to hormones and behavioural changes.

Signs your pet may be overweight

• Your pet doesn't have a 'waist' when viewed from above.
• There's no abdominal 'tuck' when seen from the side.
• There's more than a thin layer of fat over its ribs.

Helping your pet lose weight

If your pet is over its ideal weight, a 20% reduction in calories is considered a good starting point for weight loss – though regular monitoring is needed. A more precise figure involves calculating your pet's resting energy requirement (RER), based on its ideal weight. To calculate RER, use the equation RER = 70 x (ideal body weight in kg)^0.75. As a guide, to lose weight, a dog should have an energy intake equivalent to its RER; a cat, 80% of its RER.

It's important that your pet gets enough protein – a rule of thumb for dogs is 2.5g of protein per 1kg of ideal body weight. For cats, it's 5g/kg the ideal body weight.  This is particularly the case when a pet is on a reduced energy diet – protein helps your pet maintain muscle mass while losing body fat, and make your pet feel less hungry.

Rather than give your pet less of their regular food, formulated weight-loss foods may be a better option. Well-formulated ones can compensate for smaller servings by increasing the proportion of protein and other nutrients to ensure your pet still gets what they need. They may also have a lower energy density – by including more fibre, for example – so your pet is eating only a little less in terms of volume, but getting a greater reduction in energy.

We looked at weight-control or weight-loss varieties of popular supermarket and vet-supplied foods for dogs and cats, and compared the serving size and calorie content against their regular counterparts. For cats we also included 'indoor' foods, as these are often lower in calories, gram for gram, than regular food.

A note on calories

In animal physiology studies, calories, rather than kilojoules are used. And since many multinational companies use calories in their nutritional information (to cater for the US market, where calorie information is required on labelling), we will use calories rather than kilojoules. You may also see the term metabolisable energy used in relation to pet food. This takes into account the energy losses from food in faeces (due incomplete digestion of food), urine and gases (byproducts of the digestion process).

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What we found: dog food

A 10kg typical neutered adult pet dog needs about 630 calories (though working dogs and intact dogs need more). An overweight dog that's on a weight-loss diet to achieve an ideal weight of 10kg needs approximately 390 calories per day.

The total daily calories in the pet food we analysed, based on recommended serving size for a 10kg dog, ranged from 342 to 735 for regular food. Most came in at less than the recommended 630 calories, meaning it wouldn't be a disaster if you inadvertently overfed your pet a little, or gave them a treat on top. However, some were so low in energy that a nutritious top-up would be recommended for most dogs.  

For weight loss food the range was 358 to 550 calories, suggesting that the weight loss could be quite slow when eating some brands at the upper end of the scale.

Even within brands there were some anomalies in the calorie content of diet vs regular foods. My Dog Lite with Chicken, Brown Rice and Spinach, for example, had more calories than some of its regular chicken-based counterparts when served as recommended for a 10kg dog. And the Nutro Natural Choice Lite Lamb & Rice had a larger serving size than the regular Nutro Natural Choice Lamb & Rice, so despite having fewer calories per 100g, the larger serve meant total daily calories were greater when served as directed.

As these anomalies and the generally broad range of figures demonstrate, relying on feeding guidelines may not ensure your dog's energy intake is ideal. And the actual amount of energy your dog needs depends on many individual factors, including breed, activity levels, age, individual metabolism, skin and coat thickness, and living conditions (whether they're an indoor or outdoor dog, for example).

However, the guidelines are at least a starting point – which some manufacturers also point out – and you can work up or down from there after observing whether your pet is keeping a healthy weight, or losing weight if that is what's needed.

In terms of protein, we found many of the 'lite' foods had the same or less protein than their regular counterparts, though some had more. However, in most cases the protein needs for a 10kg dog would still be met if fed as recommended.

 * For a 10kg adult dog when fed as directed. When the recommended serving size is provided as a range, the calculations correspond to this range. Weight control products are italicised.

* For a 10kg adult dog when fed as directed. When the recommended serving size is provided as a range, the calculations correspond to this range. Weight control products are italicised.

What we found: cat food

A typical 4kg neutered adult cat needs around 240 calories per day (intact cats need more, as do cats that are very active outdoors). If it has to lose weight to achieve 4kg, it needs 160 calories per day. The recommended serving sizes of most of the regular cat foods we looked at came in at under 240 calories. The weight-management foods were above 160 calories, but generally under 200 calories, which is the RER of the typical 4kg cat, and could be expected to result in weight loss.

However, most cat foods don't contain the recommended protein level when fed as directed. This was true for both diet and regular foods – though they did meet the voluntary Australian standard. A 4kg cat needs about 20g or more of protein per day. We found levels ranging from 14g to 33g, with the higher levels found in wet food.

Vet Julie Summerfield recommends wet food for cats that need to lose weight. She consults for manufacturer Safcol, which produces only wet food, but she argues that wet food is better than dry food for cats because it's higher in protein from animal sources, whereas the protein in dry food also comes from cereals and other plant sources. Cats (unlike dogs) lack certain enzymes required to metabolise non-animal sources of protein effectively and if the diet is restricted in calories, the cat may not get enough useable protein to maintain muscle. Another advantage of wet food is the water content – studies have shown that cats given dry food and access to water don't get as much total water as cats given wet food.

 * For a 4kg adult cat when fed as directed. When the recommended serving size is provided as a range, the calculations correspond to this range. Weight control products are italicised.

* For a 4kg adult cat when fed as directed. When the recommended serving size is provided as a range, the calculations correspond to this range. Weight control products are italicised.

Getting started on weight loss

• Consult your vet to find out your pet's current weight and ideal weight, and a strategy to achieve the latter.
• Get the whole family (and possibly neighbours, if your pet is sociable) on board – there's no point if the main feeder is carefully monitoring meals while others are giving treats throughout the day!
• Make sure you're feeding your pet the correct amount – don't just guess. It's a good idea to weigh dry food, as a measuring cup may not be accurate, especially for small amounts of food.
• Just as for humans, exercise is an important aspect of weight loss and control.

Pet treats

We give our pets treats for all sorts of reasons: as training rewards, to keep them entertained when they're home alone, or just because making our pets happy makes us happy. If those aren't reasons enough, some manufacturers have turned treats into 'functional' foods, offering added benefits such as dental care (tartar removal), fresh breath, joint care, hairball control, skin and coat health, and urinary tract health.

And the treats market is booming. Supermarket sales alone were worth over $200 million for dog treats in 2014, up more than 8% on 2013 expenditure, and cat treats almost $4 million (up $19%).

But this boon for manufacturers is a mixed blessing for our pets.

Obviously the occasional treat won't do much harm, but some pets are getting regular treats throughout the day. And according to the US-based Association for Pet Obesity Prevention, the calories in pet treats are a major factor in the rise of pet obesity.

Many are highly processed – this isn't in itself a bad thing, but some contain large amounts of added sugar, fat, salt and colourings. Even natural products, such as pigs ears and bone marrow, can be high in fat. 

It's recommended that treats contribute no more than 10% of a pet's daily energy requirements. This is based not only on controlling your pet's weight, but also on the nature of treats – they're not a complete food, so a diet consisting of more treats and less proper food may leave them short of essential nutrients. Some treats, such as rawhide, may also be dangerous in large quantities.

Packaging recommendations like "Treats should not contribute more than 10% to your pet's diet" may serve as a helpful reminder, but there are two problems: there's often no calorie information provided on packaging or websites, and most people have no idea how many calories their pet should be eating each day. So it's fair to say there's quite a bit of guess work involved!

Some manufacturers will provide a feeding guide based on the weight of your pet and the number of the treats they can be given each week – so a 10kg dog can be given 'up to seven' Schmackos Twirls per week, for example. At about 50 calories each, that's less than 10% of their approximately 600 calories recommended intake per day.

However, if this is in addition to your dog's regular food, they could well gain weight over time. If they also get their joint care cookie, dental care stick and fresh breath chew, they've added even more energy to their daily intake.

Many treats we looked at were more than 10% of the daily recommended intake for a 10kg dog, so couldn't be given daily. A Schmackos Everlasterz, for example, contains 295 calories – almost one half of our 10kg dog's daily requirements, and more than 10% of the recommended intake for most dogs. It's recommended to be given only once a week, but you also need to be mindful of treats given on other days.

Be treatwise

• Find out how many calories your pet should be eating per day, and how many calories are in their usual treats.
• Don't give human treats, such as bacon, hot dogs, sweet biscuits and so on – they're very energy-dense. Purina has some printable posters demonstrating ice-cream equivalents (for humans) of human treats when given to dogs.
• Keep tabs on how many treats your pet is eating throughout the day.
• After major treats, such as a meaty bone, reduce the amount of food you'd normally give them at meals.
• Consider keeping aside some food from their meal to give out as treats for training, or for filling a food-entertainment toy.