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You may see ‘Ash’ or ‘Crude Ash’ labelled on pet food packaging and have wondered what this is. A common myth is that ash is an ingredient added to pet food for bulk. However, ash is not an added single ingredient but a measure of a group of nutrients, in this case minerals, the same as you would see protein, fat or fibre labelled. In fact, ash is simply a technical term for the essential minerals your pet needs including magnesium, iron, zinc, potassium, phosphorus, calcium and more.
Therefore, a pet food cannot be produced without ‘ash’ as these minerals are necessary for your pet’s survival. There are however some health conditions where the ash content of pet food has been of particular interest and we will discuss this below along with how the level of ash in a pet food can help to determine the quality of the pet food you are feeding.
In order to work out how much protein, fat, fibre, carbohydrate and minerals are in any pet food scientists will test the diets using a standard piece of equipment called a bomb calorimeter. This analyser measures how much energy in total is released by the food and by each type of nutrient when it burns, calculating the energy density Kcal/100g, total amount of protein (crude protein), total amount of fibre (crude fibre) and total fats (crude oils and fats) in the diet. What’s left at the end of the process is anything that doesn’t burn, and this is the mineral content of the diet referred to as ash or inorganic matter.
This technique is only used to determine levels of nutrients in pet food. Pet foods do not contain burnt ingredients. The term ash is used to describe the total mineral content of the food.
Seeing the word ‘ash’ on your pet food packaging should not be cause for concern.
Back in the 1970’s - 1980’s researchers made a correlation between high levels of ash (minerals) and urinary tract conditions in cats. They were specifically concerned with urinary crystals. Crystals are collections of minerals in the bladder and urinary tract. They may be tiny, like grains of sand that irritate the bladder, or they may be larger and then there is a risk of causing a blockage. Large urinary crystals (stones) can block the urethra (the tube in male cats that carries urine from the bladder through the penis) preventing urination and causing death within 24 hours if not treated.
However, it has since been discovered that urinary conditions are not as simple as this and the total ash content in pet food is not to blame, instead some of the individual minerals can be a problem. For example, Struvite crystals are a type of urinary crystal (there are several types) formed from magnesium, phosphorus and ammonium and occur when the urine has a very alkaline pH. There are several health reasons that may cause an animal to have a more alkaline pH than normal including infections, kidney disease and genetic conditions to mention a few, fundamentally however, diet plays a big role. Dogs and cats naturally have slightly acidic urine due to the protein content of their diet, diets low in meat-based protein can result in a more alkaline pH and predispose for crystal formation. However, meat when added to a diet brings with it a mineral/ash content and this can vary greatly depending on the quality, so it is about a careful balance. In addition to an alkaline pH the other factor required for crystal formation is an excess of specific minerals which might be caused by them being more concentrated in the urine through the pet not drinking enough or diets containing excessive levels of ash/minerals. So, this means that for pets without urinary problems a high-quality meat-based diet which is low in ash will maintain an optimum urinary pH (acidic) and should help to prevent crystal formation. However, for those with crystals or stone formation then specialised diets are recommended to aid treatment with reduced levels of specific minerals (rather than total ash content). Again, if we use Struvite stones as the example the recommendation is to feed a diet with a reduced magnesium, phosphorus which in order to achieve means the diet is low in protein content and has urine acidifiers added to keep the pH of the urine normal.
High mineral levels in the diet do not cause kidney disease. However, once a dog or cat has developed kidney failure controlling the dietary amount of some of the essential minerals can be extremely important in the management of this condition. The most important mineral with regards to kidney disease is phosphorus. Several studies show that feeding a diet with restricted phosphorus level to both cats and dogs with kidney failure can increase their lifespan (when compared to patients fed on a normal pet food). However, feeding a diet which is low in ash in order to reduce the overall phosphorus level is not recommended as this mean you are also restricting other minerals. For example, although some kidney patients also need a diet with reduced potassium levels, others do not and feeding a diet with reduced mineral (ash) and therefore reduced potassium levels could be detrimental to these dogs or cats.
Although many joint conditions are thought to be genetic due to the fact that certain breeds are more prone to specific conditions e.g., the German Shepherd and hip dysplasia, diet and exercise are also thought to be factors in whether dogs go on to develop skeletal problems.
It was originally thought that excessive levels of protein in puppy food caused skeletal problems such as hip and elbow dysplasia, panosteitis and Osteochondritis dissecans (OCD) in dogs however there are no recent studies that support this claim. Instead, it has been shown that larger breed puppies that eat too much food (resulting in a very fast growth rate) or those fed on a diet with high calcium or Vitamin D levels can unfortunately develop severe joint problems. This means that again, the overall ash content of the diet is not the crucial factor but the levels of individual minerals, in this case calcium and phosphorus (which is required with calcium in the correct ratio for healthy bone growth) need to be carefully controlled to prevent any disruption to healthy bone formation.
A common myth is that ash is an ingredient added to pet food for bulk.
The minerals that form the ash content of pet food come mainly from the meat and bone content of the food but also vegetable sources and additional minerals added by the manufacturer. In general, white meats such as chicken and fish naturally have a lower ash content than red meats such as beef and lamb. Most importantly, higher quality meat, especially if it is fresh, will have lower levels of ash because it contains less bone than poorer quality meat sources and dry meat meals.
High quality pet foods may also use chelated minerals. These are minerals that have been combined with other organic molecules such as amino acids. They are more expensive to produce but are thought to be more easily absorbed by the body, meaning that you don’t need to add as many of them to the food. Using chelated minerals therefore results in a pet food with a lower ash content.
Seeing the word ‘ash’ on your pet food packaging should not be cause for concern. It is an essential part of your pet’s diet and gives you an indication of the total amount of minerals in the food and can help you determine pet food quality. Unfortunately, though, for many health conditions it’s the individual mineral levels that are required and you may need to contact the manufacturer for this specific information.
Fascetti, A.J and Delaney, S.J (2012) Applied Veterinary Clinical Nutrition, Chichester: Wiley Blackwell.
Introduction to Bomb Calorimetry