Rats as a species seem to be extremely susceptible to the effects of rapid growth rate, early maturation and excessive weight gain, therefore weight management is an important issue. A rat’s propensity towards weight gain is also affected by maternal diet and early dietary influences. This is because early overfeeding permanently alters aspects of fat metabolism (making it more efficient), which often results in fatter adult rats. Some rats may also have specific genes that predispose them to obesity, usually by affecting appetite control, but these are less common. Regardless of the influences of your rats’ early kittenhood, you will also have a major effect in determining adult weight, according to how much you feed as they grow. Rats fed to their maximum growth potential, tend to grow quickly and reach maturity early, both of which are linked to reduced lifespan.
Fat deposition seems to follow the same pattern in rats as it does in humans. Some seem to primarily lay down their fat underneath their skin. These rats tend to feel soft and squishy and when they become overweight you can clearly see and feel this. Other rats may actually have very little subcutaneous fat (beneath their skin) and tend to deposit fat within their abdominal cavity, around the organs. These rats are normally big rats who still feel firm to the touch. This kind of fat deposition is actually more detrimental to health, so don’t rely on squishiness alone to decide whether your rats are overweight.
Preventing weight gain
It is much easier to prevent rats from getting overweight in the first place, than it is to get them to lose weight once obese. Prevention involves a combination of dietary consideration and activity.
Diet and prevention
The key to preventing obesity in rats is to encourage a slower path to maturity, by not over feeding the juvenile rat. This can be achieved by giving a diet full of necessary nutrients, but fewer calories. When feeding kittens at the end of their rapid growth phase (around 8 to 10 weeks) you may find that this is best achieved by limiting their dry mix - as for adult rats - though they will need roughly the same volume of food as an adult rat, to account for growth. You can then supplement the dry mix with fresh vegetables, a little extra protein, and a good vitamin and mineral mix.
Once the rapid growth phase is over (10 weeks plus), an adult diet with some calorie restriction (say from a daily evening fast period, or feeding half rations every third day) is sufficient, so long as you remember not to offer too much volume overall. The diet by this stage can be primarily grain mix, with some fresh vegetables. If you want to continue feeding more fresh food the easiest way to do this without overfeeding, is to make a meal (perhaps once a week) based on a whole grain carbohydrate food, with some added protein and vegetables, and feed this instead of their dry food. Remember when feeding fresh foods the volume should be increased, as fresh food is largely made up of water.
Scatter feeding is also extremely effective as a means of weight management. Within groups where the rats differ greatly in size and weight, it can help underweight rats to thrive, whilst at the same time ‘dieting’ those who are overweight. When using a bowl, those rats who are more driven to eat have easy access to the food and often stash, dominate the food bowl and overeat. With scatter feeding, all of the rats have an equal chance of finding food, and lean rats are often fitter and more willing to actively work to this end. Hence the weights at both extremes begin to normalise. This method of food delivery will be discussed fully later in this chapter.
One group of rats who merit special attention in terms of preventing weight gain are those who are post neuter, be that a castration or spay. These rats are generally prone to putting on weight after the operation, and will benefit from reduced rations or extended lean periods, once they have recovered from the surgery. The propensity to gain weight post-neuter is thought to be due to a reduction in metabolic rate, secondary to alterations in hormone levels. This means that most rats will gain weight following a neuter if they eat the same amount of food as they did before the surgery.
Activity and prevention
Maintaining good activity levels is an essential part of wieght management but outside fo the scope of an article on dietary management.
As well as the opportunity for activity, the environment that a rat lives in may also have an effect on weight management. Brown rats (Rattus norvegicus) are, as a species, hardy creatures who are naturally adapted to living outside, and well able to cope with colder temperatures. This was one of the advantages they had over the Black rat (Rattus rattus), who colonised the UK first, but have now been effectively eradicated from this island. While humans played a part in this, the Brown rat was certainly better adapted for over-wintering here, and our inclement weather.
Our rats thrive in cooler conditions and tolerate a cold environment much better than a hot one. Environmental temperature has a big influence on weight gain, and managing weight is easier when the rats are kept in cool surroundings. Because of their rapid metabolism, rats produce a lot of internal heat and in a cool environment this is easily lost. This results in the rat feeling comfortable, energetic and able to engage fully in activity without overheating. Even when the environmental temperature drops to the point where the rat feels cold, it can easily produce the energy needed to keep it’s temperature at a comfortable level. All of this conspires to keep the rat fit and lean.
In a warm environment the rat’s internal heat cannot easily be lost. The rat feels less comfortable, conserves energy so as not to generate more heat and as a result becomes less active. Less calories are burnt overall, both as a result of reduced activity and not needing to create energy to keep warm.
The amount of food offered needs to be adjusted to account for changes in environmental temperature, meaning we should feed less in the summer than in the winter, unless our rats are kept in a warm, heated room, in which case it may be necessary to feed less all year round to maintain normal weight.
Weight reduction diet
Despite our best efforts, some rats seem predisposed to becoming obese, and these rats have a shortened life expectancy and a less active and rich life, because their size often makes them lazy and slow. Overweight rats are more prone to many illnesses such as diabetes, bumblefoot, mammary lumps, strokes, pituitary tumours and respiratory problems. It is also more likely that their kidneys will lose function earlier and more rapidly than their slim friends and relations. Obesity cannot really be ignored as it is likely to lead to future health problems and an early death.
If despite your best efforts you realise that a number of rats in a colony are becoming overweight it is very likely that you are simply feeding too much. Cut the amount of diet right back whilst weighing the rats once a week and settle with the volume of feed that achieves a weight loss of a few gramms a week (this should be up to approximately 5g). Once you have determined how much is needed in terms of volume, stick to that amount and try scatter feeding, as weight control is much easier using this method. Make sure that they are only fed once a day (late in the evening), and that they have a lean period in the afternoon when there are only scraps in the cage. Don’t give any treats or fresh food except for a small amount of low calorie vegetables, but not peas and sweetcorn, because of their higher energy content.
If you already feed a restricted amount and are still seeing weight gain, cut amounts further, replace some dry mix with low calorie vegetables, and employ scatter feeding as your most effective weapon in the battle to keep your rats lean.
You can extend this regime further by introducing lean days (when you feed half rations), or not feeding the night before a cage clean out, so that any missed food within the bedding is eaten. On these days you can feed extra vegetables. I tend not to recommend complete fasts, as these can tip the rat into ‘starvation’ mode, so that their metabolism slows and they become even more efficient at using calories and holding onto fat.
If only one rat in a colony is getting obese, the easiest thing to do it to remove that rat from the cage at feeding time. Encourage him to exercise and give him some vegetable treats. Only put him back with the others when they have had the opportunity to consume a proportion of the food. Use this method alongside increased activity and scatter feeding. It may take weeks, or even months to make a real impact, but perseverance should eventually result in overall weight loss and will bring health benefits to your rat.
Choosing a grain mix for weight loss
Weight reduction is best achieved on a diet that is high in unrefined carbohydrates, but whatever mix you choose success relies heavily on restricting quantities. Rats are extremely efficient at processing and digesting food and can gain weight on most diets, if over fed. A mix based on straight grains and minimally processed human cereals, a low fat protein source and vegetables is ideal. You will find it easier to succeed in helping your rats to lose weight if you also choose a low fat mix and stay away from starchy, fatty or sugary fresh foods, including quantities of fruit. Scatter feeding is the one single most effective tool that I have found in the management of weight, and it is helpful regardless of which grain mix you use.
Low calorie ‘filler’ foods
Some rats, especially those with genetic factors that influence appetite, may be driven to eat almost continually. For these rats, the usual diet should still be restricted to normal levels as far as is possible without the other rats starting to lose weight. Scatter feeding will help this, however, these obesity prone rats will benefit from being offered an extra supply of low calorie foods to satisfy their constant appetite. Possible low calorie fillers include: cucumber, water melon, celery, rice cakes, plain (fat free) popped corn, broccoli, kale, red pepper, pak choi, dandelion leaves, tomatoes and frozen prawns. Also try ice cubes made from plain water or Daily Essentials, with pieces of fruit or vegetable in them.
Diet to promote weight gain
Sometimes a rat becomes undernourished, perhaps before it comes into rescue, or during a period of ill health. Many rats also lose weight as they age. When trying to increase the weight of a group of rats who can eat normally, it is best to rely to a large extent on a good quality, varied grain based mix, fed ad libitum. A daily portion of fresh, higher calorie, nutrient rich foods can be given, such as banana, avocado or nuts, as well as the usual vegetables. Where there is not a pressing reason to normalise weight, this approach is preferable to overfeeding lots of high fat, high calorie foods; weight gain will be slower, but the negative impact on long term health will be much less.
Interestingly, I have found that in a group of rats of mixed weights scatter feeding helps the underweight gain weight, just as surely as the overweight lose weight. This seems to be because the food is not readily available in a dish for the over-eater to ‘hoover up’ and the leaner rats have an equal chance of finding the food they need, before it is eaten by someone else!
In the case of sick or elderly rats a move towards a higher calorie softer diet will often help. It may also be helpful to supplement the diet with a high calorie nutrient paste, such as Nutrical, which not only boosts calorie intake but helps to stimulate appetite.
Paediatric Research November 2000 - Volume 48 - Issue 5 - Perinatal Feedings Adversely Affect Lipogenic Activities but Not Glucose Handling in Adult Rats - Balonan, Lino C.; Sheng, Hwai-ping
American Journal of Clinical Nutrition, Vol 41, 1332-1344 - Dietary habits and the prediction of life span of rats: a prospective test - MH Ross, ED Lustbader, G Bras
Physiology & Behaviour, Volume 22, Issue 3, March 1979, Pages 583-593 - Gonadal effects on food intake and adiposity: A metabolic hypothesis - George N. Wadea and Janet M. Gray
Journal of Nutrition Vol. 122 No. 3_Suppl March 1992, pp. 774-777 - Exercise and Food Restriction in Rats -John O. Holloszy.
Adapted with permission from The Scuttling Gourmet
Author: Alison Campbell
Articles relating to feeding rats.
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