Both horses and people can become insulin resistant. This blog looks at what it is, how it develops in people and horses, and why it's become such a big problem in the modern world.
I suspect I have two different groups of people coming to this blog, those that have an interest in horses, and those who have, or are close to someone who has Type 2 diabetes. Some of you may well be in both groups. What you'll have in common is that you may have heard of the term insulin resistance. With this in mind I'm not going to assume any knowledge of the condition in people or horses.
Insulin resistance unsurprisingly involves insulin. This is a hormone that is produced by the pancreas, which also has a role in producing the enzymes that help us digest food. The pictures below show where the pancreas is found in the human and horse digestive system - it releases digestive enzymes into the small intestine and hormones into the blood.
Insulin is vital for us to use the carbohydrate foods in our diet. Carbohydrate comes from sugary and starchy foods. Sugary foods include table sugar, honey and syrup, foods with added sugar like cakes, biscuits and sweets and natural sugar in fruit and milk. Starchy foods include cereal, bread, rice, pasta and potatoes. All carbohydrate foods are digested into simple sugars the small intestine, the most common being glucose. This is released into the blood stream causing the blood glucose levels to rise.
When it detects the blood glucose levels rising the pancreas releases insulin. The insulin is needed for glucose to get into the body's cells. We often describe insulin as the key that unlocks the door into the cell to let the glucose get in. Here it is either burnt for energy or if there's more than needed it is stored either as glycogen in the liver and muscles or turned into body fat.
Our bodies need a constant supply of glucose, even when we are inactive. The brain and heart in particular prefer to use glucose as fuel. Blood glucose levels are generally kept stable by insulin and another hormone called glucagon, that causes the liver to release some of its glucose stores if blood glucose levels start to drop.
This happens in both people and horses. But they are very different in the food the they have evolved to eat and use for fuel.
Horse and Human Diets.
People evolved to eat quite a diet quite high in starchy foods such as roots, tubors, grains, and fruit in season. We also need protein and fat in the diet from nuts, seeds, pulses, fish, eggs and meat. But this carbohydrate was much less processed than today and included tough whole grains, roots, tubers and vegetables that are hard to digest and release glucose much more slowly.
Horses evolved out on the great plains of North America where they ate grass that was much higher in fibre and lower in carbohydrate than modern pasture. Protein and fat also came from the plants the horses ate.
What both horses and people have in common is that they evolved to live on a diet very different from what they get today. They also needed to use a lot of energy either to hunt down and process their food, or walking to find new grazing. Remember this exercise; it means that any glucose released into the blood is taken up and used immediately, and so cells have evolved to need less insulin when the body is active,
So what happens when you move horse and man to modern life. Well a great deal less exercise for a start, as food is easily available either in the supermarket or provided by owners (or tiny horse slaves as we are known).
People bred new plants that grow more quickly and produce better yields, and developed ways to process food to make it tasty, high in fat, sugar and salt, and easy to eat in large amounts. There's more about this in my blog about how our environment affects what we eat
We've also improved grasses for pasture that allow the animals such as cows to gain weight quickly. You might hear people talk about sugar in grass, particularly fresh growth in spring. There is some simple sugar such as glucose and fructose, but much of the carbohydrate isn't sugar - it is fructans. These are 3 or more sugar molecules joined together - different from sugars, but much shorter than the long glucose chains that make up starch. Modern pasture grass makes a lot more sugar and fructans, and so is higher in carbohydrate than the traditional horse diet. We also feed our horses starch in the form of grains such as barley and oats, which release yet more glucose in the blood.
It's oh so easy for both species to eat more than we need and start to put on extra weight. We've also go so used to seeing both ponies and people carrying a few extra pounds that it is difficult to remember what our lean, fit ancestors looked like, with their metabolisms working perfectly to extract every scrap of nutrition from the sparse food they relied on. And the weight loss of long harsh winters with limited food, and the need to burn calories to keep warm, is a thing of the past.
People tend to use glucose as their main source of energy, although they also burn fat, particularly during extended periods of low intensity activity. Horses however evolved to eat very little carbohydrate. They eat large amounts of is fibre rich foods such as grass and hay. This contains a lot of fibres such as cellulose and which people are unable to digest. We would die pretty quickly eating grass.
Horse and Human Digestion.
In both people and horses carbohydrate, protein and fat are digested in the small intestine into the simple sugars, amino acids and fatty acids that the body can use and store. However fibre cannot be digested in the small intestine and passes into the large intestine, and this is where the horse's fabulous hind gut comes in.
At the beginning of the hind gut horses have a massive fermentation vat called the caecum, followed by an enormous large intestine. Horses, like people, cannot digest fibre, but the caecum is full of microorganisms including bacteria and yeast that ferment this fibre and into substances known as Volatile Fatty Acids (VFA). Any carbohydrates that make it through the small intestine into the caecum are also fermented into Lactic Acid. Volatile Fatty Acids and Lactic Acid are absorbed into the blood in the large intestine.
Horses do need a certain amount of glucose for the brain and heart and intense exercise, some of which comes from carbohydrate digestion in the small intestine. But they can convert some types of Volatile Fatty Acids and lactic acid into glucose in the liver, so do not need to get much glucose from their diet.
The majority of a horse's exercise in the wild is walking while grazing, powered by these Volatile Fatty Acids, with short amounts of high intensity running away from predators. In cold weather a lot of energy from fermentation is also used to keep the horse warm - they lived outside in all weathers and had to make their own heat, unlike humans with caves/houses, clothes and fire to keep warm.
Humans don't have a caecum. It has become the tiny appendix (see picture above) that's not much good for anything, so we can't digest most fibre. We do have some bacteria in our large intestine that ferment soluble fibre from things like oats and pulses to produce Volatile Fatty Acids. These are used by the cells that line our gut and seem to be important for gut health, but don't provide energy to the rest of the body. Most fatty acids in the human diet come from animal fats and plant oils which are quite different from the Volatile Fatty Acids produced by fermentation.
When things go wrong.
So let's look at what goes wrong in people first.
There are genetic differences in how well our carbohydrate metabolism works. Some people inherit a tendency to store fat around the middle, including in the liver and pancreas when they put on weight. This can lead to insulin resistance which is when your cells don't listen very well to the insulin that the pancreas produces.
Remember that the body needs insulin to get glucose from carbohydrate foods from the blood into the cells to be used for energy. The modern diet also tends towards large amounts of easily digested carbohydrates, causing lot of glucose being released into the blood at once. So more insulin is needed to counter large amounts of blood glucose combined with insulin resistance. When the person is young, it isn't a problem. The pancreas just produces more insulin and the glucose is taken up by the cells.
But as we older everything slows down a bit. Insulin is the hormone that causes fat storage when there is more fuel than is needed, and so high levels do not help with weight loss. So we might have put on more weight, and become less active making insulin resistance worse. At some point the pancreas can't keep up or starts to slow down and blood glucose begins to rise. Doctors use a clever blood test called the HbA1c to measure blood glucose levels. This looks at how much glucose is stuck to the red blood cells to tell how high it has been over the last 2-3 months. At a certain level this is called a diagnosis of Type 2 diabetes.
Short term side effects such as feeling tired, thirsty or going to the toilet more often happen if the blood glucose is very high. But many people when they get a diagnosis of Type 2 diabetes don't actually feel unwell at all, and in fact they are not. But high blood glucose, and storing weight around the middle often goes with the other characteristic of metabolic syndrome including high blood pressure and cholesterol, blood clotting more easily and more inflammation in the body.
These can cause issues longer term such as damage to the blood vessels increasing the risk of them becoming blocked. For the large blood vessels this can increase the risk of heart attacks and strokes or poor circulation to the extremities such as the feet. It also causes problems for the tiniest blood vessels in the body found in the kidneys and eyes and so can cause loss of sight or kidney failure. Finally it can damage the nerves which can lead to numbness and pain especially in the feet, which combined with poor circulation increases the risk of ulcers or even amputations.
We now know that, certainly in the early days, Type 2 diabetes is reversible. Recently there has been a big study called the DIRECT study that has shown that people who have had their diabetes less than 9 years who lose at least 15kg have a 80% chance of returning their blood glucose levels to normal without medication. It is thought to work when fat is lost from in the liver and pancreas reducing insulin resistance. However weight loss isn't always straight forwards and people can eat for many reasons other than hunger. More about this in my second blog on why we eat.
Physical activity is also very effective at reducing insulin resistance as the cells need less insulin to take up glucose when we exercise. In a younger person with metabolic syndrome this can be very effective, but older adults often have health problems that limit the activity that they can do.
People with type 2 diabetes also often reduce carbohydrate in the diet and choose less processed foods that raise blood glucose more slowly (low glycaemic index). The theory is that reducing the amount of glucose being released into the blood reduces the amount insulin the pancreas needs to produce, but low carbohydrate diets usually lead to weight loss and so may work that way as well. It's also important to eat for a healthy heart and I talk more about this on my blog on the Mediterranean Diet.
There are also many different medications used to treat Type 2 diabetes including those that reduce insulin resistance, help the pancreas to produce more insulin or the body to get rid of excess glucose.
Metabolic Syndrome in Horses.
Horses also suffer from insulin resistance and metabolic syndrome with similar causes to people, but it has some important differences.
Remember we discussed that humans are evolved to live off a fairly high carbohydrate diet, if not as high in easily digested processed carbohydrates as in modern life.
Horses on the other hand. mostly use volatile fatty acids which are produced when fibre from their food is fermented by bacteria in their hind gut. In the wild there was very little carbohydrate from the tough grasses they evolved to live on. So horses naturally produce less insulin and are prone to insulin resistance.
The modern horse lifestyle has many changes that we'd recognise from ourselves. Improved cow pasture produces more carbohydrate in the form of sugars and fructans, and we feed starch in the cereals in hard feeds. The horse feed industry, like the human food industry can make more money by "adding value" to cheap ingredients to make tasty, easily eaten foods that we love to see our horses enjoying. We also rug our horses to keep them warm so they burn less calories and they no longer walk many miles a day to find grazing.
Eating more calories than they burn leads to the obvious problem of excess calories are stored as fat increasing insulin resistance. But there is also another problem. People are designed for big meals with long gaps in between, horses just aren't. They need to trickle the food in to digest the very small amounts of carbohydrate that their natural forage contains. When too much carbohydrate is fed at once the horse small intestine cannot cope with digesting very much and it passes into the caecum where it is fermented to produce lactic acid (more on this later).
Horses, perhaps even more so than people, can have genetic differences in their metabolism. Everyone has met the native pony or cob that gets fat on fresh air, who are probably have metabolisms similar to their ancestors.
We have of course selected some breeds to be more able to sprint for longer duration such as thoroughbreds, or long medium intensity such as Arabs. These horses are fed much larger amounts of carbohydrate in bucket feeds, and burn it off with regular exercise. They may be less prone to metabolic syndrome that their hardier cousins. However there was some interesting research done a few years back that found standardbred racers can perform just as well on a forage only diet and are less prone to stomach ulcers which can be increased risk of happening with large, irregular meals.
In people waist measurement is a pretty good indicator of risk of metabolic syndrome. Those who tend to store their excess fat around their waist tend to have higher fat in their liver and pancreas which is linked to insulin resistance.
Horses do store excess weight round their middle as a grass belly, but fat stored in other places can also be a bad sign, for example on an apple shaped bum or big crest. This is why body condition scoring such as that recommended by the British Horse Society was developed to allow riders to asses how overweight their horses were. We don't know enough about how fat stored in different areas affects insulin resistance in horses.
Another difference is how much fat each species stores, Opinions differ but at healthy weight humans have around 8-19% for men and 21-33% for women. A healthy weight horse is only 5% fat - so horses get problems at much lower body fat percentages. It is worth mentioning here that in that both in people and horses there are those that aren't overweight that also have metabolic syndrome, but this is less common.
In both people and horses fat cells release inflammatory hormones (adipokines) which are linked to the complications of insulin resistance and metabolic syndrome such as damage to nerves and blood vessels.
So back to what happens to cause metabolic syndrome in horses and how is it diagnosed and treated? As with people, the insulin resistance that some breeds are prone to is made worse by lack of activity and gaining weight.
Once upon a time diabetes in people was diagnosed with an oral glucose tolerance test. In this test the blood glucose was measured first thing before they had eaten (fasting). They were then given a drink full of glucose (about 75g) and their blood glucose tested two hours later. In someone with normal insulin function blood glucose returns to normal, but with insulin resistance it stays high as the body struggles to produce enough insulin to bring it down. With people we have mostly moved onto Hba1c as described above, but in horses the OGTT is still used. I did once ask my vet about doing an HbA1c on a horse but she hadn't heard of it.
There is also another condition in horses that is often linked to metabolic syndrome particularly in older horses and that is Cushing's syndrome (PPID). This is a disorder of the hormones produced by the pituitary gland and can also be linked to insulin resistance and high blood glucose. A form of Cushing's syndrome can also occur in humans linked to Type 2 diabetes, but it is much less common and does not produce quite the same hormone imbalances as the equine version.
The treatment of metabolic syndrome in horses has a lot in common with its treatment in people. Weight loss is very important, although difficult with horses digestive system designed to graze continuously. So ideally hay from poorer quality pastures, soaked to remove as much of the water soluble sugars and fructans as possible is fed in small holed nets to slow down eating. In some ways it is easier for horses as we are able to restrict what they eat and they can't break out to go off shopping, whereas people eat for reasons other than hunger.
Exercise is also vital. Horses are designed to walk many miles a day in search of food, burning off glucose and VFAs as they enter the blood stream. This is much more difficult in the modern stable and small paddock lifestyle. Articles about the benefits of track system living, while true, just aren't always practical for people who rely on keeping their horses in livery. However, horses do seem to be much better at burning calories during and activity and regular long hacks can really help to keep down the weight. But one of the main complications of equine metabolic syndrome, laminitis, can make this very difficult.
There are many medications that are used for the treatment of diabetes in humans. One in particular, metformin, has been used for many years in both people and horses. This medication helps to reduce insulin resistance, but it's main role is stopping the liver releasing its glucose stores into the blood and making things worse. But newer diabetes drugs area also being trialled, but I suspect the large amounts needed to treat horses and the cost of research studies will make them too expensive.
Complications of Metabolic Syndrome in Horses.
As we've previously discussed, people with Type 2 diabetes can get problems with their feet. But this usually takes many years of high blood glucose to develop a combination of damage to the nerves in the foot and problems with the circulation caused by blockages of blood vessels.
Horses, however can develop a foot condition called laminitis. Laminitis is a very painful condition which is inflammation to the laminae, which are soft tissues under the hoof in horses. It can come on of very quickly and be so severe that horses have to be euthanised.
There are a number of theories for why this happens including high insulin increasing inflammation, disturbances to the blood flow, or concussion from excess weight. Body fat also produces hormones caused adipokines which cause an increase in the steroid hormone cortisol, which can increase blood glucose levels and inflammation,
Although it is linked to damage to the blood vessels and tissues, involving inflammation, oxidation and clotting, it doesn't seem to be the same thing as the long term blood vessel damage cause by high blood glucose levels in people.
There is also another cause linked to a large amount of carbohydrate being eaten. As I've previously mentioned, horses' digestive systems are really not designed for large amounts of carbohydrate at once. Rich grass, high in fructans or cereal meals high in starch, can mean that the small intestine can't cope with digesting it all. It passes into the hind gut where it is fermented by microorganisms to create a large amount of lactic acid. This lactic acid can kill many of the good bacteria, which releases toxins into the blood causing the inflammation around the body, but particularly in the feet.
What about the other human long term complications of high blood glucose? Horses are less prone to heart disease and the build up of fatty plaques that can block the arteries in humans. It is not entirely clear why this is, but could be because they do not eat large amounts of saturated fat from animal products and processed fats.
I am also not aware that horses get damage to the tiny blood vessels in the backs of the eyes and kidneys - but if we do not look for it we may not see it. It may also be that they just don't live long enough for these problems to occur; generally they can take 10-20 years in people.
Alike but different.
So insulin resistance and metabolic syndrome in people have similar mechanisms and outcomes. But with people the main concern is long term damage to blood vessels increasing the risk of heart disease, and problems with the eyes, kidneys and feet, whereas with horses it is the acute problem of laminitis.
Both sets of complication involve inflammation, problems with clotting and damage to blood vessels, but the mechanism doesn't seem to be the same.
Treating the condition in people and horses is similar; a lower energy and carbohydrate diet, weight loss and plenty of exercise. But reducing weight in horses is complicated by their need to eat near continuously, whereas humans the challenge can be their need to eat for reasons other than hunger. Both groups can also struggle to get the exercise they need.
My name is Paula Gallon and I am a HCPC registered dietitian. I have worked in the NHS for over 13 years, most recently as a Diabetes Specialist Dietitian, helping people make changes that work for them. My main area of specialisation is Diabetes including diabetes prevention, type 2 diabetes and heart health. I can help you to understand your condition and its treatment, and what changes to your diet and lifestyle will work for you. I also specialise in body positive approaches to diet and lifestyle including intuitive eating and health at every size. If you have found that restrictive diet and exercise programs have left you tired and disillusioned, I can support you to make personalised healthy lifestyle changes that work for you. I can also help you if you have lost weight and would like advice on healthy ways to regain it.
I believe changes should be practical and fit in with your lifestyle and family, and that you shouldn't have to stop doing the things that you enjoy. But you need the information to make the right choices for you, and advice should be based on the best evidence for what works. Dietitians are trained to look at the scientific evidence on diet and lifestyle and translate it into straight forward, practical advice that works. I am currently offering online video consultations in the comfort of your own home.