How does pasture cause lamintis?

Fructans and starch are not responsible for most cases of laminitis, Dr Teresa Hollands told delegates at the Laminitis Awareness 2010 seminar.

Most cases of laminitis occur when horses are eating grass. Pasture associated laminitis accounts for 66% of cases occurring in the UK. But what causes the laminitis?

Experiments have shown that giving large meals of starch or fructans can cause laminitis. Large amounts of these carbohydrates suddenly arriving in the horse’s large intestine disrupt the normal population of bacteria in the gut, leading to a cascade of inflammatory and toxic events.

However, Dr Hollands, nutritionist at Dodson and Horrell, explained that this process is unlikely to be involved in the majority of cases of pasture-associated laminitis.

Firstly, grass contains little starch. Of the pasture plants commonly found in the UK, only clover has significant amounts of starch. Grasses store glucose that they can’t use straight away as fructans.

It has been shown that laminitis can be induced by giving a large bolus of fructan  (5g-12.5g fructan/kg body weight).  That’s about 3.75kg fructan for a 500kg horse.

How much fructan would a horse eat when grazing? Grass contains higher levels of fructan during the winter. Mixed pasture might contain 150g fructan/kg dry matter of grass in the winter (compared with 6.6g/kg in the summer). If a 500kg horse eats an amount of grass equivalent to 2.5% of his body weight, (12.5kg), his total intake of fructans would be about 1.9kg.

So the full daily intake falls short of the levels that have been shown to cause laminitis. And what’s more, as horses are “trickle feeders”, that fructan intake is spread out over 24 hours. So even in the winter when the fructan levels in the grass are highest, the horse is only likely to eat something like 50g fructan/hour. In the summer the figure is likely to be about 5g fructan an hour - a thousand times less than the amount needed to cause laminitis.

What’s more, recent work has shown that fructans are fermented in the small intestine, and so are even less likely to reach the hindgut in sufficient quantities to cause food-induced laminitis.

So how does grass cause laminitis? “We need to move away from thinking about individual components of the diet "  Dr Hollands suggested. “In the end it is the calories that are the main risk factor.”

“Grass provides more than enough calories for most horses in light work.”

She explained that recent work with World Horse Welfare and Napier University used alkanes to measure grass intake.  Every day, some horses ate an amount of grass equivalent to 5% of their body weight. Some individuals increased their body weight by 4% a week.

“So you can see that grass easily provides horses with excess calories,” she said “ leading to gradual accumulation of fat. Excess calories over time equals fat. Not all fat is the same.”

She explained that research had shown that some adipose (fat) tissue is metabolically active. The fat cells (adipocytes) release numerous biologically active substances (adipocytokines), which affect glucose and fat metabolism. When present in excess they can lead to insulin resistance, which in turn can result in laminitis.

“The slow insidious eating of excess grass over years is the problem; not the grass they ate today. People get insulin resistance and diabetes because they have been on a bad diet for years, not because they ate a doughnut today!”

What can you do to prevent laminitis associated with insulin resistance?

First you need to reduce the fat. Give thirty minutes exercise daily with a heart rate of 80 beasts per minute. (“Turnout in the paddock is not enough” she says). Accept that the horse or pony will lose weight (fat) over the winter.  Indeed encourage him to do so by using lightweight rugs only - so that he burns fat to keep warm.

Do not starve the horse. Maintain the bulk intake (at 2.5% of body weight) - otherwise he will be prone to developing stereotypies or gastric ulcers. But reduce the calorie intake - soak the hay for 12hours or feed oat or barley straw.

Make sure horses do not get too much grass. Consider using a muzzle; increase the number of horses (and /or cattle and sheep) on the pasture. It’s important to feed a balanced diet.  Make sure the horses receive adequate proteins, minerals and vitamins by feeding a low calorie commercial feed balancer.

The solution is to reduce obesity, ensure nutrition is optimal, and increase exercise.