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For generations, we’ve agonised over how best to meet horses’ energy requirements. But what is energy, how does it provide fuel and is it the same as calories?

Although the term ‘energy’ is often associated with performance or behaviour and ‘calories’ with weight, they are exactly the same thing. Unfortunately, this means it’s impossible to find a feed that is high in energy but low calories or vice versa! Put simply, calories are a measurement of energy. In human nutrition, energy is measured in kilocalories, hence the commonly adopted term ‘calories’. In the UK, energy in equine diets is measured in megajoules (MJ) of digestible energy (DE) per kilogram (kg). One MJ of digestible energy equals 239 kilocalories — about one bag of peanut M&M’s!

What is energy?

Energy is not a nutrient itself, but is derived from the nutrients in food. In physics we’re taught that ‘energy can’t be created or destroyed, only changed from one form to another’. In the same way that energy derived from coal or oil must be converted to electricity before it can power your television, chemical energy in food must be converted to a form that can be utilised by the body (ATP — adenosine triphosphate).


Sources of energy

The main energy sources in the horse’s diet are fibre, oil, sugar and starch (found largely in cereals).


Protein is not a primary energy source and is generally only utilised as a last resort such as in cases of illness, exhaustion or starvation in which case lean tissue is broken down to try and meet demand. Excess protein is broken down and excreted in the urine. It will not cause excitability or increase the risk of clinical conditions such as laminitis or tying up.

From food to fuel

Muscles require ATP to contract and produce movement. As muscles only contain enough stored ATP to last a few seconds, they must continually regenerate ATP.

  • Starch and sugar is broken down into glucose in the small intestine and absorbed into the blood stream. If not used or converted to ATP immediately, glucose is stored as glycogen (a long chain of glucose molecules) in the muscle (90%) or liver (10%) for use later. Glucose and glycogen are converted to ATP via a series of complex chemical reactions or ‘energy pathways’ — either with oxygen (aerobic metabolism) or without oxygen (anaerobic metabolism). Aerobic metabolism is slower but more efficient and is predominately used for low-intensity exercise, while anaerobic metabolism is faster (but also less efficient) and used largely for short bursts of high-intensity exercise or rapid acceleration. However, these energy pathways are used in combination with each other depending on the type of exercise and the fuel stores available. So, at any one time, some muscle fibres will be working aerobically and some anaerobically.
  • Fats are broken down into fatty acids in the small intestine, absorbed into the bloodstream and then stored in adipose (fat) tissue, muscle or the liver. Fat can only be converted to ATP aerobically and while this process is very slow, it is also highly efficient and yields more ATP than glucose or glycogen. If your horse is overweight, walking and trotting for long periods will burn fat!
  • Microbial fermentation of fibre in the large intestine produces volatile fatty acids, namely acetate, propionate and butyrate, which are absorbed into the bloodstream. Acetate can be used immediately by several body cells or used in fat synthesis. Butyrate is converted to acetate and propionate is converted to glucose in the liver.

Meeting energy requirements

The easiest way to assess whether any horse or pony is consuming the right amount of energy for their workload is to look at their body condition. Those maintaining a healthy weight/body condition are consuming the right amount of energy, while underweight horses are not consuming enough and overweight horses are consuming too much — regardless of their ‘ridden energy’ levels or performance! Horses therefore need to be in ‘negative energy balance’ (consume less energy than they burn) to lose weight and in ‘positive energy balance’ (consume more energy than they burn) to gain it.


Slow versus quick release energy

Fibre and oil are broken down and absorbed relatively slowly. Consequently, they are often referred to as ‘slow release’ energy sources. Sugars and starches are absorbed into the bloodstream more quickly and are often referred to as ‘quick release’ energy sources.

Energy and behaviour

In general, diet can negatively affect behaviour in one of two ways: by supplying excess energy or by providing high levels of starch (and sometimes sugar), which is the most common cause of diet related excitability. While high levels of sugar in grass may sometimes lead to exuberant behaviour, compound feeds, fibres and balancers contribute very little sugar to the total diet, even if they contain molasses.


A lack of ridden energy is often mistaken for a lack of energy in the diet when, in truth, horses in good or overweight condition are already consuming enough or too much energy for their workload.

Other factors that may influence energy levels include:

  • Temperament
  • Fitness — poor stamina or running out of energy is often a reflection of the horse’s fitness and/or body condition
  • Weather
  • Excess weight gain or obesity
  • Type of work

In the case of lethargy which is out of character, seek veterinary advice to rule out an underlying clinical issue.

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