This has been well researched in equine studies and the science doesn't support what you believe. Wheat bran is an intestinal irritant that does not have any laxative effect in horses and does not improve intestinal motility as many once thought. If the horses already have sand abrading the intestinal lining and producing inflammation, feeding wheat bran will only worsen the inflammation, and will do nothing to clear the sand. I urge you to stop feeding wheat bran.
Metamucil is psyllium, just like the equine product called sand clear. Extensive studies showed that psyllium only clears 50% or less of the accumulated sand even when fed in high doses daily over a period of weeks, and even when combined with massive doses of mineral oil.
On the other hand, just feeding good grass hay 24/7 was shown to clear 98% of the accumulated sand, without any added psyllium or mineral oil. So, start by providing 24/7 access to grass hay.
Do anything you can to stop them from ingesting more sand. Provide ample fresh water at comfortable drinking temperature 24/7 along with the hay, and access to plenty of white salt free choice (sodium chloride) as in a white salt block.
I would supplement the hay with a good quality senior feed. I recommend Purina Senior, I've fed it for years, and know many people who feed it to their hard keeper thoroughbreds to keep weight on.
It is a well balanced complete nutritional feed that can be fed dry or as a mash, and it is well digested, absorbed and utilized pound for pound, so the body cells get the most nutrients out of it.
I would only feed as much hard feed as you deem absolutely necessary because you want the horses to keep eating the hay to keep the sand consistently moving out of the tract. it is the long stem fiber in hay that increases intestinal motility and clears the sand. I would find the best quality grass mix hay you can to entice them to eat as much of it as possible.
If your horses are losing fluids, vitamins and electrolytes through continual diarrhea, you need to consult with your vet on potential dehydration and vitamin and mineral replacement requirements.
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Add........................ The horses lose weight because the absorptive surface of the intestinal lining is inflamed or blocked by the sand, so nutrients are not being absorbed into the blood, and are just passed in the manure.
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Add 2............................... While beet pulp can be fed as part of a weight building program, it is not beneficial in cases of sand colic since it replaces long stem fiber needed to move the sand. Beet pulp does not contain long stem fiber, and since it is not a complete nutrient source like Purina Senior pellets are, it is not of benefit in your situation. I would not recommend feeding beet pulp in your situation.
Of course you should have the diagnosis of sand accumulation in the intestine confirmed by your vet to assure you are not missing some other issue that could be responsible for your horse's symptoms.