Several factors influence how much energy it takes to maintain normal body temperature. Air temperature, humidity, wind velocity, solar radiation and precipitation are all factors that affect how much energy is expended to maintain normal temperature for animals that are exposed to these elements.
We know that horses use more Calories to stay warm in the winter as cold weather has been estimated to increase Digestible Energy requirement about 2.5% for each degree Centigrade below -10 C (14 degrees Fahrenheit) per the 2007 NRC Nutrient Requirements of Horses, page 10-11. The “thermal neutral zone” for horses is estimated to be from about 5 degrees C/ 40 degrees F (Lower Critical Temperature) to 25 degrees C/77 degrees F (Upper Critical Temperature).
It takes horses 21 days to adjust to a higher or lower ambient temperature, with most of the adjustment taking place in 10-14 days and more adjustment taking place over a longer time, so the actual range may depend on what the horse has acclimated to over longer periods of time.
We do not have good horse data for the impact of temperatures above the Upper Critical Temperature. If we assume even a 0.5% increase in DE for each degree C above the Upper Critical Temperature (a fairly conservative estimate based on known cold weather changes and other species information), then if the ambient temperature is 35 degrees C/95 degrees F, we would need 5% more DE at the higher temperature just to maintain body weight. As most horses do not eat more at higher temperatures and may actually consume less, the higher DE requirement for just maintenance might be expected to produce an actual weight loss just from the extra energy required to keep cool.
Higher ambient temperatures also increase daily water requirements, particularly if horses are working. The increase can easily be 50 to 100% higher, depending on the combination of factors present on a given day. If they do not get enough water, they may also lose weight.
To put it in a human perspective, I was judging horse shows this summer when it was 95 to 100 degrees F, 80+% humidity, very little breeze and bright sun. In addition to a bit of sunburn, I lost some weight and required 24-48 hours to get fully rehydrated even while drinking water all day!