Before industrialization, people used to have their main meal earlier in the day, rather than as the last meal - this was probably the practice in most cultures and places. People everywhere were more tuned into the cycles of nature that we’re a part of. In Ayurveda, this is framed as the digestive 'fire' mirroring the peak of the sun in the day. So you might think that digestion is stronger in the summer with the more intense sun, but actually the opposite is true.
It's the digestive fire that generates a bigger appetite in winter. It’s stronger when there's more need for warmth in the body’s core and to help digest heavier, richer foods that add a warm layer (which nature helps to shed in spring with light, cleansing greens).
In the summer, the heat moves to the surface and out through sweat which cools us down, so the digestive fire is weaker. The complicated thing is that we're naturally going to want coolness, but too much coolness in our gut further dampens the digestive fire. And that could result in congestion in the gut or lungs, which can lead to summer colds or to greater susceptibility to colds a few months later. Inadequate digestion of food results in gunk, known as ama in Ayurveda, which is the starting point for discomfort, and if it builds up over time, for more serious disease.
Even plain, room temperature water dampens digestive fire, which is why people who are trying to manage their appetite better may be advised to drink water if they feel hungry between meals. Many fruits are also energetically cooling even if they are not chilled, as are salads - things people are drawn to in warmer months.
This is why in India people often add some kind of warming spice, like black salt or cumin, to fruit or water in summer. The Indian version of fruit salad has salt and black pepper at minimum, and often has some cumin and red chilli also. Raw food - with the exception of ripe fruit and very light greens - is harder on the digestive system for many people, so that's another thing to be wary of. Balancing energetically cool things with warming spices, or bean or roasted veg salads are good ideas, as is limiting the amount of chilled food and drink - but fine to have sometimes!