You may have had encounters with people who urge you to eat a "sattvic" diet, implying that foods that are "sattvic" are somehow more virtuous or inherently better for you than foods which are labeled "tamasic" or "rajasic". Perhaps as a Westerner my opinions are less rooted in the traditional teachings regarding these gunas but in my personal experience with food and with my teachers (who learned from traditional teachers) I have a different perspective.
Continuing with the definitions given in the previous post there can be no inherent virtue in eating or avoiding certain foods; in Ayurveda there is a concept called "satmya/astatmya" and as you might infer the root Sanskrit for this concept is shared with the Sanskrit word for "truth", "satya" (as well as with the word for "balance", or "sattva"). Every culture has foods and recipes passed down from ancient times that are rooted in the place in which that culture evolved. For example, peoples that are native to tropical islands tend to have diets rich in starchy roots like taro, coconuts, fruit, and seafoods. Those who lived in high mountain regions or other cold areas tended to be hunters of large fatty game and only included plant foods when they were naturally abundant in warmer seasons. Others may have been blessed with long warm seasons and a large variety of plant foods to gather and harvest, supplementing the animals foods from the hunt.
Some cultures learned to make use of the milk of certain animals (to make cheeses, yogurt, lassi, kefir, or other fermented products) while others never consumed milk or cheeses. Descendants of cultures which consumed milk products may do well or even prefer them while descendants of cultures which NEVER consumed dairy lack the enzyme necessary for digesting it, having evolved lactose intolerance. When someone consumes a food that they tolerate or are used to, it is called "satmya' while consuming a food that is not tolerated is "asatmya". The foods themselves are not inherently good or bad but if someone can't digest a food that food is not good for them no matter how healthy that food might be for someone else.
Another instance is the "yogic prohibition" of consuming "rajasic" foods like onions or garlic; that teaching considers those foods to be disturbing to the mind while many yogic traditions teach that milk from certain species of cow are highly supportive to yoga practices. However, other cultures noticed that onions and garlic are excellent for supporting the immune system (and science supports this).. My mother, an elementary school teacher, noticed that her students of Korean descent who consumed traditional fermented foods that included LOTS of garlic NEVER got sick while the White students who didn't eat that much garlic got sick every winter. Traditional Chinese Medicine makes use of may varieties of mushrooms while Ayurveda tends to think of mushrooms as being tamasic in nature, tamasic foods being minimized or avoided because of their possible effects on the mind.. Just as Ayurveda recommends that one should eat in order to balance one's constitution and current state of balance, our ancestors' experiences with food may help us find food ways that help us do the same.
`A more contemporary view of food also takes into consideration the source of the food: where was it grown (if a plant food), how was it raised/fed/slaughtered (if an animal food), how processed or refined is the final product or is it a whole natural food with minimal or no processing? Was it genetically modified, subject to spraying of pesticides or herbicides, was it harvested from poor soil or before it was fully ripe? Was the animal living in poor conditions, fed foods not natural for that species, or subject to extreme stress on its way to slaughter? Many people want to consume a meal free of "violence", ie a fully plant based diet, without considering that if their soy based meat substitute came from a monocropped field their Tofurkey came from a farm that lacked natural biodiversity, may have resulted in the killing of beneficial microbial soil communities or the deaths of beneficial insects along with the pests. Others may consume large raw veggie/fruit smoothies on a daily basis without realizing that the bloating they feel and lack of hunger is from their inability to fully digest large amounts of raw plant foods. In some cases, the antinutrients in plants (the lectins, oxalates, phytates, goitrogens) cause serious health effects by blocking absorption of needed nutrients. Cooking often removes or deactivates these antinutrents and makes the nutrients more bioavailable to the human body, the inclusion of animal fats in many traditional recipes can also help with nutrient absorption.
Ayurveda uses ghee, a form of butter made from cow's milk or yogurt that is cooked to remove the sugars and proteins int so only the fat remains. Ghee helps make turmeric, a root or rhyzome noted for its anti-inflammatory properties, more bioavailable, and black pepper or ginger are often also used with the turmeric and ghee to improve digestion. Ayurvedic texts also teach the use of animal foods for medicinal purposes (as does Traditional Chinese Medicine), meat soups and stews being very grounding and settling for "Vata" individuals who are naturally high in the "Space" and "Air" elements - for them, tamasic foods help to stabilize them. "Kapha" individuals are rich in "Earth" and "Water" elements and would find animal foods to be heavy; these people do better on plant based diets because the plants help lighten them.
One of my yoga teachers once said, after a student asked him if he had to eat a certain way in order to be a good yogi, "eat what you feel is appropriate for you". May you find the the highest quality, most deeply nourishing, and emotionally satisfying foods that balance your body AND your mind.