Healthy food is a reasonable amount of food from different groups, with no products of chemical synthesis. Organic food tastes better to me.
Humans are omnivorous. That means that any diet excluding any type of food completely cannot make for healthy food habits. Healthy food (or, as grammar Nazis would have it, "healthful food") is something people turn to when they want to alter their perceived unhealthy manner of eating. Most people turn to healthy food because they are not happy with their weight. Healthy food is not all-grain diet,or no-carb diet, or any of that. To get thinner, a person must eat less, that's it. The problem with this is that organism tends to keep stable weight (with a window of ten to twenty pounds), and the stable weight for some people is higher than modern aesthetic standards suggest. Diets can be fun and they help to focus on getting thinner. People like to feel they do the right thing before getting any obvious results.
Modern healthy food science changes its opinions with disturbing frequency. Dietary fiber first appeared in early twentieth century as a magic cure for masturbation; now it reduces the risk of cancer. Cod liver oil used to prevent rachitis, was forgotten, and reappeared recently to reduce the appearance of autistic behavior, as well as reduce the risk of cancer again. Chocolate used to be nutritionists nightmare, but now it is healthy food. I believe there is a chance for reliable data, but for now I'd rather trust my own reason. Human diet consists largely of carbohydrates, fats and proteins. Carbohydrates and fats are used for energy immediately, carbs easier than fats, and for storage. Complex carbohydrates are polysaccharides, or starches. Sugars are disaccharides (sucrose and lactose) and monosaccharides (glucose, fructose). Same dry weight of carbs carries same amount of energy: four calories per gram, or hundred calories per ounce, be it rice, pasta, or sugar. Fats have twice more calories than carbs, and they are also processed differently. Vegetable oils are unsaturated fats and are considered quite harmless. Animal fats, including butter, can contribute to unhealthy cholesterol levels and should be taken with more caution. Recent studies suggest that reasonable amount of alcohol taken with meat makes fats in it harmless. Note that people used to drink wine with meat long before healthy food science appeared. Poisonous trans-fats are type of fats that don't occur naturally. Trans-fats are an unfortunate product of artificial hydrogenation, or hardening of liquid vegetable oils. Avoid chemically generated food, and you'll be fine. Proteins in foods are essential for building proteins of the body. Adults need lesser proportion of proteins than children, but still a considerable amount, since proteins are constantly used up and replaced.
Speaking of sources of food, people eat grains, fruit and vegetables, meat (including fish and poultry), and milk and milk products. Some adults cannot process fresh milk, which makes certain sense, and a fraction of those don't tolerate cultured milk products either. Some people don't eat meat for religious, traditional or health reasons. I believe it makes perfect sense for people who come from long generation of vegetarian culture, since their metabolism is inherently adjusted to vegetarianism. I doubt an omnivorous American will automatically benefit from refusing meat, if only because getting all necessary proteins from plants takes very careful planing of meals.
I believe that healthy food is minimally processed food. However, cooking, chewing and digesting take time and energy. Humans invented various ways to simplify these tasks. Wines, breads and cheeses were around for very long time, and all three are cultured, partially digested healthy foods. The problem with processed food is not the processing, or cooking, but the unnatural combinations of tastes and nutritious components. An obvious example are soft drinks: they are very sweet, therefore more satiating than we expect from drinks, but actual amount of sugar is concealed by sharpness of gas and acid.
Now, I consider my cooking healthy food. My recipes are not low-calorie, or intentionally low in fat. However, cooking with organic ingredients and trying to preserve the original taste makes for less fat and plenty of grains and vegetables. I noticed that I reduced or replaced butter and cream in my dessert recipes because it tasted better. Milk fat smoothes taste, but most desserts are consumed with tea, coffee or milk to wash them down, and these desserts are fine with a bright, strong taste.
See also: kitchen, ricotta, bread flour.