Originally posted by: BaliBabyDoc
Shouldn't we decide what a healthy diet is by what condition it puts your body in?
You are on to something but the truth is typically too complex for layperson consumption. For instance, there's ABSOLUTELY nothing good about saturated fat but all saturated fats are not the same. There's a clear hiearchy where sat'd fat from avocados ain't too bad, sat'd fat from dairy sources is bad, and sat'd fat from meat sources is horrible. The simplest interpretation is that there's something else about avocados that make them OK to eat (if not actually good to eat).
Polyunsaturated fat is generally good for you but monounsaturated is actually better. Unless of course you are talking about docosahexanoic acid (DHA) and eicosapentanoic acid (EPA) these two omega-3 fatty acids are actually more vital to the human diet than monounsaturated ones. In fact, they are so important the human mammary glands concentrate them to a level 10,000x that of serum . . . breastfed babies are healthier (and happier) babies. But not all sources of omega-3 are comparable.
Even some saturated fats are good. For instance, medium chain triglycerides (MCT) are too short to be stored as fat so your body essentially burns them like carbs. Babies get a lot of this in breastmilk, too. Of course, if you have a surplus calorie diet your body will just convert excess carbs or protein into fat. The rationale behind high carb diets instead of high fat diets evolved from simple physiology. It's far easier for your body to reprocess dietary fat into adipose than convert carbs and protein into fat. So it made sense to recommend low fat diets. Unfortunately, as you tune down fat intake your body adjusts by increasing endogenous fat production and increases efficiency at reclaiming lipids and cholesterol. In addition, the body's most powerful anabolic hormone is insulin. Diets high in simple carbs create spikes in serum glucose that promote higher insulin secretion. Insulin is equal opportunity so it drives muscle and adipose synthesis while simultaneously decreasing the breakdown of fat (lipolysis) and stored carbs (glycogenolysis). The take home is that excess calories in ANY form has negative consequences unless you are a competitive athlete trying to gain muscle mass.
There's nothing inherently good about a Krispy Kreme doughnut . . . well except for the taste. But if you managed to replace the processed flour with a soy, whole wheat blend and the glaze with a sugar-substitute like sucralose . . . you've taken a 200kcal high-glycemic index snack (granted who eats just one) with no fiber, no real micronutrient content, and minimal protein . . . into a 100kcal low glycemic-index snack with several grams of protein (plus soy isoflavones) and several grams of fiber.
In sum, all foods produce the same basic pattern of physiological response in the majority of the population. That's why broad recommendations (such as the food pyramid) are useful guidelines. But clearly food preparation and consumption patterns can have dramatic effects on the ultimate outcome.