These three items were the centerpiece of my early science life. I wrote my honors thesis at Williams College about arteriosclerosis which at that time was considered to be heavily influenced by dietary fat. Then after medical school I entered a wonderful time of life learning much about F, Cs, and Cs. The 1960s overflowed.
The high point of those years was those spent in the intellectual and physical embrace of two multi-million dollar research grants from the NIH. The first dealt with the control of cholesterol synthesis, and the second "The Effect of Diet on the Metabolism of Fat In Man." This second grant supported a five-bed metabolic study unit at the Lankenau Hospital in Overbrook, Pennsylvania. The study beds were staffed with full nutritional and biochemical backup as we fed specific formula diets of varying calorie amount and constituency to our volunteer experimental subjects whom we paid $10 a day for all the inconvenience and discomfort they endured for the five to 60 weeks of the particular study. These generated dozens of high impact research papers in the best journals, book chapters, and international lectures.
Along the way we spent an amazing year, 1962-3, in Munich where I did research with Professor Feodor Lynen again dealing with fatty acid synthesis. He won the Nobel Prize the next year.
Fat and carbohydrate are made of the same three elements, carbon, hydrogen, and oxygen, in specific amounts and conformations. One critical difference lies in the fact that most fats are not soluble in water whereas carbohydrates are. This simple fact accounts for their very different distribution in the body. Most fat is found in storage depots, and in the cell membranes where it facilitates transport of different metabolites.
Carbohydrates, which are in far less abundance in the body, are present in muscle and liver to provide short term energy needs.
Another important difference between carbohydrate and fat is the significant disparity in their caloric value. Fat yields nine calories for every gram that is burned, while carbohydrate yields only four. An even larger difference is noted when considering the substances as they occur in the body or in food. Since fat abhors water it yields 3,500 calories per pound of body tissue, whereas carbs yield only 400 calories per pound as the rest of the tissue weight is water.
When dieting to lose weight, it is therefore necessary to accumulate a 3,500 calorie deficit. If a person requires 2,000 calories to maintain weight and undertakes a 1,000 calorie diet it will take one week to lose two pounds (7 X 1000 calories per day. (1)
The observant dieter usually notes an initial 5- to 8-pound loss in the first week, only to note no further, or even a gain, the next week due to the artifactual fluid loss that occurs as the carbo reserves are mobilized, first. Any lo-cal, lo-carb weight loss diet results, at first with a rapid loss, however this must be repaid when carb is resumed.
This ping-pong pattern of weight change is discouraging for the earnest dieter. Mark Twain famously observed, "losing weight is easy. I've done it a thousand times."
Bottom line: It's not the type of calorie that matters in the long run, it's the amount.
Reference: 1) Bortz, W. The Predictability of Weight Loss, JAMA,1968; 204:101-105.