DEAR DR. BLONZ: How long does it take calories in the foods we have eaten to become body fat? I have a physically demanding job, and I tend to gain weight during holidays when I am less active and tend to overeat. This seems to disappear once I return to work, along with better eating and activity. I am wondering at what point the excess fat becomes "permanent." -- D.M., San Jose, California
DEAR D.M.: Short-term body weight changes can be misleading and should not be assumed to parallel gains or losses in body fat. First, some background: Energy is the most essential commodity of life, and the body conserves and stores as much as it can, as body fat. While essential for survival as a species, this trait has an obvious downside if unchecked.
The calorie, a measure of heat, is the unit we use for the energy value of foods. Carbohydrates and protein provide about four calories per gram, while fats provide nine calories per gram. You mention holidays, so we must mention alcohol, which provides seven calories per gram. (For reference, there are about 14 grams of alcohol in a standard drink: 12 ounces of regular beer or five ounces of wine.) Like other animals, humans are designed to be mobile, so the body turns virtually all excess energy into fat, the most concentrated form.
One of my favorite examples is the plant world, where the priority is growth, not mobility. Plants evolved storing their energy as carbohydrates to get the most growth from their calories, helping them grow fast and spread their leaves to begin collecting energy from the sun. The exception for plants is their seeds, where mobility is again the priority. It explains why we find fat in seeds; this concentrated energy makes them light enough to be carried in the wind or by animals to a new destination with sufficient energy to fuel the growth of the new plant.
Your question is about immediate weight changes. There can be transient fluctuations in body weight due to shifts in body water, which can be affected by what you've been eating and drinking. Then there are unabsorbed materials in the queue for elimination, reflecting meal size and composition. Please enjoy your holidays, and don't fret short-term variances in numbers on the scale. Within a week or so, you'll see if things are there to stay.
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