* The bare minimum is about 1200 calories. Under that, you can't balance types of foods to get proper nutrition. Special suppliments are required.
* 1800 - 2000 seems to be low-activity caloric needs. With this, you still should specially plan out meals in order to get proper nutrition.
* 2300-2600 seems to be for a moderately active person.
* 2800 - 3600 is for someone trying to bulk up and who exercises regularly and substantially.
* Up to 6k for hard training in competitive sports, active military, etc.
All of these numbers are for "average". Each individual will vary substantially. Assuming the wrong number leads to poor nutrition or obesity.
There may be specific dietary adjustments for diabetics, such as being more careful about too much protein, etc.
Regardless of the purpose, everything I've found for all different levels of activity, diet should generally be around 60% carbs, 10-20% protein, and 20-30% fat. Minimum protein is .45g per kg of ideal body mass.
WHO and FDA both have info, as well as many exercise, nutrition, health, and even most of the "buy our awesome product" sites tend to stick to these numbers.
Most health sites say that 4-5 meals 3-4 hours apart is best. They don't necessarily say why.
It takes 600 grams (2400 calories) of carbohydrates to fully replenish glycogen reserves of a 160 pound man in good health. If recovering from hard exercise, then ideally, this replenishment occurs over 2-4 days. Flooding the body with glucose faster than it can be coverted to glycogen (glycogenesis) causes an increase in the amout converted to fat.
Basically, your muscles run off of ATP (Adenosine TriPhosphate). Each muscle cell has enough reserves for one hard pull, or 2-3 quick pulls.
Once that's depleted, Creatine Phosphate is used to recharge ADP into ATP. There is enough CP in each muscle cell for about 2 seconds of exertion.
After that, anerobic glycolysis converts each glucose into 2 lactate and 2 ATP. There is enough glucose in the cell to last about 2 minutes.
After that, assuming your blood is circulating and oxygenating your tissues, your liver and muscle glycogen reserves are used. Glycogen is quickly converted back into glucose. Aerobic glucose metabolysis recharges 36 ATP per molecule. Glycogen reserves generally last about 2 hours.
After that, fatty acids and aminos are converted. Fatty acid metabolysis produces 130 ATP per molecule, but each molecule is much larger. The overall process is much slower, and availability of free fatty acids is lower. Aminos can be metabolized to recharge 15 ATPs.
Fatty acids and aminos are basically repair parts for cells. Some comes from diet and some comes from apoptosis of older or damaged cells.
I've read that alcohol reduces or prevents glycogenesis.
RESULT: Running constantly with too little glucose/glycogen available decreases the body's availability of repair parts, as well as decreasing stamina.
Other than muscles, your brain is the primary consumer. Although the brain represents only 2% of the body weight, it receives 15% of the cardiac output, 20% of total body oxygen consumption, and 25% of total body glucose utilization. With a global blood flow of 57 ml/100 g·min, the brain extracts approximately 50% of oxygen and 10% of glucose from the arterial blood.
Your brain only runs off of glucose. Lactose and pyruvate could be used at lower efficiency if they crossed the blood-brain barrier sufficiently. Mannose could be used, but isn't consumed or produced normally inside the body.
RESULT: Running at low levels of glucose can impair the availability of fuel required for the brain to function.
CALORIES/GRAM Some fun numbers I've found online:
Fat: 1 gram = 9 calories
Protein: 1 gram = 4 calories
Carbohydrates: 1 gram = 4 calories
Alcohol: 1 gram = 7 calories
FUEL SOURCESThis page tells the order and sequence of where that fuel comes from:
It doesn't tell how much slower fat/protein metabolysis is than from carbs. Nor does it tell that aminos and fats for metabolysis generally come from natural cellular breakdown (Apoptosis).
True starvation robs cells of repair parts. Eventually, they fail and undergo apoptosis prematurely. The body doesn't generally directly attack/digest otherwise healthy cell.
Apoptosis in JWK's Biology Pages
Apoptosis in Wikipedia