As I see it, the basics are simple: Subject to certain limits, the fewer calories you consume, the better your health and longevity. Benefits are proportional. The primary payoff is avoiding, or vastly delaying, the major killer diseases: heart, stroke, diabetes, cancer (also see: Calorie Poisoning: Civilization's Exploding Killer Disease). Other benefits include a stronger immune system, increased fitness, and a greater appreciation of food.
Over the longer term, the simplest way to judge calorie intake is weight. However, getting to the minimum, healthy weight should be done slowly - roughly no more than 2% per month for overweight people, 1% for others. In any case, preferably, weight should not fluctuate by more than about 3% (+- 1.5%), measured first thing in the morning.
The lower limit of healthy weight is most easily determined by body fat and BMI (see below).
However, lower calorie intake and low weight are not enough: Good nutrition is absolutely crucial - this is not about starving! Most nutrients should come from foods, not supplements.
There is also good evidence that increased fitness (cardio & muscle) through additional exercise are factors in longevity - even for people doing CRON successfully.
CRON is a lifestyle, not a diet - (i.e. habits you change permanently, not just a short-term weight-loss program However, as your weight gets closer to optimum, focus shifts increasingly from cutting calories to improving nutrition). This implies that you need to work out a way to enjoy doing it - it isn't something you just have to 'get through'. It also suggest that slow, gradual changes are more likely to work - new habits have to be formed. This does not mean one can't go 'cold turkey' on some sub-optimal eating habits.
Make CRON fun, a journey of discovering new tastes and habits. Don't focus on what you shouldn't do or eat, but rather on what great new foods you can bring into your life. Nurture a new appreciation of food as you become more aware of tastes, ingredients, and options. Don't beat yourself up over failures or perceived limitation in your program.
Become knowledgeable about calorie and nutritional content, health markers, and your own psychology (what you find easy or difficult, enjoyable or not, etc). Learn various 'tricks' and techniques that make CRON easier and more fun (see: Easy CRON and My CRON experience).
CRON does not have to be difficult or unpleasant, or as detractors like to say: 'a longer life, not worth living'. Quite the opposite: A sound approach to CRON will increase one's appreciation of food, and generally lead to a better quality of life. Hunger does not have to be an issue; many people on CRON actually struggle to eat 1500 calories a day - feeling too full!. Something else to remember is that it becomes easier over time, both because physically and psychologically we adjust to it, and because your relative calorie restriction actually reduces as you reach your target weight.
CRON is not an all or nothing thing. This counters the objection that severe CRON is just too difficult or impractical, or that one can't do certain sports, or look buff, or whatever. Benefits are proportional to the degree of restriction /nutrition - in fact, for most people the greatest risk reduction percentage occurs with reasonably modest CR and nutrition improvements.
CRON does not have to be a lot of trouble. You don't have to spend hours shopping for special foods and preparing them. Most CRON foods are readily available in your local supermarket, and many of them require little or no preparation. You don't have to weigh everything you eat! Just educate yourself on how many calories various portion sizes of given foods have. The more you do this, the better you'll understand the tradeoffs.
Being on CRON does not mean you can't eat out, go to parties, or eat some high calorie, low-nutrition foods. Firstly, one can usually find some healthy fare pretty much anywhere and/ or eat less. Furthermore, because CRON is a lifestyle, not a diet, there is no risk of 'falling off the wagon'.
CRON does not require super-human willpower. It requires some education, and a few conscious choices that enable you to change your habits. On of the key decisions is what foods to buy and have around the house! As simple as that.
Most important: Lots and lots of low calorie, high nutrient, and high fiber vegetables. Secondly, fruit. Then, small amounts of fish, lean meat, or other protein, like nuts, beans, soy, egg (white).
Try to eliminate beverages containing sugar - (herbal) teas are much better. Dramatically reduce processed foods high in sugar (and fat): pastries, candy bars, many 'cereals'.
Reduce fats - lots of calories, few nutrients, and some fats are rather harmful: Trans fats are bad news - minimize absolutely (anything fried, most baked goods, and most processed foods). Cut back on saturated fats (switch to nonfat dairy & to lean meats) and/ or change to monounsaturated oils (olive, nut). The only type of oil that most people do not get enough of is omega-3 fatty acid (fish, flaxseed).
Beans are good. Rice, pasta & bread are borderline - relatively high in calories and low in nutrients - eliminate, or dramatically reduce portion size. Brown/ whole wheat are best, try to avoid white/ highly processed.
Some people (especially unfit/ overweight) are overly sensitive to high glycemic foods (foods that rapidly raise blood sugar levels). For those well into a CRON program this is not usually a problem. However, eating many small meals rather than a few large ones, plus having balanced meals - with protein, (mono) fat, and fiber - seems to mitigate metabolic spikes.
Important: Discover for yourself what specific mix of good foods you enjoy, make you feel healthy, present no big hassle, and minimize hunger.
Not much research has been done, but a number of fairly simple points emerge:
If you feel weak, faint, or more tired than usual, you are probably overdoing it. Also, because (moderate) exercise is crucial to good health, you need to consume sufficient calories to be able to do & enjoy it.
Watch biomarkers such as blood/ urine analysis; strength, endurance, balance & cognition tests, and (important!) bone density. If anything deteriorates abnormally, investigate! On the other hand, various markers - such as reduced basal metabolic rate - are indications that CR is working.
If your BMI drops below 18.5, or your body fat below 5% male (8% female) you are probably close to the limit.
Important: If CR becomes difficult, or if it makes you unhappy, relax & eat a little more!
Feeling cold more easily (dress more warmly). Having difficulty sitting comfortably (sit on a cushion). Being hungry a lot while in the weight-loss phase (see: Easy CRON ). Fortunately, these symptoms tend to go away as the body adjusts.
Social pressure on looks or food discipline. Others people may discourage your CRON efforts by unhelpful comments on how skinny you are, or by trying to pressure you into eating as they do. Many people (men and women) also think that masculinity is tied to being a certain (heavy) weight. Dealing with these issues requires the confidence of knowing that CRON is a smart thing to do if you value your life.
Reduced libido. If this is a problem, reducing the severity of calorie restriction and /or conventional libido-enhancing techniques may overcome this.
Anyone embarking on serious CR should carefully monitor their health (look for worrisome symptoms, watch biomarkers), and get professional advice. Ideally you want a comprehensive baseline test of biomarkers before you start. (There are now many clinics and services that specialize in health screening).
Hating it. If you're miserable doing CRON, reduce, change, or discontinue it. Perhaps get some ideas/ advice from others happy with CRON.
For more info on CRON see: Calorie Restriction FAQ and Benefits of Calorie Restriction