“Junk food”is difficult to define.White sugar is probably the nearest contender for the title.It contains plenty of calories for energy but not muchelse,and is often described as an “empty  calorie”food.Alcohol is also high incalories,but beer and wine contain some of the B vitamins and wine is a goodsource of iron,so even a teetotaler could not describe all alcohol asuseless,nutritionally speaking.Calories measure the energy we derive from the food we eat,and sugar and alcohol are sometimes described as having a high energy density. There is a limit to the amount of energy we need each day(2,000-2,200 calories is the average for women and 2,500-

3,000 for men)and ifwe eat too much sugar and alcohol there is no appetite left for the vitamin-rich foods we need—fish,meat,fruit and vegetables.Buying vitamins can bepredicted by psychological as well as nutritional motives and it is prudent to investigate why we think we need them and what benefits we expect from them before we rush off to the health shop to make our purchases. Most of us buy vitamins for one of three reasons.Either we believe that they are prophylactic,that is they will ward off advancing ill,or they are therapeutic and will deal with the  ills we have already,or finally we may believe they are wonder drugs and will lift us into a state of super health,with all its attendant delights.We are protected from some of these wild imaginings by the laws which control advertising but even without false promises we still believe that  vitamins will“do us good”.Belief is a very potent state of mind and the power of the placebo pill is never underestimated in clinical trials used to test new drugs.A placebo is a harmless substancegiven to one group of patients in the trial and it is similar in taste and appearance to a new drug which is given to a second group ofpatients.Theoretically the drug should cure or relieve any symptoms and the placebo should have no effect.Often these trials produce surprising resultsand the placebo group recovers as well as the group taking the new drug.It isa“mind over matter”philosophy and for some of us it works. Vitamin pills can sometimes fall into this category. Vitamins B and C cannot be retained in the body,so if we take more than weneed of these,they are soon excreted in the urine.The possible exception hereis the theory about the increased body “pool”of vitamin C,but even this is limited and is still largely unproven. Taking too much of the fat solublevitamins can be dangerous and vitamins A and D should never be  taken indiscriminately.Vitamin E has not been found to have any toxic effect inlarge doses,but neither do there seem to be any noticeable benefits.

[问答题]

[问答题]

[问答题]

[问答题]

[问答题]

[问答题]

[问答题]

[问答题]

[问答题]

[问答题]

[问答题]

[问答题]

[问答题]

[问答题]

[问答题]