The U.S.Food and Drug Administration concluded in late October that meat and milk from cloned animals are safe to eat, exciting farmers who are developing clones of cows and the creatures. But almost immediately, one of the FDA’s advisory committees sharply questioned the conclusion and the science behind it.
In evaluating food from cloned animals, the agency had two approaches in October. One approach compared clones at different stages of life to healthy common animals to decide whether they appear the same. The second and more vigorous approach could involve tests on the meat and milk of clones.
When the FDA concluded that cloned animals appeared to be safe to eat, some people criticized that the agency seemed to rely almost exclusively on the first approach.
“We evaluate many things for safety. We can identify what the danger might be.” said Stephen Sundlof, director of the FDA’s Center for Veterinary Medicine. “When you’re talking about cloning, it’s very different because we have not been able to identify a particular danger.”
Other people said, “We don’t know if these animals are healthy. What if they appear healthy, and there’s some metabolic difference caused by genetic defect? They don’t have those studies.”
The key to weight loss is regular physical activity. And surprisingly, you don’t have to give up eating or make the gym your second home to see long-term, positive effects.
Your body needs a certain amount of energy to maintain basic functions such as breathing, blood circulation and digestion. The energy required to keep your organs functioning is referred to as the resting or basal metabolic rate.
Any time you are active, additional energy is required. It is obtained from glycogen (肝糖) (carbohydrates) and fat stored in the blood, liver, and muscles. The key to losing weight is to draw on the fat rather than on the carbohydrate reserves.
Which of the two energy sources you use depends on the intensity and duration of your activity. The higher the intensity, the more your body will pull from the stored carbohydrates. The lower the intensity, the more your body will rely on fat as its fuel.
Aerobic exercise (any activity that increases oxygen intake and heart rate) is most effective for weight loss. When you perform aerobic activities you repeatedly contract large muscle group such as your legs and arms. Walking, running, rollerblading, swimming, dancing, and jumping jacks are all forms of aerobic activity.
Surprisingly, if your aerobic activity is low to moderately intense and of long duration, you will burn more fat then if you had participated in a short burst of high-intensity exercise. In short, a brisk 30-minute walk will burn fat while a 100-yard sprint(疾跑) will burn glycogen.
