**5.1. Evidence from animals**

In last 50 years, farmers have been using low doses of antibiotics to promote growth and feed efficiency of pigs, cows, sheep, and poultry [102]. Different antibiotics have been demonstrated to have these effects independently of its class, chemical structure, and mode of action and spectrum of activity. Moreover, the effects on growth are greater when animals receive antibiotics early in life than if the exposure occurs later in life [103–105].

Also, studies in mice using multiple types of antibiotics have further confirmed this association, as well as identifying early life as the key period for microbe-mediated programing of host metabolism [106, 107].

Experiments with germ-free animal models have provided direct evidence of the key role of the microbiota in the association between low doses of antibiotics exposure and growth promotion. In 1963, Coates et al. showed that in germ-free chicken antibiotics alone have no growth promoting effects [108]. Recently, Cox et al. showed that germ-free mice who received the microbiota from mice treated with low dose penicillin gained more weight and fat mass than mice colonized with microbiota from control animals [107].

Then, there are two main findings from these experiments. First, early life is a critical time for metabolic development of the host, and second, the microbiome has a key role in this process and its disturbance duty to antibiotic exposure at this time affects the course of growth and development [109].
