A variety of positional and geometrical isomers of linoleic acid are included in the general term conjugated linoleic acids (CLAs). Rumen bacteria produce CLAs [1, 2] from dietary linoleic acid, and as a result, red meats and dairy products are the main sources of cis-9, trans-11 CLA in the human diet. In foods, CLAs are in the triglyceride form.
Experiments in which humans were fed CLAs have reported loss of body weight , reductions of % body fat and sagittal abdominal diameter [3–5], and a positive impact on some coronary artery disease risk factors . Various mechanisms have been presented to explain the mode of action of CLAs, either in animals or humans [4–9].
The gastrointestinal tract is inhabited by a large and diverse microbiota [10, 11]. This microbial population is relatively stable, but changes occur due to age, disease status, use of medications such as antibiotics, and diet [12–14]. It has been speculated that changes to the intestinal microbiota could explain alterations in lipid metabolism [15, 16]. In vitro experiments have shown that CLAs can inhibit growth of some bacteria and alter bacteria membrane lipid composition . However, very few studies have been published in which the effects of the lipid component of the diet on the gastrointestinal (GI) tract microbiota have been reported [18–20].
It is believed that the microbiota that inhabits the GI tract influences a wide variety of digestive, metabolic, and immune functions . Carman et al.  pointed out that changes in the intestinal microflora population may not be easy to achieve, but in any case, it is changes in 'microflora associated characteristics' (MACs) that are better indicators of effects of diet on the host. A change in enzymatic activity may be one MAC that has long term implications on health. The activities of various fecal enzymes have been reported to be influenced by dietary fat , carbohydrates [23–27], and consumed bacteria [25, 28, 29].
β-glucuronidase activity in feces comes from Bacteriodes and other bacteria. β-glucuronidase hydrolyzes a variety of glucuronides, liberating carcinogenic aglycones. Fecal β-glucosidase activity also comes mainly from Bacteriodes, but many streptococci and lactobacilli also have high β-glucosidase activity. β-glucosidase is responsible for the hydrolysis of plant β-glycosides, releasing into the intestinal lumen aglycones which are mutagenic and carcinogenic [30–32]. Nitroreductase enzyme acts on aromatic nitro-compounds resulting in the formation of harmful amines . Urease enzyme can act on urea releasing ammonia and carbon dioxide; high urease activities have been found in some Eubacteria and Peptococcus bacteria . Ammonia has been shown to promote the growth of tumors in the colon; it facilitates the growth of pathogenic bacteria and contributes to mucosal tissue damage . Decreases in fecal β-glucuronidase, β-glucosidase, nitroreductase and urease activities are thus considered desirable because of their links to the production of carcinogens .
Goldin & Gorbach  reported that fecal β-glucuronidase, nitroreductase and azoreductase enzyme activities did not change when human subjects consumed 500 ml of low fat milk per day for 30 days. Conversely, in one Yakult feeding trial reported by Tanaka , β-glucuronidase enzyme activity decreased in 4 of 10 control subjects, β-glucosidase enzyme activity decreased in 3 of 10 control subjects, and reductase enzyme activity decreased in 3 of 10 control subjects consuming unfermented milk (240 ml/day) compared to pre-experiment values. In a second trail, the reductions of β-glucuronidase and β-glucosidase were statistically significant in subjects consuming unfermented milk (300 ml/day). Feeding lactose (20 g/day or 40/day) did not affect fecal β-glucosidase or β-glucuronidase activities in elderly subjects .
Lactobacilli along with bifidobacteria have received much attention as ingredients in probiotic products [39–41]. Feeding milk containing lactobacilli has been shown to successfully increase lactobacilli numbers [42, 43]. However, feeding just lactose 20 g/day or 40 g/day has been shown to reduce significantly lactobacilli numbers .
It is apparent that CLAs are potent bioactive lipid ingredients in many foods. As CLAs pass down through the GI tract, they may be bringing about changes to the intestinal microbiota, which may in turn be contributing to their whole body effects.
This study was undertaken to determine whether the consumption of CLAs effected the intestinal microbiota population composition and function. Fecal samples were collected from subjects who had consumed different forms and amounts of CLA to determine whether the consumption of the experimental milks affected the numbers of various fecal bacteria, fecal enzyme activity or fecal composition. Data presented here were obtained during a larger experiment in which the effects of CLA consumption on lipid metabolism and body composition were studied.