Abstract
A systematic review and meta-analysis were conducted to predict and identify ways to increase conjugated linoleic acid (CLA) formation in ruminant-derived products to treat human health issues with dietary tannins. The objective was to compare and confirm the effects of dietary tannins on CLA formation by analyzing in vitro and/or in vivo studies. We reported the results of the meta-analysis based on numerical data from 38 selected publications consisting of 3712 treatments. Generally, via multiple pathways, the CLA formation increased when dietary tannins increased. Concurrently, dietary tannins increased Δ9 desaturation and the CLA indices in milk and meat (P < 0.05 and P < 0.001, with average R2 values of 0.23 and 0.44, respectively), but they did not change the rumen fermentation characteristics, including total volatile fatty acids (mmol/L) and their acid components. In vitro observations may accurately predict in vivo results. Unfortunately, there was no relationship between in vitro observations and in vivo results (R2 < 0.10), indicating that it is difficult to predict CLA formation in vivo considering in vitro observations. According to the statistical meta-analysis results regarding animal aspects, the ranges of tannin levels required for CLA formation in vitro and in vivo were approximately 0.1–20 g/kg dry matter (DM) (P < 0.001) and 2.1–80 g/kg DM (P < 0.001), respectively. In conclusion, the in vivo method was more suitable for the direct observation of fatty acid transformation than the in vitro method.
Highlights
There is a substantial demand for ruminant-derived products, such as milk and meat, and quality, especially fat content, has become increasingly important to consumers recently [1]
A possible reason for this result was the inconsistent parameters of conjugated linoleic acid (CLA) synthesis and the types
Regarding the selected articles (Tables 1–3), sensitivity is depicted in a forest plot (Fig 2), and risk of bias is presented in a funnel plot; the risk of bias for the CLA data in all the types of studies in the present meta-analysis was not significant (P = 0.067) (Fig 3)
Summary
There is a substantial demand for ruminant-derived products, such as milk and meat, and quality, especially fat content, has become increasingly important to consumers recently [1]. Saturated fatty acids (SFAs) are present in higher concentrations than polyunsaturated fatty acids (PUFAs) in milk and meat [2]. The relationship between dietary SFAs and the risk of coronary heart diseases (CHDs) is strong; CHDs result in 2155 coronary-related deaths among 344,696 persons annually and are caused by SFA accumulation in the human body when energy intake is unbalanced, resulting in unhealthy levels of low-density lipoprotein. A meta-analysis of tannin level effects on ruminal biohydrogenation (LDL) and high-density lipoprotein (HDL) cholesterol [3]. Over the past 2 decades, many nutritionists around the world have conducted relevant studies to solve this problem and have identified a natural PUFA with conjugated linoleic acid (CLA) in milk and meat as a natural key element. Most of the researchers found that CLA has a health-promoting value in humans concerning the prevention of CHDs, abating or eliminating cancer, improving immunity and treating obesity through the development of fixed lean body mass [5,6,7]
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