Meal replacement shake mix powders and beverages have become increasingly popular among health-conscious individuals who consider meal replacement shakes an integral part of a healthy diet. Moreover, the growing emphasis of muscular and fit male and female images in the media has greatly boosted the use of muscle-enhancing or weight-loss shakes and beverages among adults and teens alike. In fact, a U.S. study that surveyed 2,793 middle school and high school students to determine their muscle-enhancing behaviors found regular use of protein powders or shakes among 35% of the teens surveyed (Eisenberg, Wall, & Neumark-Sztainer, 2012). Protein, mainly in the form of whey, rice, and pea, is the most common ingredient in commercially available shake mix powders. A number of "whole food" botanical shake mix powders and elixirs also have become popular that contain fruit and/ or vegetable extracts, vitamins, minerals, and various other ingredients.
Shake mixes are manufactured by multiple brand names and are readily available at retail stores, especially those that specialize in sales of organic and genetically modified organism-free foods and nutritional supplements. In contrast to over-the-counter drugs, the quality of dietary supplements, including shake mixes, is largely unregulated. With the large variety of ingredients of unknown origin, quality, and processing methods used, there is a potential risk of contamination by harmful and toxic elements such as heavy metals that could have adverse health effects for consumers of these products. Heavy metals are not readily metabolized and excreted; therefore, they can bioaccumulate over time, which poses additional health risks through repeated exposures.
According to a Consumer Reports study (2010), 20% of 15 protein powders/drinks that were tested for heavy metal contamination exceeded the maximum allowable limits for arsenic (As), cadmium (Cd), and lead (Pb) set forth by the nonprofit U.S. Pharmacopeia (USP) for three daily servings (USP, 2015). USP is a federally recognized authority that sets voluntary standards for the identity, strength, quality, and purity of medicines, food ingredients, and dietary supplements. Arsenic and Cd are classified as Group 1 carcinogens by the International Agency for Research on Cancer (IARC) (World Health Organization [WHO], 2012), and Pb is a known neurotoxicant in humans, particularly in children (Sindhu & Sutherling, 2015). Thus, the findings of potentially harmful heavy metals in shake mix products warrant further studies.
Though not included in the Consumer Reports 2010 analysis, hexavalent chromium [Cr(VI)] is a human and animal carcinogen and contaminant of agricultural water and soil that is readily absorbed by crops (WHO, 2012; Witt et al., 2013). Oral exposure to Cr(VI) is widespread and is known to adversely affect many people worldwide (Sun, Brocato, & Costa, 2015); thus, high levels of chromium (Cr) found in foods might also be of toxicological significance.
To further investigate the potential relevance of toxic heavy metal contamination in commonly consumed protein- and botanical-based meal replacement/shake mix powders, we purchased 30 powdered shake mixes from large nationwide chain stores located in the Birmingham, Alabama, area and tested them for As, Cd, Cr, and Pb.
Materials and Methods
A total of 30 powdered shake products were analyzed: 23 protein mixes and 7 botanical mixes. The protein sources consisted of whey, pea, hemp, sprouted brown rice, egg albumin, rice, milk casein, soy, and whey. The various botanical mixes contained wheat, barley, oat and kamut grasses; alfalfa; fruit, vegetable, and various plant extracts and powders; coconut seeds; spirulina; apple fiber; sprouted barley malt; and whole leaf wheat grass powder.
All sample preparation was performed in laminar flow hoods known to be free of contamination from trace metals. In the first round of analyses, 10 protein and 5 botanical shake powder products were pooled in 5 pools (3 per group) in quantities proportional to their respective serving sizes and screened for As, Cd, Cr, and Pb by inductively coupled plasma dynamic reaction cell mass spectrometry (ICP-DRC-MS). Pool 4 was found to contain elevated levels of As and Cd, according to recommended limits set forth by federal and state regulatory agencies as detailed in the Maximum Recommended Thresholds section; Pb levels were not obtained for that pool. In the second round of analyses, the three samples from Pool 4 and an additional 15 new shake mix samples were analyzed individually for As, Cd, Cr, and Pb by ICP-DRC-MS.
Applied Speciation and Consulting, LLC, performed all ICP-DRC-MS analyses. Briefly, all water used for dilutions and sample preservatives were monitored for contamination to account for any biases associated with the sample results. A known mass of each sample was weighed into a polypropylene vial. All samples were then digested with aliquots of concentrated HN[O.sub.3] and [H.sub.2][O.sub.2] in a hot block apparatus, in accordance with U.S. Environmental Protection Agency (U.S. EPA) Method 3050B. All sample analyses were preceded by a minimum of a 5-point calibration curve spanning the entire concentration range of interest. All calibration curves associated with each analyte of interest were standardized by linear regression, resulting in a response factor. All sample results were instrument-blank corrected to account for any operational biases. Prior to sample analysis, all calibration curves were verified using second-source standards that are identified as initial calibration verification standards. Ongoing instrument performance was monitored by the analysis of continuing calibration verification standards and continuing calibration blanks at a minimal interval of every 10 analytical runs.
The sample digests were analyzed using ICP-DRC-MS. An aliquot of each sample digest was introduced into radio frequency plasma where energy-transfer processes cause desolvation, atomization, and ionization. The ions were extracted from the plasma through a differentially pumped vacuum interface and travel through a pressurized chamber containing a specific reactive gas that preferentially reacts with either interfering ions of the same target mass to charge ratios (m/z) or with the target analyte, producing an entirely different m/z, which can then be differentiated from the initial interferences. A solid-state detector detected ions transmitted through the mass analyzer and the resulting current was processed by a data handling system. In accordance with many promulgated methods (e.g., U.S. EPA methods), the instrument is set up to collect three replicate measurements or readings for each analyte; the result that is reported for each analyte for each sample is the average of three replicate measurements.
Maximum Recommended Thresholds
Currently, there are no federal guidelines with set limits for metal contamination in dietary supplements. Several agencies at the state, federal, and international levels, however, have issued recommendations on estimated safe maximum daily exposures for As, Cd, Cr, and Pb for adults, as outlined below.
As: U.S. EPA limits total As intake to 10 [micro]g/day (in 1 L of drinking water), including the inorganic and the less toxic organic forms (U.S. EPA, 2017). California Proposition 65 requires a warning on labels for a daily serving in excess of 10 [micro]g inorganic As (Office of Environmental Health Hazard Assessment [OEHHA], 2002, 2016a, 2016b). The American National Standards Institute (ANSI)/ National Sanitation Foundation (NSF) International's International Dietary Supplement Standard #173 recommends a maximum of 10 [micro]g/day of As (NSF...