1. Introduction

In our previous analysis, we tested the lithium levels of ten American foods. 

All ten foods were found to contain levels of lithium above the limit of detection, but some foods contained a lot more than others — ground beef contained up to 5.8 mg/kg lithium, corn syrup up to 8.1 mg/kg lithium, and goji berries up to 14.8 mg/kg lithium. 

But of the ten foods we looked at, eggs appeared to contain the most, up to 15.8 mg/kg lithium when analyzed with ICP-OES: 

So for our next study, we decided to look at more eggs. 

The first reason to look at more eggs was to confirm the results of our first study, and confirm that these numbers could be replicated.

The second reason to look at more eggs was to start getting a better sense of the diversity of results. Where the first study gave us a small amount of breadth by comparing several foods, the second study would give us a small amount of depth by comparing several eggs. 

The third reason to look at more eggs was that we might be able to find an outlier, a sample of food that contains far more than 15 mg/kg lithium. Eggs containing 15 mg/kg lithium are somewhat of a public health concern; how much more concerning would it be to find eggs that contain 50 mg/kg, or 100 mg/kg. 

(There are reports of such outliers in other foods, in particular from work by Sievers & Cannon in the early 1970s, who reported an “extraordinary” lithium content of 1,120 mg/kg in wolfberries from the Gila River Valley.)

As in the previous study, this project was run with the support of the research nonprofit Whylome, and funded by a generous donation to Whylome from an individual who has asked to remain anonymous. General support for Whylome in this period was provided by the Centre For Effective Altruism and the Survival and Flourishing Fund. 

Special thanks to all the funders, Sarah C. Jantzi at the Plasma Chemistry Laboratory at the Center for Applied Isotope Studies UGA for analytical support, and to Whylome for providing general support. 

The technical report is here, the raw data are here, and the analysis script is here. Those documents give all the technical details. For a more narrative look, read on. 

2. General Methods

2.1 Eggs

First, we collected a sample of eggs from grocery stores around America.

We started by purchasing several cartons of eggs from grocery stores near Boulder, Colorado. We bought several different brands, and tried to get a fair mix of eggs, both white and brown, conventional and organic. 

However, this was still not enough diversity for our purposes. So in the meantime, we asked friends from around the country to mail us cartons of eggs. 

Fun fact: Eggs don’t actually require refrigeration, Americans are basically the only weirdos who even keep them in the fridge. Especially when it’s mild outside, they keep for many weeks at room temperature. So shipping these eggs was relatively easy — really it’s just about packaging them with lots of padding so they don’t break. Most of the eggs arrived intact and we’re very grateful for the great care in packaging and shipping taken by our egg donors (ha). 

The list of eggs is summarized in greater detail in the technical report.

From most cartons, we took two samples of 4 eggs. This gave us two measurements per carton, which should give us some sense of how much variation there is within an individual carton.

Each sample was homogenized/blended with a stick blender for 1 minute to obtain a smooth, merengue-like texture. The blended mixture was then transferred to drying dishes and dried in a consumer-grade food dehydrating oven.

We also pulled out one brand for more testing, to assess individual egg-to-egg variability. From the carton of Kroger Grade AA, we took two samples of 4 eggs as normal. Then we took three more samples of individual eggs. The single eggs were blended and dried just as the larger 4-egg samples were. 

When all samples were dried, they were crumbled into a powder, weighed, put into polypropylene tubes, and shipped off to the lab for further processing.

2.2 Digestion

Food samples need to be digested before they can be analyzed by ICP-OES. Based on our results from the previous study, we used a “dry ashing” digestion approach, where samples are burned at high temperatures, and the ash is dissolved in nitric acid. 

Incineration causes organic compounds to exit the sample as CO₂ gas, but elements like sodium, potassium, magnesium, and lithium are non-volatile and remain behind in the ash.

2.3 Analysis

ICP-OES generates a tiny cloud of high-energy plasma, the “inductively-coupled plasma” of the acronym, and injects a cloud of liquid droplets into that plasma (hence the need for digestion). ICP-OES then examines the light that is emitted by the plasma as the liquid sample hits it.

In addition to lithium, we also analyzed all samples for sodium. Sodium is chemically similar to lithium, and most foods contain quite a lot, which nearly guarantees a good signal in every sample.

This makes sodium a useful point of comparison. At every step, we can compare the lithium results to the sodium results, to see if general patterns of findings match between the two elements.

3. Results

All samples were analyzed as one project, but for clarity of understanding, we’re going to report this project in two parts, as two studies.

In Study One, we look at the main body of results — eggs analyzed as four-egg batches from a single carton.  

In Study Two, we look only at the Kroger Grade AA eggs — analyzed as two four-egg batches and three one-egg batches, to assess individual egg-to-egg variability.

3.1 Study One

 For starters, here is a histogram of the distribution of lithium measurements in our egg samples: 

We’ve previously speculated that the distribution of lithium in food would be lognormal, as it is in drinking water, and indeed this looks very lognormal. 

For comparison, here’s the distribution of sodium:

Note that the x-axis is extremely different between the two plots! This is not surprising; eggs contain a lot more sodium than lithium.

For a sanity check, the USDA says that “Egg, whole, raw, fresh” contains 142 mg sodium per 100 g egg. Converted, that’s 1,420 mg/kg, which approximately matches these results, though the mean in this sample is much lower at only 987.3 mg/kg. The median is 963.0 mg/kg, and the standard deviation is 288.8 all told.

Slightly surprising are those three samples that (according to the analysis) contain almost no sodium — their values in the data are 7.6 mg/kg, 1.5 mg/kg, and one measurement below the limit of quantification. 

3.1.1 By Batch

More interesting is the breakdown by batch.

As a reminder: each carton of eggs (aside from the Trader Joe’s eggs, due to an oversight) was used to create two batches of four eggs each. Then, each batch was tested in triplicate, so each carton was tested six times. Here, each bar indicates a batch. Each batch has three dots, representing each of the three results from the tests done in triplicate: 

The main finding is that lithium was detectable in nearly all eggs. This suggests that ICP-OES is more than sensitive enough for this type of work, and that in general, eggs contain appreciable levels of lithium. 

Most egg samples contained between 0.5 and 5 mg/kg. The few readings of “zero” in the plot actually mean “less than about 0.04 mg/kg moist weight”.

Hypothetically speaking, the batches were all well-mixed. Eggs were blended with a stick blender for a full minute (to a very creamy consistency, think meringue), then dried and crumbled, and the dried bits mixed up. So it’s quite surprising that after all that, there’s so much variance within the batches.

Some of the batches show close agreement between different samples from the same batch. Both Simple Truth AA batches have only a very small amount of variation. Whole Foods Batch 2 is bang on every time. 

But other batches show a lot of variation. Batch 1 of Organic Valley and Batch 1 of Eggland’s best both contain one sample that is a huge outlier. You might dismiss these as some kind of one-off analysis error. But some of these cases, like both CostCo batches or the first Land-O-Lakes batch, show disagreement between all three samples. 

We wondered if this might mean that these batches were imperfectly blended. This would be quite surprising, given the lengths we went to to ensure that the batches were well-mixed. 

If the batches were perfectly blended, then all three samples should contain identical levels of lithium. The only differences between the results would then be errors in the analysis, not real differences in the samples. But if errors were the only source of noise, you would expect to see similar levels of variation in every batch. 

Two explanations seem likely.

First, lithium is very strange. In our last study, we saw that sometimes you get very different numbers for the exact same piece of food. Maybe the differences between different samples from the same batch comes from the fact that it’s hard to get accurate measurements for lithium levels in food.

Second, perhaps eggs are just goopy. It’s possible that despite our best efforts to completely blend the samples, they are still less than perfectly mixed, so some samples from the same batch contain more or less lithium than others. 

We can test these explanations by comparing the lithium results to the sodium results for the same set of batches and samples. If the variance is the result of a problem with lithium detection, then the sodium results should be much more consistent within batches. But if the variation comes from the eggs being imperfectly blended, then we should see similar variation in the sodium results as in the lithium results. 

3.1.2 Sodium

Here are the sodium results: 

Sure enough, there is a lot of variation between sodium levels, even within single batches. This suggests that the variation we saw in the lithium results is not the result of something weird about lithium. It’s probably something general about the samples or the analysis. 

Some of the variation in sodium lines up with the lithium results. The Whole Foods batches show great precision for both lithium and sodium, suggesting that they are especially well-blended or homogenous or something. But there is also some disagreement. For lithium, Organic Valley Batch 2 was much more precise than Organic Valley Batch 1. For sodium, it is the opposite. 

Sodium does show something unique — three very clear outliers with readings of almost exactly zero sodium (specifically 7.6 mg/kg, 1.5 mg/kg, and one reading below the limit of quantification). 

These look like errors of the analysis rather than real measurements. All three are outliers from the sodium data in general, more than three standard deviations below the mean. All three are from different batches and starkly disagree with the other samples from that batch. And we have strong external reasons to expect that any bit of egg will contain more than zero sodium.

In addition, we notice that these three cases with exceptionally low sodium levels are the exact same three cases that registered as below the limit of quantification for lithium. This suggests that none of these readings are real, that there were three samples where something went wrong, and the analysis for some reason registered hugely low levels of sodium and no lithium. If true, that means that all real measurements detected lithium above the limit of quantification.

The other variables we considered, like location, egg color, and whether or not the eggs were organic, didn’t seem to matter. Maybe differences would become apparent with a larger sample size, but they’re not apparent in these data.

3.2 Study Two

You might expect that hens from the same farm, eating the same feed, would all have roughly similar amounts of lithium in their eggs. For the same reason, it seems likely that any two eggs in the same carton wouldn’t be all that different, and would contain similar amounts of lithium.

All the above seems likely, but we actually have no evidence. It’s an assumption, and exactly the kind of assumption that could really confuse us if we assume wrong. It’s worthwhile to check.

Certainly the results from Study One call the assumption into question. A thoroughly blended mix of four eggs seems like it should have homogenous levels of lithium throughout. But empirically, that isn’t what we saw. We saw a lot of variation. Maybe the variation within those 4-egg batches comes from differences between the four eggs.

To test this, we did another round of analysis, focusing on a single carton of Kroger eggs. As before, of the 12 eggs in the dozen we took two groups of four to create two four-egg batches.

In addition, we took three of the remaining four eggs, and used them to create three one-egg batches, mixing and sampling just that single egg. The one-egg batches each consisted of a single egg from this carton, blended well. The one-egg batches were also tested in triplicate, i.e. three samples from the same egg. 

Here are the results: 

These four-egg batches look much like the four-egg batches tested in Study One. They show a lot of variation between the samples tested in triplicate.

The single-egg batches, on the other hand, did indeed have lower variance than the 4-egg batches. There was much closer agreement between different samples from the same eggs, than samples from different eggs. Certainly we see a difference between the egg used for Batch 3, which all samples indicate contains about 1 mg/kg lithium, and the egg used for Batch 4, which all samples indicate contains about 5 mg/kg lithium

This suggests that there really may be appreciable egg-to-egg variation. This could be the result of other factors, including simple randomness, but the tightness of the single-egg analyses is suggestive. And the fact that the variance seems much lower in single-egg batches implies that the mixed four-egg batches are imperfectly blended.

The sodium results for these batches seem to confirm this, with greater variation in sodium in the four-egg batches than in the one-egg batches: 

Again, this suggests that the patterns we observe in the lithium data are the result of actual results in the world, or the analysis in general, rather than some artifact of the lithium analysis in particular.

4. Discussion

Nearly all egg samples contained detectable levels of lithium, and around 60% of samples contained more than 1 mg/kg lithium (fresh weight). These results appear to confirm that eggs generally contain lithium.

If you accept the argument that the three samples with conspicuously low sodium readings are the result of a failure of analysis, then all egg samples contained detectable levels of lithium. 

In terms of diversity of results, samples varied from as much as 15 mg/kg Li+ to as little as less than 1 mg/kg Li+. Variation did not seem to be related to the geographic purchase origin of the eggs. Nor were there any obvious differences between organic and non-organic, or white and brown eggs. This suggests that these are not major sources of variation. 

However, we did see evidence of a lot of variation in lithium levels between individual eggs, even between individual eggs from the same carton. 

While there was a lot of variation between samples, some samples showed a great deal of consistency, especially samples from single eggs. This suggests that dry ashing followed by ICP-OES has high precision when analyzing food samples for lithium. Though these results do not speak to whether or not this analytical method is accurate for such samples, they do suggest that these are real measurements and not merely the result of noise or analytical errors.  

One of our hopes for this study was to find an egg that contained more than 15 mg/kg lithium, that we could subject to other, less sensitive analytical methods. This would let us get a sense of accuracy by triangulation, comparing the results of different methods when analyzing samples of the same egg.

We did in fact find eggs that contain such high concentrations. Above we reported the lithium concentrations in fresh weight, because those are the numbers that are relevant if you are eating eggs. But in terms of analysis thresholds, the numbers that matter are the dry weight. For dry weight, some of these egg samples contain as much as 60 mg/kg lithium. That’s more than enough to be above the sensitivity of a technique like AAS. 

As we are quite interested in trying to confirm the accuracy of lithium analyses in food, one next step will be to replicate these analyses using other analytical techniques like AAS.