Are the carbohydrates from shellfish—oysters, clams, shrimp, squid, etc—the same as carbohydrates from sugar or fruit or grains? I’ve long suspected that they are not.
Did you even know that shellfish had carbs? Many keto dieters and people with diabetes are surprised to learn that certain meats and shellfish in fact have large amounts of carbohydrates. Liver, for example, has quite a few carbs: a 4 ounce portion of beef liver has about 4 grams, about as much as the same amount of spinach or broccoli. A cow’s or chicken’s liver contains carbs for the same reason that your own liver contains carbs: the organ makes and stores a type of sugar, glycogen, that is necessary in all sorts of complex metabolic processes. These sugars remain in the liver after it’s been butchered and packaged and cooked. Animal muscles also store sugars, but they dissipate after death, leaving most meat essentially carb-free.
Shellfish also make and store glycogen, but unlike fish and livestock they do not have livers, and the glycogen remains in their muscles when you eat them. Some shellfish, anyway, is eaten moments after the death of the animal, or even before it, which would not allow the muscle sugars time to dissipate even if they were inclined to do so.
There’s always been some confusion about these unexpected carbohydrates in the low-carb community: do they count as regular carbs? Are they slow or fast? Are 8g of carbs from a pot of mussels just the same as 8g of carbs from a cracker, or a tomato?
Luckily, I have a body that’s designed to answer these questions, because I have Type 1 diabetes. For keto readers that are less than familiar with the condition, here’s a quick explanation: my body has essentially lost the ability to produce insulin. When a regular healthy person eats carbohydrates, the pancreas secretes insulin in response, which allows the body’s cells to utilize the sugars that enter the blood, maintaining a healthy and steady blood glucose level. When I eat carbohydrates, I usually take an insulin shot along with my meal in order to mimic the pancreas’ natural response. But if I neglect to take that shot, I’ll see my blood sugar rise directly in response to the food I’ve eaten. In a sense, a person with Type 1 diabetes is a perfect natural experiment, because he or she can see the blood sugar impact of foods without any interference from responsive insulin.
For most keto dieters, these questions are largely academic, but for people with diabetes that need to manually control their body’s insulin response, they can be significant. I myself began to suspect that the carbohydrates in shellfish are unusually slow-acting after experiencing several bouts of hypoglycemia when eating squid or clams or lobster, indicating that I had used too much insulin for the meal. I’ve asked others experienced people with diabetes, and found that some careful eaters do discount the carbohydrate content of shellfish.
I decided to put my own body to the test. On a recent sunny afternoon, I ate 48 raw oysters, without using any meal-time insulin.
Why raw oysters? According to at least one source, raw farmed Eastern oysters, like the ones I ate, have the highest carbohydrate content per calorie of any common shellfish ingredient. And as it happens, I live in an area known for its oysters. My home is near a salt-water bay, the banks of which were once packed with millions upon millions of oyster shells, shucked and piled by Native Americans over a period of at least a thousand years.
The USDA’s nutrient data states that one such oyster has about .75g of carbohydrates, which may not sound like much. But with four dozen on my platter, that added up to 36 grams, a substantial total, more than what you get out of a banana or two slices of Wonder Bread. That’s enough, reportedly, to kick many a keto dieter out of ketosis.
Under normal circumstances, if I were to eat 36 grams of carbs without using any insulin, I would expect my blood sugar to rise some 100-200 mg/dL, spiking me to the unhealthy levels I do my best to avoid. I hoped and expected, though, that the oysters meal wouldn’t spike me to that extent.
And so, armed with a traditional glucose meter and a Dexcom continuous glucose monitor (CGM), I shucked and ate my 48 oysters, and I watched what happened. Here are the numbers from my glucose meter:
- Hour 0: 82 mg/dL
- Hour 1: 107 mg/dL
- Hour 2: 124 mg/dL
- Hour 3: 139 mg/dL
You can see in the banner photo that my CGM registered 100 mg/dL as I shucked the first oyster (a discrepancy of 10-20 points between two different devices is not uncommon). According to the CGM my blood sugar experienced a very gradual rise, peaking at 145 mg/dL, more or less confirming the evidence from the glucose meter. (It did take a while to eat all of the oysters—I shucked as I ate.)
Three hours after beginning the experiment, with my blood sugar plateauing, I decided I had had enough. After this point, any blood glucose rise might well be attributed to the effect of the protein I’d eaten (about 65g) instead of the carbohydrates, so I felt good about closing the experiment. I took a small correction bolus of insulin in order to bring my blood sugar back down.
What did I learn? 48 oysters raised my blood sugar by about 50 mg/dL. One point per oyster, which is both easy to remember and significantly less than the carbohydrate count itself would have predicted. You will recall that I had supposedly eaten enough carbohydrates to create a rise that was two to four times as large.
And the rise was extremely gradual, far more gradual than it would have been for most traditional sources of carbohydrates, be they candies or cookies or lentils or blueberries. I have no doubt at all that if I had bolused with rapid-acting insulin for 36 grams of carbohydrates, I would have been struck by a raging and potentially quite dangerous hypoglycemic episode.
I understand that any N=1 experiment is of limited value. Someone else’s body may way react differently, and even my own body might react differently on a different day. With that said, I did what I could to ensure that the experiment was as “controlled” as it could be. I hadn’t eaten any breakfast and my blood sugar levels were extremely steady going into my oyster lunch. I didn’t exercise or do anything else that would have been likely to impact my blood sugar. And I ate nothing else, with the bare exception of the juice from a single wedge of lemon (0.4g carbs) and some prepared horseradish (0g carbs), because what kind of savage would eat 48 oysters without some of the traditional accoutrements? I was, to tell the truth, dying for a few glasses of bracing white wine, but I heroically stopped myself in the interests of science.
It’s also true that the precise carbohydrate content of my meal was impossible to verify. The same nutrient database claims that wild oysters have about 30% fewer carbohydrates per ounce, which seems questionable: farmed oysters are the same species as wild oysters, and grow in the same areas, eating the same foods. Other estimates may vary too.
My meal may have fallen well short of some of the famous historical feats of bivalve gluttony—my lunch would have been a mere appetizer to Diamond Jim Brady, nor would it have impressed the writer Jim Harrison, who claimed to have eaten 144 oysters once, just to see if he could do it: “I could. I got gout the next morning.” Luckily, dear reader, I experienced no gout.
But my experiment was at least enough to strengthen my opinion that shellfish carbohydrates are not like other carbohydrates. In the future, I will count on carbohydrates from oysters to have a strength and speed far reduced in comparison to other carbohydrates. I suspect the same will be true of other shellfish, too. But how about beef liver? Perhaps some more gluttonous experiments are called for.