If you want to make keto really work for you, it helps to understand a little bit about how the diet does its magic - and one of the big players here is the hormone insulin. Insulin does a whole lot of different things, but it’s best-known as the hormone that you make to metabolize carbs.
Insulin gets a really bad rap in low-carb circles, to the point where it can get really oversimplified. There’s more to weight gain than insulin! For general health, insulin isn’t necessarily bad, and it’s actually necessary for some health-related goals (for example, if you want to gain muscle, insulin is definitely your friend). But keto isn’t just about general health. Keto is about a specific metabolic shift. If your goal is ketosis specifically, insulin is bad news - here’s what you need to know.
Insulin: Keto Enemy #1
The whole point of the ketogenic diet is that you’re forcing your body to use ketone bodies for energy, instead of fat and carbohydrate. That’s what makes the diet work.
Insulin suppresses ketone production. So if you want to get into ketosis and stay there, you want to minimize insulin as much as possible. Unless you’re taking outside insulin, the easiest way to do this is by changing what you eat. Insulin is produced in response to different foods, so by changing your diet, you can minimize insulin production. That’s the point of a ketogenic diet.
Eating for Low Insulin Production
The ketogenic diet minimizes insulin production by restricting both carbs and protein - the diet keeps carbs as low as possible and supplies just enough protein to meet your needs, but not more.
To reduce insulin production, lower carbs
Carbs raise insulin levels because you need insulin to metabolize carbs (use them for energy). The more carbs you eat, the more insulin you need.
It works like this: when you eat something carb-heavy, the glucose (carbohydrate) in that food raises your blood sugar. But having high blood sugar all the time is dangerous, so when your body senses that you’ve eaten carbohydrates, your pancreas makes some insulin to take that glucose out of the bloodstream and store it somewhere safe (your fat cells) for later use.
This is all fine and good, and if you’re going to eat carbs, you want it to work exactly like that. If you eat carbs but can’t produce enough insulin (e.g. people with type 1 diabetes), you’ll be in very deep trouble. But the flip side of this is that if you want to reduce insulin production, you’ve got to lower carbs.
To reduce insulin production, lower protein
This one is a bit more of a surprise - most people know that carbs have something to do with insulin, but not a lot of people know that protein can also trigger an insulin spike. But it’s true!
For example, in this study, adding whey protein to a mixed meal increased the insulin response to the meal. Other types of dairy protein are also insulinogenic - that’s why keto focuses on high-fat, low-protein dairy foods (like butter), not high-protein, low-fat dairy foods (like Greek yogurt). It’s all about reducing the insulin spike.
It’s not totally clear how protein causes this insulin response. It’s possible that protein causes an insulin spike by stimulating the secretion of another protein, GLP-1, which then goes on to stimulate insulin secretion.
What is clear is that if you want a diet that minimizes insulin production, you’d also want to reduce protein. In practice, it’s not practical or even desirable to get protein down to almost nothing, the way you can with carbs. Protein does more than just cause insulin spikes, and protein deficiency causes problems of its own. So on keto, the goal is to have adequate protein: enough to keep you healthy and meet all your needs, but not more.
Keto: low in carbs, moderate in protein
All of this research on carbs, protein, and insulin explains the whys and hows of keto pretty well. If the goal is to use ketones for energy, you need to reduce insulin production, which means minimizing carbs, reducing protein by quite a bit, and relying mostly on fat for calories because, well, fat is what you have left. Fat raises insulin levels very minimally, especially if it’s not in the context of overeating. A high-fat, adequate-protein, low-carb diet doesn't totally prevent insulin production, but it reduces it by enough that the very low level of insulin left doesn’t stop you from making ketone bodies.
And the results are pretty impressive. Eating a keto diet significantly reduces fasting insulin and postprandial insulin (insulin levels right after a meal). In patients with diabetes who rely on outside insulin, a ketogenic diet reduces the need for insulin.
Keto, Insulin Resistance, and Insulin Sensitivity
On the topic of keto and insulin in general, it’s worth noting how keto affects insulin sensitivity and insulin resistance.
Insulin gets glucose out of your bloodstream (where it’s dangerous) and into your muscles, liver, and fat tissue (where it’s not). But for insulin to do that effectively, it has to be able to “persuade” all of these tissues to let the glucose in.
In people who are insulin sensitive, these other tissues are receptive to the insulin signal: they’re happy to open the door when insulin comes knocking. In people who are insulin resistant, these tissues resist the insulin signal. If insulin can’t do its job, all that glucose stays right in the bloodstream (after all, where else is it going to go?) and the insulin resistant person ends up with high blood sugar all the time. When this gets diagnosed as an actual disease, it’s called type 2 diabetes.
Now take a look at what happens when you throw a keto diet into that mix. Blood sugar control improves and people can reduce the amount of insulin they have to take as medication. This is especially true for people with higher blood sugar to begin with. Keto is impressively therapeutic for people with insulin resistance and problems controlling their blood sugar - on top of people with type 2 diabetes, it also works for women with polycystic ovary syndrome (PCOS), another disease that’s marked by insulin resistance.
Basically, because a keto diet doesn’t require you to be able to make insulin or use it normally, it’s very therapeutic for people who can’t do that.
In Conclusion: A Specific Diet for a Specific Purpose
Understanding how and why a diet works can help you apply it in your own life. For example, if you just think of keto as “low-carb,” you might be tempted to use a lot of whey protein powder, because everyone knows protein is the perfect macronutrient that makes you thin, right? But whey protein is actually very highly insulinogenic, so it’s not a great option for keto!
A ketogenic diet minimizes insulin by reducing both protein and carbs - that’s how it takes the brakes off ketone production and lets you really hop on the the ketosis train.