Should we become obese, fat can spill out from our overbloated fat cells back into our bloodstreams and get lodged in our muscles. When this happens, it can interfere with insulin signaling, causing our muscles to become less responsive to insulin, a phenomenon known as insulin resistance. Normally, our muscles take up blood sugar in response to insulin, but if they become resistant to the effects of insulin, the sugar remains in the blood and can build up to dangerous levels. To prevent this, our bodies produce even more insulin to force more blood sugar into our muscles. But all that extra insulin in our system can cause additional fat storage and result in a vicious cycle: obesity leading to insulin resistance, which leads to higher insulin levels that then lead to more obesity and even more insulin resistance.
When insulin resistance gets so bad that our insulin production can no longer keep up and overcome it, our blood sugars start creeping up and we become prediabetic and then progress to full-blown diabetes. Instead of treating the cause — insulin resistance — with lifestyle medicine to try to reverse the diabetes, what do most doctors do? Prescribe even more insulin, which can perpetuate the cycle. With injections, insulin levels can be forced so high that even resistant muscles will concede, but what effect will all that extra insulin have on our fat stores? Within the first year of starting insulin, type 2 diabetics typically gain between three and nine kilograms of “insulin-associated weight gain.”
The Insulin-Obesity Cycle
Obesity can kick-start the vicious cycle leading to the insulin resistance, which leads to elevated insulin levels, which leads to more obesity.
You can also become insulin resistant without being obese. The fat that ends up clogging our muscles and causing insulin resistance can come from the fat we wear or the fat we eat. Normally, we may have as little as 100 μmol/l of free fat floating around in our bloodstreams at any one time, but those who are obese may have up to 800 μmol/l. However, skinny people can reach 800 μmol/l just eating a high-fat diet. In other words, a thin person eating a low-carb diet can have the same level of fat in their blood that an obese person does.
Infuse fat into people’s veins through an IV, and, by using a high-tech type of MRI scanner, you can show in real time the buildup of fat in muscle cells within hours, accompanied by an increase in insulin resistance. The same thing happens when you put people on a high-fat diet. Feed folks even a single high-fat meal, and within six hours, their insulin sensitivity can be cut in half, meaning their insulin resistance shoots up. Do this day after day, and insulin levels can rise to compensate, which can then lead to weight gain as the vicious cycle starts turning.
Some people are born with higher-than-normal insulin levels and naturally have a higher propensity to gain weight, suggesting that the cycle can also start with an elevated insulin level, leading to elevated obesity and, subsequently, elevated insulin resistance.
The causal role of high insulin levels in obesity was established in a study published in 2018 entitled “Reducing Insulin via Conditional Partial Gene Ablation in Adults Reverses Diet-Induced Weight Gain,” but the adults they were talking about were adult mice. You can prove insulin drives obesity by genetically engineering a low-insulin mouse that is essentially immune to obesity, but how can you prove it in people? The closest we’ve come is the demonstration that drugs that lower insulin levels do cause weight loss in obese adult humans, but the drugs also may have direct effects on body fat that could account for the benefit. So while we don’t know for certain, I think there’s enough evidence to try to keep our insulin levels within the normal range.
Break the Cycle by Improving Insulin Sensitivity
One way to lower our insulin levels is to make our insulin work better. The reason our bodies pump out so much insulin is to overcome any resistance, so by improving the insulin sensitivity of our muscles, we can make a little insulin go a long way. How? You can do it with exercise (both endurance and resistance training), weight loss, or reducing your intake of fat—but not necessarily all types of fat. While the monounsaturated fats concentrated in nuts, olives, and avocados appear more likely to be detoxified or safely stored away, the saturated fats concentrated in meat, dairy, and junk can create the toxic breakdown products in our muscle cells thought responsible for the development of insulin resistance.
Experimentally shifting people from animal fats to plant fats can improve insulin sensitivity even without changing the overall quantity of fat eaten. Thinking this may help explain why those eating more plant-based diets have less insulin resistance, researchers at Imperial College London set out to compare the amount of fat clogging the muscle cells of vegans versus omnivores. So as not to give the vegan group an unfair advantage, the researchers recruited omnivores who were as slim as the vegans. The researchers wanted to know if plant-based eating had a direct benefit beyond simply indirectly pulling fat out of the muscles by helping people lose weight in general. The vegans were found to have significantly less fat trapped in their muscle cells, which can translate into less insulin resistance and lower insulin levels.
So can switching fats help with weight loss? Amazingly, Australian researchers found that even if you feed people about the same number of calories and the same amount of fat, but switch out meat and butterfat for olive oil, nuts, and avocados, you lose nearly three more kilograms of fat in a single month. You can certainly overdo plant fats, though. On high-fat diets, the type of fat appears to matter less. Have people drink nearly a half cup of oil, and you can temporarily triple their insulin resistance in a matter of hours.
Break the Cycle by Lowering Insulin Spikes
In addition to making our insulin work better by improving our sensitivity to it, we can lower our insulin exposure by choosing foods that cause less of an insulin surge. Although those who are born with or develop higher-than-normal insulin spikes after meals are at elevated risk of weight gain and obesity, anyone can get an exaggerated insulin response to Wonder Bread.
High glycemic loads are the primary stimulus for insulin release as our bodies desperately try to sock away blood sugar from the rapidly digested sugars and starches. Randomised controlled trials clearly show that swapping out refined grains in favour of whole grains reduces insulin spikes (likely due to the fibre content). Remember, though, insulin doesn’t just deal with carbohydrates after a meal but protein and fat as well.
To their credit, low-carb and paleo diet advocates identify insulin as playing a role in the obesity epidemic, but they often don’t appear to recognise the broader scope of insulin triggers. Since carbs increase insulin, the argument goes, we should eat lots of meat, which is just fat and protein with zero carbs. That wouldn’t cause an increase in insulin, right? Wrong. We’ve known for more than a half century that if you feed people a steak, their insulin levels go up. Pretty much pure protein (like whey powder) and pure fat can have a similar effect. Have people eat some lentils with butter, and you get a 60 percent higher insulin reaction to pure sugar compared to lentils alone. That’s why we need more than a glycemic index. We need an insulin index. We need to feed people dozens of different foods and just measure what kind of insulin reactions they get. And that’s exactly what researchers did.
What do you think causes the biggest insulin reaction: a large apple, an orange, a cup of oatmeal, a cup and a half of white-flour pasta, four chocolate chip cookies, a bunless burger, or a fish fillet?
Is that your final answer?
Well, surprisingly, beef and fish cause more insulin to be released. In terms of meat, the original study looked only at beef and fish, but subsequent studies found the insulin response to chicken and pork was just as high. It turns out meat protein causes almost exactly as much insulin release as pure sugar. So, based on their own logic, low carbers and paleo folks should be reaching for big bowls of pasta rather than meat.
Those eating plant-based diets average significantly lower insulin levels and have less insulin resistance, even compared to non-vegetarians at the same body weight. In fact, those who eat meat have up to 50 percent higher insulin levels in their bloodstreams. Might that just be because they’re more sedentary or something? Researchers from the University of Memphis decided to put it to the test by placing men and women on a plant-based diet and got significant drops in insulin within just three weeks. But add some egg whites to the plant-based diet, and you can cause a “dramatic” rise in insulin output—as much as 60 percent within just four days.
Fish and poultry may be even worse than the egg whites. Add about half a can of tuna to some spaghetti, and induce about a 70 percent higher insulin spike in diabetics. Skinless chicken breast and white rice cause an insulin reaction closer to straight sugar than rice alone. Compared to chicken, the meat-free tastes-like-chicken Quorn causes up to 41 percent less of an insulin reaction within fifteen minutes.
So to reduce insulin levels to potentially facilitate weight loss: Rule #1: Avoid High-Glycemic foods; Rule #2: Make plant protein your preference; and Rule #3: When you do eat animal protein, try to pay particular attention to Rule #1.
This is an edited extract from How Not to Diet: The Groundbreaking Science of Healthy, Permanent Weight Loss by Dr Michael Greger. Available now for $34.99, published by Bluebird - macmillan.com.au