Dear Doctor: I've read stories about Girl Scouts selling hundreds of boxes of cookies outside pot shops to people who are anticipating the munchies. My question is: What is it about cannabis that causes that kind of hunger?

Dear Reader: It's a toss-up which effect of using cannabis is better known -- the fact that it alters mood and perception, or the raging hunger that users experience, known as the munchies. Research into the psychoactive effects and mechanisms of the drug dates back decades. Now, thanks to research published in 2015 by scientists at Yale University, we finally have an answer to the munchie conundrum.

According to the World Health Organization, cannabis is the most widely used and trafficked illicit drug in the world. In the United States, it is recognized as a Schedule I controlled substance. However, growing numbers of states are passing laws that permit medical and/or recreational use of the drug, which is how some enterprising Girl Scouts came to set up shop outside cannabis dispensaries.

Cannabis looks and grows like a weed, but it's actually quite complex. Researchers have isolated more than 400 different chemical compounds and entities within the plant, a number of which are being studied for medicinal qualities. In fact, the mechanisms revealed in the research that decoded the munchies show promise in helping people who need help with appetite, such as cancer patients undergoing chemotherapy.

The active ingredient in cannabis is tetrahydrocannabinol, or THC. It is chemically similar to anandamide, a neurotransmitter that naturally occurs in our brains. In fact, the molecular structures of THC and anandamide are so similar that THC can attach to and activate sites known as cannabinoid receptors, which are located on neurons throughout the brain. Once THC enters the blood, either by smoking or ingesting, it takes just a short time for it to bind to receptors and begin to affect a range of cognitive processes in the brain.

In 2014, a team of European researchers discovered that THC fits into a neural structure in the brain known as the olfactory bulb. The result is an increased sense of smell, which the scientists believed could lead to an increase in appetite. A year later, a team of neuroscientists from the Yale School of Medicine published a surprising set of findings that focused on the drug's effect on the appetite centers of the brain.

Working with genetically modified mice, the scientists found that some of the compounds in cannabis actually trick the appetite center of the brain. Through a series of complex steps, those compounds subvert the very mechanism that, when working properly, tells the mice that it's time to stop eating. As the lead researcher of the study put it in a press release, "It's like pressing a car's brakes and accelerating instead."

Scientists are intrigued by this newest study. A compound that flips a biochemical switch and makes a brain process do the opposite of what was intended is indeed unique. However, the consensus is that this, like that olfactory bulb discovery, is just one piece of the complex munchies mystery.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: I take medication daily for hypothyroidism and borderline high blood pressure. My question is: Do I benefit from iodide in table salt or should I buy plain salt for cooking?

Dear Reader: Your question highlights how different Western diets are from those in other parts of the world, where iodine deficiency is a significant issue. (For this column, we'll focus on the element "iodine," but "iodide" is the safely ingestible form of it.) That deficiency is due to inadequate dietary intake of iodine, which is found in fish, seafood, kelp, vegetables grown in iodine-rich soil, dairy products -- and salt.

Although sea salt also contains a small amount of iodine, many table salt brands in the United States contain much more. That's because both Europe and the U.S. have encouraged the fortification of salt (and some other foods) to maintain adequate iodine intake in the population. Because of those efforts, launched in the U.S. in 1924, we've reduced health problems linked to low iodine intake. That's not to say mild iodine deficiency can't been found, especially with more people consuming unfortified salt, just that it's rare.

In any case, iodine is necessary for the formation of thyroid hormone. With low levels of iodine -- a condition known as hypothyroidism -- the levels of the thyroid hormones T4 and T3 drop. In trying to produce more thyroid hormone, the thyroid gland enlarges; the externally visible swelling is called a goiter. Low thyroid levels can be risky during pregnancy, especially in the first 12 weeks, when the mother's thyroid hormone is necessary for the development of the fetal nervous system. Severe iodine deficiency in pregnant women can lead to their child's intellectual disability, deafness, inability to walk appropriately and muscle spasms. Prenatal vitamins often contain potassium iodide to prevent iodine deficiency. But it is also important that pregnant women not take too much iodine because this may suppress the fetus's ability to produce thyroid hormone on its own.

As for whether you should consume iodized salt for additional boosting of your thyroid, I wouldn't worry too much about salt's impact on your thyroid levels. Although I'm not certain as to the cause of your hypothyroidism, the most likely explanation -- autoimmune thyroiditis -- is not due to low iodine levels. In this country, hypothyroidism due to low iodine intake is rare. Rather, you have low thyroid levels for other reasons.

I'm not suggesting you consume copious amounts of salt. Because of your high blood pressure, you should be careful with your intake. Reducing salt consumption has been shown to lower blood pressure in those with abnormally high intake. Instead of salt as a source of iodine, I would recommend increasing your intake of fish, seafood and seaweed snacks. You can also get iodine from fortified breads, cereals and waters.

Overall, the recommended minimal intake of iodine for a non-pregnant adult is 150 micrograms per day; for pregnant women, the recommendation is 220 to 250 micrograms. The average intake in the United States is between 240 and 300 micrograms.

But whatever you do, monitor your blood pressure with changes in your salt intake -- and, for good measure, keep an eye on your thyroid levels. If your doctor is concerned about your iodine levels, a urine test can measure it.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: Apparently one size does not fit all when it comes to the Mediterranean diet, because a recent study found that rich people benefit from it and poor people don't. It can't be as simple as that. What gives?

Dear Reader: We confess that after reading several of the news reports generated by the release of this study, we were tempted to write our own headlines. The gist would be: "Let's Ignore Significant Details So We Can Come to Flawed But Provocative Conclusions!"

It's true that the researchers did find a link between income and the health outcomes of the subjects of their study who followed the Mediterranean diet. Specifically, individuals with more money reaped measurable health benefits, and those who had less money did not. But as you suggest, the how and why of these results is more nuanced than simply rich versus poor.

Whether it set out to or not, this study adds to a body of research that examines how economics and education affect our health. People with lower income levels not only have less money to spend on food, but they also have fewer options when it comes to shopping. It's an important topic that the National Institutes of Health classifies as environmental justice. But let's pull back a bit and get to the details of the study at hand.

First, the Mediterranean diet. It cuts out refined and processed foods, unhealthy fats and limits sugar. Instead, the emphasis is on a variety of fresh fruits and vegetables, leafy greens, nuts, legumes, unrefined grains and cereals, olive oil, and modest consumption of fish, lean meat and dairy products. Past studies have tied the diet to improved cardiovascular health, lower cancer risk and increased longevity. So when researchers in Italy analyzed four years of self-reported dietary data from 19,000 women and men, they weren't surprised that those who followed the Mediterranean diet had a lower incidence of heart problems.

But there was a catch. After the participants were further sorted according to levels of income and education, the benefits of the diet were not universal. People with lower income levels didn't show a reduction in cardiovascular risk. Digging deeper, the researchers learned that while everyone in the study adhered to the basic principles of the Mediterranean diet, when it came to the specific foods being eaten, marked differences emerged. That's where education -- yet another interesting twist -- comes in.

According to the study, participants with more education ate the widest range of fruits and vegetables. Individuals with higher income levels ate the most nuts, whole grains, fresh fruits and fish. The diets of those on the lower end of the education and income scale included more meat, less fish, less variety in fruit and vegetable choices, and cooking methods that were less healthful.

The authors acknowledge the pitfalls of drawing conclusions from studies with self-reported data. At the very least, when it comes to the health outcomes of the Mediterranean diet, we've learned that details matter. And in the bigger picture, the differences in income and education meant everyone in the study wasn't on the same diet after all.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)