WHAT IS LEPTIN?
In 1994, Jeffrey Friedman, MD, PhD, and his colleagues at Rockefeller University discovered this peptide and named it appropriately “leptin,” from the Greek word for thin, leptos. This discovery and the awareness that one could become resistant to the chemical messenger was a major breakthrough in understanding obesity.
Leptin’s main role is to tell the brain how much energy is on hand and how much may be needed. Leptin is highest after a meal, when it tells the hypothalamus to reduce food intake. Because it is secreted by body fat, leptin levels generally rise in proportion to total body fat mass. Women tend to have higher leptin levels than men thanks to their higher body fat percentage. Studies suggest leptin increases during the luteal phase of the menstrual cycle due to the effects of estradiol and progesterone.
LEPTIN AND IMMUNITY
Leptin would seem to be the good guy of chemical messengers. The trouble is that when a surge of leptin is prolonged, the brain receptors designed to receive the message become desensitized or resistant to leptin and are no longer able respond to the signal. Despite an excess of leptin, the person with leptin resistance actually suffers from symptoms of low levels of the hormone. The overweight person may be feeling hungry more often and store fat too readily. Instead of feeling satisfied, his or her brain instructs the body that it needs more energy.
More recent research suggests that inflammation and intestinal microbes may also increase leptin levels, and leptin is emerging as a key player not only in the regulation of fat storage but also in the regulation of immune function. For instance, increased leptin release from fat cells effectively disables the security guards (Treg cells) of the immune system, setting the stage for a flood of immune cells around adipose tissue, leading to chronic inflammation, insulin resistance, and impaired fat burning.
LEPTIN DRIVES INFLAMMATION, INSULIN RESISTANCE
However leptin is not a kamikaze pilot set out to destroy your waistline by enabling sugar-burning immune cells to swarm the fat cells of your abdomen. Leptin actually enhanced the longevity of our cavemen ancestors by averting infection during times of famine.
In ancient times, starvation and infection were two driving forces of premature death. A shortage of food is a surefire way to thwart a mounting immune defense against a lethal pathogen. Leptin’s dual role as an appetite suppressor and immune system stimulant circumvented immune suppression in the face of famine. Exposure to microbes causes a leptin spike, stimulating the immune response, pushing the immune police (Treg) out of the way. Leptin also appears to help the immune response in another way, causing insulin resistance, redirecting fuel away from muscles and liver to support the sugar-loving immune system in its quest to combat invading pathogens. Indeed, after controlling for many variables, researchers have found that leptin is independently associated with insulin sensitivity in children and adults.
As is often the case, a biological process that was historically protective is paradoxically problematic in the modern era. Increased leptin levels are linked to development of metabolic syndrome, cardiovascular disorders, and autoimmune diseases—including lupus, multiple sclerosis, autoimmune thyroid disease, and rheumatoid arthritis—not to mention appetite challenges in overweight persons.
One report even showed greater intestinal permeability with increasing leptin levels. In sum, leptin is a huge link to many problems with excess fat and metabolic perturbations that we’ve discussed up to now.
A great way to reduce leptin is to exercise more. The only caveat here is leptin promotes inflammation in the joint tissues, a condition formally known as osteoarthritis, making exercise painful and self-limiting. Achy knees and painful hips were once considered to be strictly musculoskeletal; the excess pounds were stressing out the body’s mainframe. Yet when studies surfaced linking a disproportionally higher prevalence of osteoarthritis in the hands of overweight persons compared to lean counterparts, that idea was explored further. Low and behold, leptin emerged again as the nexus between fat tissue and joint inflammation.
TESTING FOR LEPTIN
Since lowering leptin is compulsory for restoring normalcy among the immune and metabolic systems, reducing leptin is as important as loosing the pound. If you have increased belly fat, or an overall body fat percentage over 25 in women and 20 in men, it’s likely that you have increased leptin above the ideal range of 15 ng/mL. To be sure, work with a healthcare professional to test your levels first thing in the morning (always retest at the same time of the day as well).