Overall, the complete answer to this question depends upon knowing whether you inherited genes that predispose you to drug addiction or food addiction. Recent research has found evidence that these two addictions are closely related to each other. What differs, and what truly matters to most people, is the consequence to smoking too much marijuana or consuming too much food. Which is worse for your body and brain? For the majority of people the answer will be obesity, not marijuana. This is why.
During the past three decades an obesity epidemic has been responsible for a 77% increase in death rates. The accumulation of excess body fat has been clearly shown to accelerate the progression of many age-associated diseases such as cancer, arthritis, diabetes and dementia. Why? A few years ago it became clear that fat cells produce inflammation by releasing specialized proteins called cytokines. The more fat cells you have the more cytokines get released into your blood. Essentially, obesity is associated with chronic, low-grade, body-wide inflammation, insulin resistance and many of the same metabolic conditions that underlie the aging process itself. The cytokines are capable of inducing shrinkage of brain regions (primarily gray matter- where the neurons live) that are used in the process of learning new things and recalling memories.
One recent study examined the relationships between academic performance, cognitive functioning, and BMI among 2,519 young people. BMI was inversely correlated with general mental ability even after controlling for demographics, lifestyle factors, and lipid profiles. Overall, obesity is implicated in lower performance on cognitive control tasks. The longer the inflammation is present, the more brain shrinkage occurs. Elderly obese people have more impaired learning and memory abilities than elderly thin people. Being obese at mid-life is also a strong predictor of dementia in later life.
Today, an overwhelming body of evidence across a wide spectrum of medical disciplines strongly argues that obesity accelerates the aging process, impairs overall cognitive function and, ultimately, is responsible for numerous processes that kill you.
Studies suggest that adults (this argument does not apply to young people) who use low to moderate daily amounts of marijuana show no personality disturbances. During the past few years some sensational studies have been widely featured in the national press; one suggested that daily marijuana use might decreased IQ (Meier et al., PNAS, 2012), the other suggested that daily recreational use caused shrinkage of brain areas that are critical for learning, memory and emotional control (Gilman et al., 2014, J Neurosci). The report by Meier et al. was immediately challenged (Rogeberg, 2013, PNAS) for failing to take into account the confounding effect of socioeconomic status, a factor which has been shown to a significantly impact on IQ score. The results of the second study have now been confronted by a more recent publication (Weiland et al., 2015, J Neurosci) that clearly demonstrated that daily use of marijuana produced no significant changes in the size or shape of brain regions involved in the control of emotion or learning and memory.
Just last month another bit of nonsense derived from poor research methods was published and then debunked (see, https://www.leafly.com/news/health/does-marijuana-cause-alzheimers). Marijuana does not shrink the brain or predispose people to Alzheimer’s disease. Research in my laboratory (copies of publications can be obtained here: http://faculty.psy.ohio-state.edu/wenk/) has demonstrated that stimulating the brain’s marijuana receptors offer protection by reducing brain inflammation. Thus, later in life, marijuana might actually help your brain, rather than harm it. It takes very little marijuana to produce benefits in the older brain. My lab coined the motto “a puff is enough” because it appears as though only a single puff each day is necessary to produce significant benefit.
I often am asked by my students whether smoking marijuana makes it more likely that to develop schizophrenia. Forty years of research has led to the following answer: if you are not genetically vulnerable to schizophrenia then marijuana use will not induce it. It appears as though stimulating endogenous marijuana receptors may be able to unmask underlying symptoms of schizophrenia as well as other mental disorders, such as bipolar disorder, if you inherited the appropriate genes from your parents.
Marijuana or Obesity?
Given recent evidence, obese people and marijuana smokers face a challenging dilemma: do they feel genetically lucky? Each person will have a different answer to the question of “which is worse.” The answer will be determined by the genetic cards you were dealt by your parents.
© Gary L. Wenk, Ph.D. Author of "Your Brain on Food," 2nd Edition, 2015 (Oxford University Press)