As coffee is the 2nd-most consumed beverage in the world (2nd only to water), there is a lot of discussion and research on its effects on health. Unfortunately, the research is generally from low-quality observational trials, but there appears to be some meaningful takeaways.
Coffee
An estimated 66% of Americans drank coffee in the past day, and coffee consumption is at a two-decade high.
Coffee contains over 700 volatile (readily evaporated) substances. Of the nonvolatile substances, caffeine is the most important one in terms of its physiologic effects on humans.
There are many potential contaminants in coffee, including pesticides, nitrosamines, organic solvents, and mycotoxins due to moldy beans.
Caffeine
A 6 oz cup of coffee contains 85-100 mg of caffeine.
For comparison:
- Tea = 40 mg
- 12 oz soda = 45 mg
- Decaffeinated coffee = 2 mg
Caffeine is completely absorbed from the gut, and its peak effect is evident in 30-60 minutes.
The daily dose of coffee intake at which caffeine withdrawal occurs is 2.5 cups per day.
Research on Coffee
The following research is mostly derived from low-quality studies – prospective, observational, correlational studies – from which cause-effect relationships cannot be derived. Also, there is significant variability in results, with studies coming to different conclusions in many areas. Thus, the following bullet points indicate which direction the research leans toward.
Interpret with caution.
Positive Effects
Increases metabolic rate https://pubmed.ncbi.nlm.nih.gov/7369170/
Increases adiponectin https://pubmed.ncbi.nlm.nih.gov/21914162/
Reduces levels of uric acid; associated with lower risk of gout https://pubmed.ncbi.nlm.nih.gov/26905267/
Improve exercise performance https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3616086/
Correlated with lower rates of colon cancer https://academic.oup.com/aje/article/147/11/1043/62063?login=false
Lower rates of Parkinson’s Disease in men, and women not on HRT https://pubmed.ncbi.nlm.nih.gov/12629235/
Moderate consumption of coffee is associated with a 31% decrease in mortality https://www.acpjournals.org/doi/abs/10.7326/M21-2977?journalCode=aim
In women, depression risk decreases with increasing coffee consumption https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3296361/
May be associated with decreased risk of dementia (inconsistent data) https://pubmed.ncbi.nlm.nih.gov/20182054/
Decreased risk of Parkinson’s Disease https://pubmed.ncbi.nlm.nih.gov/23879665/
Association with liver health benefits https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4862107/
- Decreased liver enzymes
- Inhibition of Hepatitis B virus
- Inhibition of replication of Hepatitis C virus, less severe fibrosis, decreased steatosis (fatty liver), decreased evolution of chronic Hepatitis C, better tolerance of interferon therapy
- Decreased fatty liver, decreased progression to fibrosis in NAFLD
- Decreased risk of hepatocellular carcinoma, a liver cancer
Association with decreased Metabolic Syndrome https://pubmed.ncbi.nlm.nih.gov/17070955/
Nonsignificant effect on insulin resistance https://pubmed.ncbi.nlm.nih.gov/34836231/
Decreased risk of Type 2 Diabetes https://pubmed.ncbi.nlm.nih.gov/24459154/
Association with decreased risk of gallstones in women, but not in men https://pubmed.ncbi.nlm.nih.gov/26198295/
Decreased risk of gallbladder cancer https://pubmed.ncbi.nlm.nih.gov/28376203/
Negative Effects
Increases heart rate and blood pressure https://pubmed.ncbi.nlm.nih.gov/339084/
Increases cortisol https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2257922/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2249754/
Coffee contains mycotoxins https://www.sciencedirect.com/science/article/abs/pii/B9780124095175000255
Correlated with increased risk of Parkinson’s in post-menopausal women using HRT https://pubmed.ncbi.nlm.nih.gov/12629235/
Intake of > 300 mg/day of caffeine can increase risk of miscarriage https://www.ajog.org/article/S0002-9378(07)02025-X/fulltext
Increases anxiety; induces panic attacks in large proportion of individuals with panic disorder https://www.sciencedirect.com/science/article/pii/S0163834321001614
Disrupts sleep if consumed within 6 hours of bedtime https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3805807/
Increased risk of stomach cancer https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8778943/
Summary
In moderation, 
coffee poses no threat to one’s health. Naturally decaffeinated, filter-brewed coffee poses even less threat.
Perhaps the main threat of coffee is the additives – sugar, creamer, etc. – that introduce harm. The addition of sugar is associated with weight gain. (Ref) While cream or nondairy whiteners do not associated with weight gain, there may be negative health consequences of products containing seed oils and artificial sweeteners.
Theoretically, roasting destroys 80-99% of mycotoxins. Consider mold-free coffee to avoid mycotoxins.
Avoid large amounts of caffeine during pregnancy. In the interest of caution, it is perhaps best for pregnant women to minimize/eliminate caffeine intake.
