My caffeine habit started several years ago. I never liked coffee, but suddenly I found myself in the midst of a very busy and tiring time in life (aka, dietetic internship), and those around me swore by their brain-boosting juice, coffee. I didn't like it at first, but it grew on me. Here I am today, still sipping on my morning cup of Joe. Many people turn to caffeinated beverages or food (or even candies and lipsticks now!) for that extra jolt of 'energy'. So, if someone needs that energetic boost, do I tell them to reach for a cup of Joe? Not necessarily. Believe it or not, caffeine is not actually a source of energy. The major sources of energy in the diet are carbohydrate, fat, and protein. These nutrients provide fuel for the body to function each day. Caffeine does not provide energy.
So, when someone feels the need to reboot the brain, why does s/he take a quick stroll through the nearest Starbucks? Well, caffeine is a stimulant. Even in small doses, it stimulates the central nervous system causing an increase in heart rate and mental alertness among other things. In fact, studies show that caffeine does improve mental performance by enhancing alertness and reaction time, and this happens whether someone is a caffeine addict or a newbie. Caffeine consumption in moderate doses may also temporarily enhance mood, especially for those who are sensitive or don't consume much.
One thing to note is that caffeine does disrupt normal sleep patterns for most people if consumed too close to bedtime. First, it makes it harder for one to fall asleep, and once one is asleep, it can result in temporary awakenings throughout the night. So, if you have problems with sleep, I suggest limiting caffeine consumption within 3-5 hours of bedtime. Otherwise, you may find yourself in a cycle of: caffeine consumption close to bedtime, limited/interrupted sleep, and the need for more caffeine the next day to offset the effects of disrupted sleep.
You may have heard that caffeine is a diuretic, right? Well, it does have a mild diuretic effect; however, given that coffee accounts for about 75% of the caffeine that most people consume, there's no need to worry about dehydration anytime soon. The amount of caffeine contained in most caffeinated-beverages is not enough to have a major diuretic effect. In addition, the fluid from the beverage contributes to a person's fluid needs.
So, what is my advice? Moderate consumption, which is up to about 300 mg/day (or ~2-3 cups of coffee), is okay for most people. I say most because there are some exceptions. While some recommendations suggest that moderate caffeine consumption during pregnancy is okay, I err on the side of caution here, especially in the first trimester. A recent study indicated that pregnant women who consumed at least 200 mg/day experienced more miscarriages than those who consumed less than 200 mg/day during the first trimester. If possible, I suggest avoiding caffeine or limiting intake to <200 mg/day during the first trimester especially. Moms who breastfeed may also want to avoid/limit consumption to no more than 300 mg/day because higher intakes have been associated with increased wakefulness, irritability, and poor feeding for the baby. None of which any new parent really wants, right? Caffeine may also perpetuate problems for people with migraines, so it's best for those with a genetic predisposition for migraines to limit their intake. There are also certain medications, such as Ritalin, or nutrients, such as iron, that can be affected by caffeine, so always speak with a doctor, pharmacist, or RD regarding potential drug-nutrient interactions. Lastly, I advise parents/caregivers to moderate their child's caffeine consumption. It has the same effect in children and it tends to occur more quickly and last for longer than for adults. Plus, some of the major sources of caffeine in the child's diet aren't necessarily the most nutritious either (i.e., sodas, energy drinks, candy).