Caffeine Overview

How does caffeine work?

Caffeine is a compound with known psychoactive properties. Caffeine is used universally to increase wakefulness, mental acuity, and athletic performance.

A large portion of the effects of caffeine are mediated by nervous system activity. Ingested caffeine will make its way to the liver, where it will be broken down into the three components shown below:

  • Interestingly, Theobromine is the agent responsible for the diuretic effects of caffeine, and is also the compound found in chocolate that will cause toxicity in dogs.

However, before it is metabolized into these compounds, caffeine itself demonstrates structural similarity to adenosine, and will act as an antagonist to adenosine receptors to inhibit the effects of adenosine. These are four adenosine receptors in total, and caffeine will block every one of them. The adenosine II receptor, however, is thought to mediate caffeine’s anti-drowsy properties (1). The structural similarity between caffeine and adenosine is shown below.

Caffeine will also increase other physiological functions via decreasing inhibition to medullary vagal, vasomotor, and respiratory centers, which increases respiratory rate, reduces heart rate, and constricts blood vessels (2).

Caffeine also serves as a nonspecific phosphodiesterase inhibitor. By inhibiting phosphodiesterase activity in cells, the concentration of cAMP increases which may alter cell signaling in a vast array of tissues (3). Shown below is a summarize adenylate cyclase signaling pathway, where step four represents PDE activity deactivating cAMP into AMP. This step would be inhibited by caffeine consumption.

How long will caffeine stay in my system?

This will take you to a link that simulates caffeine consumption by body weight and caffeine ingested (4).

Fun facts: smoking has shown to decrease the half life of caffeine (5), while the half life is generally extended in the luteal phase of the menstrual cycle (6).

Why do we care?

Caffeine finds its way into our diet in may different forms. Between energy drinks, soda, coffee, tea, energy shots, caffeine pills, and foods like chocolate, roughly 85% of the US population consumes some form of caffeine every day (7). Interestingly, some studies have shown that 98% of college students consume caffeine regularly (8).

But exactly how much are we consuming? On average, adults consume roughly 200 mg of caffeine per day, with males averages slightly above this value and females slightly below (9).

However, this can vary across demographics. On average, total caffeine consumption has shown to increase linearly by age group until age 65, with adults 50-64 consuming on average 225 mg/day (10)

It is apparent that caffeine is a mainstay in western culture. Our productivity-at-all-cost mentality has even made a market for coffee that tout levels of caffeine above 1000 mg per serving.

With such wide spread caffeine use, it is crucial for us to understand how this drug impacts our health. Here, we will focus on the effect of caffeine on the human endocrine system.

Effect of Caffeine on growth on Kids

The effect of caffeine on children is often misunderstood. A commonly held myth is that caffeine will stunt the growth and development of children. This notion is the product of several studies from the 80’s and 90’s that demonstrated the adverse effects of calcium handling in adult men and women. These results were over-extrapolated and people took this to mean that since caffeine will decrease calcium handling, it must impair the bone growth and development of children. Three of these studies are listed below:

Clearly, none of these studies are representative of growth during childhood. In addition to this, we know that there are many more variables than just calcium balance that determine your growth and development.

In one of the few studies that investigated caffeine consumption in adolescents it was shown that there was no mean difference in bone mineral density, total bone mineral content, height, and weight in 12 and 18 year old females who consumed no caffeine, 30-60 mg of caffeine daily, and 60+ mg of daily caffeine. This suggests that caffeine has no effect on bone growth and development in adolescents, contrary to popular belief.

Effects of Caffeine on Sex Steroids

Sex Steroids: Men

A common myth regarding the effects of caffeine on testosterone is that increased caffeine consumption is associated with lower testosterone levels. The rationale behind this assumption is that caffeine increases cortisol production (we’ll get to this later) which will lead to a decrease in testosterone via negative feedback. Although logical, The evidence suggests otherwise.

In fact, the findings in regard to testosterone and how caffeine influences it have been mixed. Some studies demonstrate that caffeine increases testosterone, while others show that it has no effect.

In one study, a group of subjects (after abstaining from coffee for two weeks) consumed five cups of coffee per day for 8 weeks. This was compared to a group who performed the same activity with decaffeinated coffee. The authors found the following (14):

  • No increase in in sex hormones or sex binding hormones at the end of the 8 weeks
  • However, males in the caffeine group demonstrated a significant increase in testosterone and a significant decrease in estradiol at week four.
  • This suggests that an acute increase in caffeine consumption may elicit an increase in testosterone, but this effect is attenuated with time as the body returns to homeostasis.

Very frequently, people take caffeine in conjunction with exercise. This may also have endocrine implications, as one study demonstrated that in professional rugby players, caffeine consumption one hour before a workout lead to a dose dependent increase in both salivary cortisol and testosterone levels after exercising, suggesting an anabolic effect of caffeine when coupled with resistance training. It is worth noting that one of the doses tested was 800 mg of caffeine (15).

  • The authors also state the possibility that this anabolic increase may be counteracted by the carbolic increases in cortisol-something to consider before taking 800 mg of caffeine before exercise.

In addition to these data, the authors showed that higher doses of caffeine potentiated the exercise induced increase in cortisol and allowed cortisol to keep increasing late in the exercise bout, when it started to decline in the placebo group.

However caffeine did not have the same effect on the time course of testosterone; salivary levels of this hormone began decreasing at the same time during the workout in placebo vs. all experimental caffeine doses.

Combined, this means that the testosterone/cortisol ratio decreases over the course of a workout when using caffeine relative to placebo, which may be counterproductive to the goal of resistance training.

In another study looking at caffeine, exercise and testosterone, the authors took competitive cyclists and made them complete high intensity intervals with and without caffeine. The authors showed that caffeine supplementation not only increased serum testosterone, but also attenuated serum cortisol later in the workout. These results are shown below (16).

Clearly, there are inconsistencies in the literature on caffeine’s effect on testosterone, with and without exercise. This topic is controversial, however the evidence suggesting a tonic low level of testosterone resulting from caffeine consumption is scarce.

Sex Steroids: Women

Again, there has been conflicting evidence regarding the effect of caffeine on estrogen and progesterone levels. These effects tend to be varied within the literature, and no mechanistic effects of caffeine on sex hormones are known. Seeing as there is a relationship between estrogen/progesterone levels and female cancers, the effects of caffeine on female hormones are of interest in order to determine if caffeine consumption impacts the rate of these diseases.

In one of the first studies to observe the relationship between caffeine consumption and breast cancer, almost 100,000 nurses were followed for a 22 year period. During this time, their caffeine intake was assessed, and also the rates of breast cancer were determined in the various groups stratified by caffeine intake. The researchers showed that caffeine consumption was weakly (but significantly) inversely correlated with breast cancer in postmenopausal women. However, these researchers did not investigate hormone levels in this population (17). Other groups have also demonstrated similar findings (18), while others show that coffee (not specifically caffeine) consumption lowers the risk for premenopausal breast cancer, but not postmenopausal (19).

To follow this up, other various groups looked at the relationship between estrogens and caffeine intake. They demonstrated the following (20):

  • Premenopausal: caffeine is inversely correlated with luteal free estrogen, but positively correlated with luteal progesterone
  • Postmenopausal: caffeine is positively correlated with SHBG (sex hormone binding globulin), which would effectively decrease estrogen bioavailability

Another group examined the relationship between caffeine and the sex hormones FSH, LH, E2, and SHGB in women 36-45 (21).

  • They showed that caffeine consumption lead to higher levels of E2 during the follicular phase, especially in conjunction with high cholesterol consumption.

These studies together suggest that caffeine has mixed effects on estrogen levels dependent on various phases of the menstrual cycle, and dependent on your menopausal status. The mechanism behind these changes remains unknown.

It is generally recommended that women avoid caffeine consumption during pregnancy, however large scale studies have shown that up to 70% of pregnant women consume some form caffeine with an average intake of just under 200 mg per day (22). In order to investigate the effects on caffeine intake on the mother and child, a rat model was used. Rats ingested (through drinking water) various amounts of caffeine per day during pregnancy and lactation, and the adrenal weight, levels of testosterone, E2, and DHT levels of the fetal and postnatal rats were measured using radioimmunoassays. The following results were observed:

  1. High doses of caffeine decreased the adrenal weight of male rats in-utero.
  2. Caffeine intake during pregnancy increased brain testosterone in both males (in hypothalamus) and females (in frontal cortex).
  3. Note: their dosage for the high dose animals where most results were found was 40 mg/kg/day. This means that for a 70 kg reference human (154 pounds), this would equate to 2800 mg of caffeine each day (or 7 x 20 oz black cups of coffee from Starbucks). This dose is not physiologically relevant, and does not reflect the average amount of caffeine consumed in pregnant women.

Effects of Caffeine on Metabolism

Insulin

What do we currently know about how caffeine affects our metabolism? You may have heard that consuming coffee decreases your risk for developing diabetes. But is this true? And if so, what is the mechanism driving this? Is this due to the caffeine, or other elements of coffee?

Seeing as we are interested in caffeine, let’s see what the literature has to offer. Recently, meta analysis from 2016 (23) and 2017 (24) suggested that caffeine intake would actually prolong a state of hyperglycemia, and decrease insulin sensitivity. However, this doesn’t really match up with the common notion that coffee intake (which has caffeine) aids in preventing diabetes.

To dive into this a little further, a group of researchers evaluated the effects of insulin sensitivity in rats. To do this, the researchers put indwelling cathoders in rats in order to administer caffeine, and also to measure insulin tolerance. Insulin tolerance was measured by the insulin tolerance test, where a bolus of insulin is administered and the change in blood glucose was measured. The researchers showed that caffeine decreased insulin sensitivity in these rats, consistent with the findings from the meta-analysis. A summary of the researchers’ findings is presented below, where the y-axis positively correlates with insulin sensitivity (25).

But what is causing this insensitivity? Another group posed that same question, focusing primarily on the GLUT4 exocytosis pathway. First, they demonstrated that glucose uptake was inhibited by caffeine only in the presence of insulin, indicating that GLUT4 exocytosis was impaired when caffeine was administered (26).

Next, by utilizing adipocyte culture and molecular techniques, this group looked at various steps in the GLUT4 pathway to see which step may be getting inhibited in the presence of caffeine. They demonstrated that at increasing caffeine concentrations Akt activity decreased, indicating that caffeine affects the phosphorylation status of the GLUT4 signaling cascade to decrease insulin sensitivity and prolong hyperglycemia.

So is coffee really associated with lower rates of diabetes? Is this just another myth? Thus far, we have no support for this notion in terms of the caffeine component.

However, it is well established that coffee does in fact lower diabetes. As shown below, the rate of diabetes decreases linearly with caffeine consumption, as demonstrated below in a sample of 1141 individuals (27).

So if coffee really is lowering our risk of developing diabetes, how is it doing this? is it the coffee, or the caffeine?

One meta-analysis demonstrated that the risk of developing diabetes decreased in the same fashion for those who consumed caffeinated vs. decaffeinated coffee, suggesting that the agent responsible for lowered risk of diabetes was not the caffeine component of coffee (28).

So what is it? It appears that other components of coffee may be lowering the risk for diabetes. This topic was reviewed by a group in 2016 (29), who discussed that many other roles that coffee may play in other health processes. For demonstration, a summary of this is shown below, however these effects cannot be attributed to caffeine, so we will not go into detail here.

Cortisol and stress axis

Our stress response is an evolutionary tool designed to prepare us for acute threatening situations. This response has two main components: the neural response, and the hormonal response. When presented with an acute stressor, our nervous system acts on the cardiovascular system to promote vasoconstriction and increase cardiac output, which raises blood pressure. When we don’t need these responses (i.e. not presented with stressor), they are inhibited. Part of this neural inhibition comes from adenosine binding to adenosine receptors to hyperpolarize these nerves, preventing the stress response signal (30). Adenosine has also been shown to cause vasodilation and a decrease in cardiac output by directly acting on these target organs (31).It has even been shown that adenosine may act pre-synaptically to prevent excitatory neurotransmitter release (32).

Seeing as caffeine is a known adenosine antagonist, it logically follows that by inhibiting these receptors, the opposite effects would be observed. This proves true, with caffeine having a known response on the neural stress response to increase blood pressure, heart rate, circulating catecholamines (33). In addition to this caffeine has been associated with an increased renin secretion of up to 50%, which also contributes to the increase in blood pressure that we see (34). One of the most profound effects of caffeine is its ability to increase cortisol production in humans.

Caffeine has been shown to increase perceived anxiety and salivary cortisol (35), and caffeine’s effect on cortisol has been shown to be attenuated with habitual caffeine consumption (36). This means that your cortisol rises associated with caffeine intake will be blunted if you consume coffee regularly, whereas non-coffee drinkers will have greater cortisol spikes in response to caffeine.

But at what step in the process of cortisol production is this increase occurring? Remember that CRH signals ACTH which signals cortisol, and dysregulation at any of these steps may result in a change in cortisol levels.

Although there is some literature characterizing the mechanism by which caffeine increases cortisol levels, there is poor characterization of the effect of caffeine on CRH levels. However, studies have been done investigating the impact of caffeine on both ACTH levels and cortisol. These studies showed that by giving healthy males the equivalent of 2-3 cups of coffee, both ACTH as well as cortisol levels increased compared to placebo over the course of three hours as shown below (37).

It is clear that an increase in pituitary ACTH is causing an increase in cortisol secretion. But is this the only part of this axis affected by caffeine? The literature would suggest no.

The effects of caffeine on the adrenal cortex directly for the first time in 2012 (38). This group of individuals]isolated adrenocortical cells, and cultured them with varying levels of caffeine. This group looked for several things that would indicate an intrinsic cortisol response: changes in cortisol production, changes in enzymes in the cortisol production pathway, and methylation of the genes of these enzymes.

When the adrenocortical cells were cultured with increasing levels of caffeine, the researchers demonstrated an increase in cortisol concentration in the culture. This suggests that these cells must be intrinsically responding to the caffeine to increase cortisol production.

Next, the researchers investigated the steroidogenic acute regulatory protein (stAR); the rate limiting enzyme in the pathway of cortisol synthesis. Upon western blot and rt-PCR analysis, the researchers found this protein to be upregulated in cells treated with caffeine, consistent with the increased levels of cortisol production observed.

Finally, the researchers looked at the methylation levels of the stAr gene promoter. They found that the stAr promoter was significantly demethylated in cells cultured with caffeine, explaining the increase in expression. However, the researchers also showed that when the cells were passaged without caffeine, the stAr promoter remained demethylated and cortisol production remained high for 10 generations of cell passaging… in the absence of caffeine.

Calcium handling

We know that caffeine will decrease calcium handling, and that excess caffeine can be a risk factor for osteoporosis. But what is the mechanism driving this? It is well established that caffeine will increase urine calcium levels, and this has been known before the millennium as evidenced by articles from the 80’s showing an increase in calcium excretion as caffeine intake increases (39).

However, at what level is caffeine affecting this process? It is well understood that caffeine impairs calcium absorption. This prevents dietary calcium from entering the bloodstream, causing its excretion in the form of urine in a dose-dependent manner (40).

However, is there more to this story? Could caffeine cause more liberation from your calcium stores (bones) as well as interfering with absorption?

There is research on the relationship between caffeine and osteoclasts/osteoblasts. In 1996, a group looked at the osteoblast population in wild-type rats and rats that had been fed a high caffeine diet for most of their lives. These researchers showed that the number of osteoblasts was significantly lower in the caffeine fed-rats. These rats had significantly less bone mass, as can bee seen below. This is a section of the femur of the rats; it can be seen that there is significantly less bone mass in the caffeine fed rats compared to wild type (41).

In addition to altering the number of bone forming cells, the number of osteoclasts was also shown to increase in bone marrow cells incubated with caffeine. These results are shown below, with purple cells expressing markers unique to osteoclasts, as represented in the bar chart.

Caffeine seems to influence calcium handing from two angles. The first, from the angle of absorption, where increased caffeine intake decreases the amount of calcium absorbed. This is further potentiated by a shift in the osteoclast/blast population, leaning towards liberating calcium from the bone. This explains why caffeine is associated with low bone mineral density, and it is important that we achieve adequate calcium if we are consuming caffeine to balance out this effect.