Caffeine: The Key to a Successful Diet?

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When I started my contest prep diet over eight months ago, I knew I was in for a long journey. By the time I hit the stage, I had dropped nearly 16 kg (35 lbs).

Even though I was committed to evidence-based training and diet practices, I was looking for any advantage I could take in order to make fat loss as efficient as possible.

Caffeine instantly came to mind. Hop online or scroll through your timeline and you’ll be smothered with #TransformationTuesday collages and cleverly-worded supplement ads praising the benefits of the latest, greatest fat-burning pill or Tummy Tea, which is all but assuredly packed with caffeine.

But even if you don’t fall for any of the flashy stuff online, caffeine is firmly cemented in the folk-lore of the bodybuilding community. Time and again, we hear people claim that caffeine suppresses appetite, boosts metabolism, burns fat and even enhances performance (it’ll even pick up your dry-cleaning for you!). 

Sure, these are all things that would make my life 1000x easier on a weight loss diet, but my inner skeptic pushed me to see what the science had to say about the “amazing” effects of caffeine. 

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For reference, I’ve highlighted the caffeine content in one cup/ glass (8 oz. / 237 ml) of various beverage, which should make the values presented in this article more relatable to your everyday life. 

Appetite-Suppressing Effect of Caffeine

Most research doesn’t show any appetite suppression at lunch when caffeine was consumed in reasonable amounts (~3 mg/kg, or 2 cups of brewed coffee) in the morning (123). 

One study noted that 6 mg/kg caffeine in the morning significantly reduced energy intake for the entire day in overweight/obese subjects, but their subjective appetite was not reduced. To make matters worse, this finding didn’t apply for normal-weight subjects. Their energy intake and appetite were similar to the control group that didn’t consume any caffeine. 

Some people may experience acute appetite suppression when drinking coffee, but the effect is unlikely to last longer than it takes to actually finish their cup. 

Strike one against the wonder-drug.

Fat-Burning Effect of Caffeine


Don’t hold your breath: caffeine’s impact on energy expenditure and subsequent fat burning effect is negligible. 3 mg/kg doesn’t seem to have any effect (123), while another study has shown that 5-7 mg/kg (about 400 mg) can increase energy expenditure by 10 kcal/hr for up to three hours following consumption. 

10 kcal per hour isn’t too impressive; it’s about equivalent to the calories in a couple of strawberries or the energy you’d burn walking for a few minutes. 

You might be thinking: if I just consume 400 mg caffeine every three hours, then I could burn 240 extra calories, right? 240 kcal is not too shabby, but drowning yourself in coffee might not be the best decision.

As with most beneficial things in life, caffeine also comes with a cost. And when you’re dabbling with extremely high amounts like you would have to in order to boost your metabolism to any relevant degree, there are several. 

Excessive caffeine consumption can increase anxiety, interfere with your sleep quality and lead to caffeine addiction. Keep up this level of intake for any amount of time and you’ll also be subject to caffeine withdrawal,complete with migraines, fatigue, decreased alertness, depression, nausea  or even flu-like symptoms. (I should also mention that 400 mg every three hours literally adds up to 2.4 grams of caffeine, which could potentially be lethal.)

Tolerance creeps up in a hurry, making smaller doses less stimulating. The most unfortunate part is that withdrawal symptoms can be observed in subjects who were using as little as 100 mgper day. 

Strike two.

Caffeine’s Impact on Strength Performance


Unfortunately, the amount of caffeine that could acutely increase strength performance is about the same as is required to increase energy expenditure. The minimal dose for an increase in muscle power seems to be 3 mg/kg, but caffeine dosages going all the way up to 7 mg/kg have been shown to increase upper body strength and maximal strength (1RM)

If you really need to push your strength performance for 1RM testing or a big lift during a competition, then a large dose of caffeine may actually be a good tool. But not only is this approach unsustainable, it typically only works for people who are naïve to caffeine (i.e. consume very little to none) in the first place.

Strike… two and a half? 

Take-Home Message: 

Caffeine basically strikes out once you dig into the data behind some of the more common claims. Very high doses of caffeine (5-7 mg/kg) may help you burn a few more calories, reduce your energy intake a bit if you’re overweight, and can even acutely increase performance if you’re not a habitual consumer. But with this many caveats, it’s hard to justify caffeine’s reputation as a wonder-drug. 

It doesn’t seem to have any appetite reducing effects; a research study actually showed that people who expect to consume caffeine (without actually consuming it) feel more energized and perform better, meaning they were subject to the placebo effect. 

High doses may also be used as an efficient tool to boost strength when necessary (e.g. powerlifting competitions), but should absolutely not be consumed on a daily basis. 

As for me, I actually didn’t use caffeine at all during my contest prep, and I managed to lose fat just fine without it. At the risk of sounding cliché, there’s no “quick fix” out there to overcome any worthy obstacle, and caffeine Is no exception.
Now that I’m not focused on dieting for a competition, I’ll occasionally opt for a hefty dose of caffeine pre-workout to give me an extra push when I’m attempting an important lift. But if you’re not pushing to break personal strength records and care more about your body composition, your best bet would be to enjoy your coffee or caffeinated beverage in moderation. 

Don’t expect any magic from caffeine, unless David Blaine gave it to you ;) 

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Scientific content creator: Anastasia Zinchenko, Ph.D.

Editor: Joe Flaherty, B.Sc., Certified Bayesian and NASM Personal Trainer, Joe's page