Talk is cheap Dr. Pieter Cohen, show me the research!
Any stimulant can cause heart palpitations and problems, it can also highlight a previously unknown condition, it also falls into the "well if one scoop made me do so much better last time, two scoops might be twice as good" category, these supplements are prone to abuse, I have witnessed too many to count and that's just in the last year, more is better seems to be a common belief.
I am strict with my clients, but many "self medicate" and make a mistake, that mistake may be just having a large dose of supplement after already having a couple of coffees in the last hour or so, some are more sensitive than others.
DMAA increases blood pressure at a dose dependant rate without increasing heart rate (Bloomer et al, 2011). This implies that DMAA is actually a vasoconstrictor. Although this does not sound ideal for exercise performance and competitions, it may actually be effective during training sessions. Blood flow restriction is an effective way to stimulate muscle hypertrophy with a much lower intensity (Loenneke et al, 2012).
There is some evidence to show that supplements containing DMAA can be effective when used as a fat burner. When combined with other ingredients such as caffeine, a commercial fat burner was found to have significantly decrease body weight, total fat, and appetite compared to those using a placebo (McCarthy et al, 2012a). It was also found that the rate of fat burning increased to over 169% compared to a placebo and the metabolic rate increased by 35% over the placebo (McCarthy et al, 2012b).
There have been numerous scientific studies that show that if DMAA is taken as recommended, it shows little to no negative effects (Bloomer et al, 2011; Bloomer et al, 2012; McCarthy et al, 2012a; Farney et al, 2012a). It is important to remember that absolutely anything (whether legal or not) can be harmful if abused, or used by people with pre-existing medical conditions. However, when used by healthy people at the recommended dosage, the available evidence shows that DMAA is safe.
Bloomer et al (2011), Effects of 1,3-Dimethylamylamine and Caffeine Alone or in Combination on Heart Rate and Blood Pressure in Healthy Men and Women. The Physician and Sportsmedicine, 3 (online)
Bloomer et al (2011b), Effect of Caffeine and 1,3-Dimethylamylamine on Exercise Performance and Blood Markers of Lipolysis and Oxidative Stress in Trained Men and Women. Journal of Caffeine Research, 1: 169-177
Farney et al (2012a), Hemodynamic and Hematologic Profile of Healthy Adults Ingesting Dietary supplements containing 1,3-Dimethylamylamine and Caffeine. Nutrition and Metabolic Insights 5: 1-12
McCarthy et al (2012a), Biochemical and Anthropometric effects of a Weight Loss Dietary supplement in Healthy Men and Women. Nutrition and Metabolic Insights, 5: 13–22
McCarthy et al (2012b), A Finished Dietary supplement stimulates Lipolysis and Metabolic Rate in Young Men and Women. Nutrition and Metabolic Insights, 5: 23–31
Loenneke et al (2012), The anabolic benefits of venous blood flow restriction training may be induced by muscle cell swelling. Medical Hypotheses, 78: 151–154