Guanidinopropionic acid (also known as 3-Guanidinopropionic acid, beta-Guanidinopropionic acid, GPA) is a creatine monohydrate analogue . This means that its structure is similar to creatine monohydrate. Guanidinopropionic acid is a synthetic product and is added to sport supplements in combination with creatine monohydrate and is said to benefit to creatine “non-responders”.
Guanidinopropionic Acid as Supplement
Oudman and colleagues  reviewing 131 studies in order to understand better the effect of beta-guanidinopropionic acid on energy metabolism and function of tissues with high energy demands. They reported that guanidinopropionic acid decreased intracellular creatine, phosphocreatine, and ATP concentrations in all tissues studied. Studies reviewed showed that in muscle tissue this effect induced a shift from glycolysis to oxidative metabolism (conversion of biochemical energy from nutrients into adenosine triphosphate [ATP]), increased cellular glucose uptake and increased fatigue tolerance (endurance).
Weight Loss Potential
It has been demonstrated that guanidinopropionic acid improves insulin sensitivity and to promotes weight loss selectively from adipose tissue in animal models . That was also noted in Oudoman and assistants review study  which reported that after guanidinopropionic acid supplementation cellular fatty acid transporter protein concentration and fatty acid oxidative capacity were increased, which may lead to fat mass reduction. Of 117 papers in animals, 41 papers reported the effect of guanidinopropionic acid on body weight (doses of GPA ranged from 0,8 to 2,5%). Average weight decrease in these studies was 10.1%. However, weight loss measured may be due to reduced food intake as it wasn’t controlled.
Some clinical studies have demonstrated that guanidinopropionic acid damages protein , can be toxic [4,5], and that it can disrupt creatine uptake [2,6].
Guanidinopropionic Acid Side Effects and Dangers
Guanidinopropionic acid is marked as safe for human use, but the effects are unclear and there is a need for more human data.
(Other common names: beta-guanidinopropionic acid or beta-GPA, GPA, N-(Aminoiminomethyl) -beta-alanine, 3-Guanidinopropionic acid)
Clark, J. F., et al. “Actions of the creatine analogue beta-guanidinopropionic acid on rat heart mitochondria.” Biochemical Journal 300.Pt 1 (1994): 211.
Oudman, Inge, Joseph F. Clark, and Lizzy M. Brewster. “The Effect of the Creatine Analogue Beta-guanidinopropionic Acid on Energy Metabolism: A Systematic Review.” PloS one 8.1 (2013): e52879.
Vaillancourt, Valerie A., et al. “Synthesis and biological activity of aminoguanidine and diaminoguanidine analogues of the antidiabetic/antiobesity agent 3-guanidinopropionic acid.” Journal of medicinal chemistry 44.8 (2001): 1231-1248.
Perna, Alessandra F., et al. “Plasma protein aspartyl damage is increased in hemodialysis patients: studies on causes and consequences.” Journal of the American Society of Nephrology 15.10 (2004): 2747-2754.
Shainkin-Kestenbaum, R., et al. “Effect of guanidino-propionic acid on lymphocyte proliferation.” Nephron 44.4 (2008): 295-298.
Eijnde, Bert O., et al. “Effect of muscle creatine content manipulation on contractile properties in mouse muscles.” Muscle & nerve 29.3 (2004): 428-435.