What is HMB?

HMB (Beta-Hydroxy Beta-Methylbutyrate) is a metabolite (by-product) that naturally occurs in the body, particularly in the muscles. It is produced during the metabolism of an essential amino acid called leucine. Leucine plays a crucial role in triggering muscle protein synthesis, promoting muscle growth, and facilitating repair. As a by-product of leucine, HMB plays a significant role, especially in the preservation and reconstruction of muscles.

HMB Metabolism

HMB is naturally produced in animals and humans from the amino acid leucine. The initial step in HMB production involves the reversible transamination of leucine to alpha-keto-isocaproate (KIC) by the enzyme branched-chain amino acid transferase (an event where an amino group of an amino acid is transferred to a keto acid). After being metabolized to KIC, it is further metabolized to HMB in the mitochondria with various enzymes. Only about 5% of leucine is converted to HMB. To put it into perspective, to obtain the typical daily HMB dosage used in human studies (3 grams), an individual would need to consume over 600 grams of high-quality protein, which is impractical. Therefore, HMB is typically increased through dietary supplementation.

Types of HMB

There are two different forms of HMB: Calcium Hydroxymethylbutyrate (HMB-CA) and Free Acid Hydroxymethylbutyrate (HMB-FA).

• HMB-CA: This is a compound found in calcium salt form, where HMB (beta-hydroxy-beta-methylbutyrate) is formulated with calcium.
• HMB-FA: HMB-FA is a compound found in free acid form. This means it represents the free form of HMB, directly HMB without being bound to another compound. There is no conclusive evidence on which form is superior.

Dosage of HMB

When considering the optimal use of HMB supplementation, both acute and chronic effects of HMB should be taken into account. It is recommended to take 3 grams of HMB-CA supplement at least 60 minutes before intense exercise. If this supplement is consumed with glucose (carbohydrates), it can be taken approximately two hours before exercise. On the other hand, HMB-FA form tends to raise plasma concentration levels faster and can lead to significant effects. Therefore, athletes can consume the HMB-FA supplement 30-60 minutes before exercise. Lastly, to optimize the chronic effects of HMB, it is recommended to consume 3 grams daily, divided into three equal portions, at least two weeks before starting workouts.
Recent studies suggest that a daily intake of 38 mg/kg/day of HMB supplementation, divided into 2-3 portions, is suitable for achieving optimal effects.

Safety of HMB

Observations from a one-month study in humans consuming 6 grams of HMB daily showed no effects on cholesterol, hemoglobin, white blood cells, blood sugar, liver, and kidney functions. Additionally, two separate meta-analyses have indicated that HMB supplementation, either alone or combined with glutamine and arginine, is safe and does not lead to adverse effects. Baier and colleagues conducted a study on elderly individuals, observing that the daily consumption of 2-3 grams of HMB-CA, along with amino acids, did not cause any changes in blood or urine markers indicative of liver or kidney function.

Effects of HMB

In a study conducted by Kraemer and colleagues, recreationally active individuals who did not engage in resistance training participated in a 12-week periodic training program. The subjects were randomly given 3 grams of HMB-CA supplementation containing 14 grams of glutamine and 14 grams of arginine, or a placebo, in a double-blind manner. The training program aimed for continuity in strength, hypertrophy, and strength endurance, utilizing three different loading models. Results showed that the HMB-CA group of previously untrained subjects experienced greater gains in lean muscle mass.
Panton et al. examined the effects of a four-week HMB supplementation during resistance training in 36 women and 39 men (20-40 years old) with different levels of training experience. The training protocols consisted of a high-intensity exercise system. Due to the high intensity of the protocol, the HMB-CA group showed greater reductions in body fat compared to the placebo supplementation group.
In a recent study on female judo athletes subjected to three days of caloric restriction, those who consumed HMB-CA showed significant decreases in body weight and body fat percentage compared to the control group.
Another study investigated the effects of additional beta-hydroxy-beta-methylbutyrate (HMB) on muscle damage resulting from intense endurance exercise. Participants were matched based on their 2-mile run times and past running experiences, and each pair was randomly given HMB (3 g/day) or a placebo. After six weeks of daily training and supplementation, all participants engaged in a long run (20 km trail). Creatine kinase (CK) and lactate dehydrogenase (LDH) activities were measured to assess muscle damage before and after the long run. The placebo-supplemented group exhibited a significantly larger increase in creatine kinase activity after the long run compared to the HMB-supplemented group. Furthermore, the LDH activity of the group receiving HMB supplementation was significantly lower compared to the placebo group. These results indicate that HMB reduces skeletal muscle damage.

Conclusion

HMB (Beta-Hydroxy Beta-Methylbutyrate) may have positive effects on accelerating recovery after high-intensity exercise. These effects seem to depend on when HMB is taken in relation to exercise, which form of HMB is used, the time of intake before exercise, the dosage, and the experience level of the athlete.
Especially, if HMB-FA (Free Acid Form) is used, it should be taken 30-60 minutes before exercise, whereas if HMB-CA (Calcium Salt) is used, it should be taken 60-120 minutes before exercise. Additionally, the impact of HMB may vary with the exercise protocol used and the athlete's experience. Even low-volume, high-intensity resistance training can cause enough muscle damage for individuals with a short training history to benefit from HMB supplementation. In addition to accelerating recovery after high-intensity exercise, HMB can help prevent the loss of lean body mass in athletes under catabolic conditions such as caloric restriction. Further studies in these areas will increase reliability.