Dog breeds were developed to accentuate behaviors that serve a specific purpose, which influenced the size and physical characteristics of the breed. Today, however, most dogs no longer serve a job in a home and some behaviors become disruptive. Millions of dollars are spent every year on pet services, including obedience training. To date, very little research exists evaluating the interactions of nutrition with behavior in dogs and other species.
Amino acids tryptophan and tyrosine are the most studied with regards to influence on behavior. Many amino acids act as precursors to neurotransmitters (e.g., tryptophan is a precursor for serotonin). Tryptophan and other large neutral amino acids (LNAA) are able to cross the blood-brain barrier, but the amount that crosses depends on the amount of free tryptophan and the amount of other LNAA available. Increasing tryptophan: LNAA can increase the amount of serotonin affecting the brain. Animals consuming high-tryptophan diets had reduced aggression (Gibbons et al., 1979; Kantak et al., 1980) and enhanced recovery after stress (Koopsman et al., 2005), but other researchers noted increased territorial behavior (Lasley and Thurmond, 1985).
Limited research is available in dogs and much of the research was not conducted in controlled environments. Lower protein diets appear to decrease territorial aggression in some but not all dogs (Mugford, 1987; Dodman and Shuster, 1998) and higher tryptophan to LNAA ratio diets may be beneficial in reducing aggressive behavior (DeNapoli et al., 2000). More work is needed to determine the mechanism, but work in other animals and limited research in dogs indicates a link.
Polyunsaturated fatty acids (PUFA), and specifically dexahexanoic acid (DHA), have received widespread attention in human nutrition due to benefits on inflammation and cognition. Lipids serve as integral components of cellular membranes, are precursors to chemical messengers and are sources of energy. Incorporation of more PUFA into the cellular membranes in the brain creates a more fluid membrane allowing neurotransmitters to flow between cells. In rodents, enriched-DHA diets improved learning ability (Lim and Suzuki, 2001), and DHA-deficient diets reduced learning capabilities (Bourre et al., 1989; Moriguchi et al., 2000).
A large-scale study evaluated 28 litters of puppies receiving either adequate- or enhanced-DHA diets fed throughout gestation, lactation and after weaning (Hoffman et al., 2005). Puppies fed the enhanced-DHA diet made fewer errors during training and had a higher training performance index as outlined by researchers. It is not yet established if a similar increase in trainability is possible in adult dogs fed enhanced DHA diets.
There is currently no published literature evaluating the influence of digestible carbohydrates on behavior in dogs, but limited research with regards to fiber and negative behaviors is available. Many companion animals exhibit food intake behaviors that can be a nuisance to owners due to feelings of hunger, such as whining and begging. While many of these behaviors are more likely learned to obtain extra food and not out of hunger, some of these may be curbed by providing a diet that is more satiating. Simple carbohydrates are digested and absorbed quickly leading to only short-term fullness and a sharp peak and decrease of blood glucose concentrations. Complex carbohydrates take longer for the body to break down and absorb, thereby blunting glucose spikes. It is thought that fiber, or indigestible carbohydrates, decrease hunger or increase satiety, due to influences on stretch receptors in the stomach (gut fill), satiety hormones and transit time. It is possible behaviors associated with hunger may be reduced when feeding a more satiating diet.
High fiber diets have reduced overall voluntary daily energy intake in dogs (Jackson et al., 1997; Jewell and Toll, 1996), but only one study evaluated any behaviors associated with feeding higher fiber diets. Butterwick and Markwell (1997) noted no differences in time spent at the feeding bowl or number of visits to the bowl 30 minutes after feeding high-insoluble fiber (up to 7% cellulose) or soluble fiber (up to 4% gum blends) diets.
The lifespan of dogs has been increasing in recent years leading to more senior and geriatric dogs. With age comes the risk of dogs developing cognitive dysfunction disorder (CDS), which is often diagnosed after an animal starts acting disoriented, has decreased social interactions, soils in the house, has sleep disturbances and/or altered activity. A combination of an antioxidant cocktail and environmental enrichment improved learning (Nippak et al., 2007), discrimination learning, and long-term retention (Milgram et al., 2002) in aged beagle dogs.
It is clear that more is needed regarding how nutrition affects behavior, but it is a growing area of research with direct implications to pets kept in homes. Evaluating nutritional interventions on aged animals or dogs with severe behavioral problems may provide insight into managing an array of behavior issues.