Fruit is a rich source of antioxidants and offers many benefits to your dog's health and well-being. Studies have shown that diets high in fruit and vegetables may help to protect against free radicals that contribute to oxidative stress. Oxidative stress contributes to cancer, heart disease, diabetes, and inflammation. Although with all good things fruit must be fed to your dogs in moderation.
Fruit contains Polyphenols.
Polyphenols are a micronutrient contained in plant-based food. They have anti-oxidant properties and help to boost immunity and fight against oxidants in the body. They help your dog's body fight against diseases. Berries contain some of the highest levels of polyphenols in fruit.
Fruit and cancer in dogs.
A healthy diet is key to maintaining a healthy dog. Research has shown that healthy diets may reduce the chance of cancer growing by 10-20% 1. Fruit is rich in cancer-fighting properties. They are high in Anti-oxidants and help to boost your dog's immunity. Antioxidants in your pet's body are constantly fighting against free radicals. Free radicals are damaged cells in your dog's body. Free radicals attempt to heal themselves by taking molecules from other cells. This damages DNA in that cell and makes your pet more prone to disease. Polyphenols have been proven to inhibit cancer growth and spread due to their high levels of antioxidants.
Anti-inflammatory properties.
Inflammation is the body's immune system response to irritants in your dog's body. Inflammation can be caused by pathogens, injuries, and chemicals/radiation. When inflammation occurs in your dog's body various cells release chemicals. These chemicals widen the blood vessels to allow the healing process to begin.
Not all inflammation is good. Above normal levels of inflammation lead to a higher chance of becoming chronically ill. Arthritis, bowel disease, and Pisorias are due to the body's cells fighting their own cells causing inflammation. This is due to the damage inflammation does to your dog's cells, tissue, and organs. Studies have shown that a diet high in fibre, vitamin C, and polyphenol increase gut health and helps to reduce inflammation.
Oxidative Stress
Oxidative stress is an imbalance of free radicals and antioxidants. When free radicals outnumber anti-oxidants this is known as Oxidative stress. Oxidative stress can be caused by health and lifestyle influences. Diet, urban environments, and exercise all contribute. A study on oxidative stress was carried out on Sparrows in urban, suburban, and rural settings. They found that urban sparrows had a much higher level of oxidative damage compared to the two other settings. They put this down to higher levels of pollution and diet3. Studies have shown this contributes to cognitive decline, skin issues, and pre-aging due to the damage done to your dog's cells and tissue2.
The best fruit to feed your dog. 
1) Berries.
Strawberries, blueberries, and blackberries are rich in Anti-oxidants, fibre, and flavonoids. They contain the highest level of polyphenols found in fruit. Blueberries are proven to reduce cognitive dysfunction.
2) Apples.
Apples are high in polyphenol and fibre which studies have shown contribute to overall well-being and help fight against stressors. They contain a lot fewer anti-oxidants compared to berries but are excellent for gut health.
Be aware dogs can not eat the seeds or core of apples.
3) Watermelon.
Before feeding your dog watermelon ensure you remove the seeds and rind. Watermelon contains lycopene and is contained in a few foods. Lycopene is responsible for the watermelon's red hue. Studies have shown lycopene to prevent cell damage and reduce cognitive decline.
4) Pineapple
Pineapple is great in moderation. Please ensure you only feed your dog raw pineapple and not canned. Pineapple contains Bromelain which fights against inflammation, cancer, and clotting. Studies have also recently shown pineapple may help reduce the effects of arthritis.
5) Pears
Are a fantastic option for increasing gut health, fighting inflammation, and can also help with weight loss.
Do not feed your dogs the seeds or stem.
6) Bananas
Bananas contain Vitamin C, are High in B6, and also contain manganese which contributes to skin health.
7) Oranges
Are an excellent source of vitamin C, Fibre, and potassium. Oranges help boost immunity, protect your pet's cells, and help collagen production.
8) Mangos
Mangos contain an excellent amount of vitamins. They contain Vitamin A, B6, E and C. Please ensure you remove the pit as this is poisonous for your dog. Mangos do contain high amounts of sugar so use them as a rare treat.
9) Peaches
Peaches are an excellent source of fibre and vitamin A for your dog. As with mangos ensure you remove the pit first and do not feed this to your dog as it contains a small amount of cyanide. Do not feed your dog canned peaches as they contain high amounts of sugar.
10) Pumpkin
Pumpkin is a healthy and excellent option to feed your dog regularly. It is fantastic for skin and coat health and aids upset stomachs.
Fruit contains a wealth of health benefits that may aid your dog's health in the long term if added to their diet. They contain excellent levels of antioxidants, aid skin and gut health, and contribute to a high level of health. They should be consumed in moderation as part of a healthy and balanced diet.
References:
1) Diet quality as accessed by the healthy eating index, the alternate healthy eating index, the dietary approaches to stop hypertension score, and healthy outcomes: A systematic review and meta-analysis of cohort studies. https://www.sciencedirect.com/science/article/abs/pii/S2212267214018711?casa_token=v3GItfNzok0AAAAA:E9yQXhzAe7oKQOrvRVEQqarihNN5Bp4miUhYJ5Bhl0Nd215L1NWc5w9n0fN_yE2WEJqcLwY_1Q1)
2) The influence of Urban Environments on oxidative stress balance: A case study on the sparrow in the Iberian penisuala.
https://www.frontiersin.org/articles/10.3389/fevo.2017.00106/full
3) Oxidative Stress in Cognitive and Epigentic ageing: A retrospective glance.
https://www.frontiersin.org/articles/10.3389/fnmol.2020.00041/full